Radiant heating system design FAQs:

This article provides questions & answers about the design & installation of radiant heating systems for homes.

The workers in the photograph at page top, where our concrete slab was being poured, were not guilty of a thing. But the contractor placed radiant heat floor tubing too deep and he omitted proper under-slab insulation. The owners ultimately had to abandon the entire radiant heated floor system.

Radiant Heat Design & Installation Questions, Answers, Arguments

These questions & answers about proper design for radiant heat systems in buildings were posted originally

at RADIANT HEAT - home - be sure to see that article.

Also be sure to review RADIANT HEAT MISTAKES.

On 2019-09-23 by ray

Do I need 4 inches of insulation under concrete radiant slab?
I'm building a 2500 sq ft house with frost wall, four inch concrete floor with hot water radiant I've insulated the exterior of frost wall with 2 inch polystyrene T&G foam down to footing and two feet out from top of footing. I using 1/2 sheeting with 1 1/2 dense foam glued to back side which will meet up with foundation foam. I'm using 4 inches of polystyrene foam under the 4 inch interior concrete floor.

On 2018-02-11 by (mod) -

For a slab on grade with no Footers or which I would call a floating slab or a momolithic with supporting Footers as the contractor might pour it, you will need both steel reinforcement and particularly at the size that you describe, you will need control joints to prevent uncontrolled cracks zigzagging through your slab.

I would suggest That you make one of those control joints at the partition between the heated and unheated slabs. At that control join you can include a couple of inches of high-R solid foam insulation that will be in fact probably under a partition wall.

Remember that you need to insulate under the entire slab as well as insulating the perimeter of the entire slab. Particularly in the Michigan climate that will be essential.

On 2018-02-11 by Mike Hoezee

I am building a 40x60 pole barn with in interior 18x30 Iiving space on one corner of the building. The living space will be heated, the other will not be heated. I'm wondering how to isolate the living slab section and insulate it so the larger slab doesn't draw heat from the smaller slab. I could insulate the entire slab but if I can't isolate the two slabs, I assume the unheated slab will just draw off the heat. How do I isolate the slabs and where do I put perimeter I insulation in this scenario. This is a slab on grade no footers in Michigan and we see temps below 20F in winters.

On 2018-01-14 by (mod) -

Ar

My OPINION is that anti-crack additives are helpful insurance but absolutely no guarantee against cracks forming in concrete pours. Control joints (that we fill with a suitable sealant) are a helpful and sometimes essential design feature to control slab cracking. In larger tile floors we match the control joints with a flexible sealant that matches the grout used in the rest of the floor.

My OPINION is that the total R-value under the floor is a key design feature when installing any radiant heated floor - despite some ridiculous claims I've come across that "once you heat the ground you stop losing heat into it" the fact is that the earth is an infinite heat sink. So insulating below and around the perimeter of a slab is important. You can add perimeter slab insulation from outside. From inside you can put as high an R-value as you can fit into the available space when making your floor. For that reason I look for high-R and foil-faced foam or equivalents.

I can't say what R-value you need - not knowing even where your building is located nor its heating loads.

To be more smart than some installers, review your design with the radiant heat floor system manufacturer - those folks have as much at stake as you in a successful installation.

On 2018-01-12 by Ar

@mrbliss
Again many many thanks for your time and kindness in considering these questions. Nothing's simple, is it? Yes, although Tyroc does claim to be a safe substrate upon which tile can be installed.. at this point we gauge it's not worth the chance and plan to remove it - but mentioned it because in the time we had it down it DID an >excellent job of insulating under LVT and was very forgiving of our mild swales, settling in nicely<. the recycled content a boon, though the manufacturer's apparent refusal to back initial buyers is a supreme disappointment.

as to the potential 'material sandwiches,' you are correct that any additional height loss is of serious concern. with average inhabitant height at six feet plus, the ceiling is indeed low to start with. the structure in question is +/- 180 yrs old and although it would be preferred to avoid plastics, that may be necessary for insulation. it's also of importance to note that adding any moisture permeable material, such as plywood, is something that needs to be avoided here to create a 'baseline plan that will not fail.'

q1a: are you aware of the use of any particular fibrous material succeeding as an slc anti-crack additive? example:
http://www.wickes.co.uk/sika-no-crack-concrete-admixture/p/154064

on a previous diy fix, about i used quikrete sand/topping mix with the suggested amount of quikcrete concrete acrylic fortifier #8610 added. the objective was to smooth out top of an exterior load-bearing retaining wall. we've watched that wall (which is backed by 36" of soil with an exposed sub-grade face on the other side) for any failure in adhesion as well as cracking and have only this year seen one hairline crack adjacent to a load-bearing masonry pillar that supports a very large porch. not bad for 6 years of shrink/swell. so,

q1b: are you aware of the success of elastic additives or acrylic fortifiers that can be used in slc? as you may note- mention of interest in your knowledge of any primers to promote slc adhesion (re:linoleum) were made in initial post.

i remain surprised that there is no appropriate floor insulation to achieve r-10 without losing 2 inches. (do note: gap in market___ here)

q2: do either of you have any experience or opinions regarding so-called reflectant foil membranes or coatings that might be put down to start? radiantguard is one product we have read of with interest.. as the mfctr' states it "reflects upwards of 95-97% of the thermal energy that hits its surface."
https://www.radiantguard.com/pages/floor-insulation
don't want to put you in the middle, but wonder if you deem the above statement bunk? it would be lovely if something that thin actually did what it claimed to.

q3: as most of the foils/bubble foils we've read about seem to be for under-slab-- to be used as a heat reflectant/moisture barrier base layer prior to pouring a new slab.. so, in our situation, isn't adding slc in combination with the layer of the modified thinset that would follow atop it to accept tile - in essence- a 'new slab' of sorts?

i understand the closed cell foam thickness as an insulator.. just really don't like plastics much and would prefer not to use that volume in this setting, so am investigating alternatives...

considering the potential stats: do thousands of current installs worldwide involving new-fangled paint on - roll out - rigid sheet - insulation materials evidence.. ?thousands of installs that may indeed have a chance off failing over time? do people really rip out tile, cable and cement every 20-25 years? we would like to avoid that.

additional -likely inane- ideas/questions : what materials could be considered as thinset additives to increase thermal mass? though this could create a self-destructing machine.. as increased heat over time may destroy initial elastic properties.. reading about that is interesting though, as it might come to address other future products that may make in-floor cable heating more effective and less costly over time.

my best to you both!

On 2018-01-10 by (mod) -

Ar

Steve and I have both thought about this; we were a bit confused by some of your question but bottom line, from Mr. Bliss

If the vinyl/linoleum is in very good condition and well bonded to the subfloor, then you can lay tile directly over it – I have done this on a couple of occasions with no problems using an isolation membrane and modified thinset. However, in this case, he would first have to lay down the electric heating mesh and pour a layer of self-leveling compound over the heating cables. Over that would go a waterproofing/crack isolation membrane and the ceramic tile.

However, I wouldn’t recommend radiant heating over R-3.2 insulation, so tearing out the tyroc and adding foam insulation would be a much better option. I don’t know how much height he has to play with, but I agree with your that 1-2 inches of foam insulation would be best. Typically extruded polystyrene (such as Styrofoam) is used under floors due to its high compressive strength. This provides R-5/inch. R-10 would be best if he can afford the space.

First, he would have to level the existing slab with the minimum amount of leveling compound to form a level base for the Styrofoam. On top of the Styrofoam I would float a double layer of plywood (1/8” to ¾”) subflooring, staggered, screwed, screwed, and glued. Over the plywood would go the heating mat in a layer of thinset, membrane, and tile.

The question is whether he could skip the plywood to save on height. I don’t think any tilesetter would recommend tiling over foam without structural concrete or plywood. He could save on height by using a product like Ditra-heat, which combines an isolation/waterproofing membrane with electric radiant heating.

I have used Ditramat (non-heated) on a couple of remodeling jobs over problem subfloors with good results.

https://www.schluter.com/schluter-us/en_US/Floor-Warming/Schluter%C2%AE-DITRA-HEAT/p/DITRA_HEAT

Regarding the wall insulation on the perimeter, any foam insulation will do. The highest rated material would be foil-faced polyiso board such as Thermax, at about R-7/in.

Let us know if anything is unclear or we missed anything.

On 2018-01-10 by Ar

Many many thanks, Mr Friedman - and to Mr. Bliss.
Patience is mine : )

On 2018-01-09 22:05:18.369430 by (mod) -

Thank you for the kind words Ar and for the interesting question.

I'm giving your question some thought and have also asked our guru friend Steve Bliss ( buildingadvisor.com ) for an opinion

On 2018-01-09 by Ar

Hello Mod and >>Thank You<< for the opportunity you've provide me and other readers to learn from your expertise!
i've been reading your site since you first started and remain amazed and thankful.

we have a 240sf room in our english basement - slab is 36" below grade. poured in the 50's - doubtless without insulation below or at perimeter. atop slab is a layer of solid color-core linoleum- old-school- set in cutback adhesive that is >extremely< i mean extremely well adhered. i see no reason to rip it up and would like to work atop it. i'd like to lay porcelain tile over electric cable or mat infloor radiant heat.
several different manufacturers have contacted me after i used their online heat loss calculators to estimate bids for the mat.. but none of them offered any suggestions re what type of insulation we should place on the slab before using their systems.

we currently have lvt plank over tyroc subfloor in this room and although that subfloor product experienced some early issues, bith with their adhesive suggestions as well as initial manufacturing plant going belly up and tyroc not honoring warranty claims regarding the initial run of material from that manufacturer, i know for a fact that using it has upped the r value of our current flooring. it cost about 650usd to purchase, but as we were laying lvt we saw no need to level (only about 1/2" out of level overall in swales here and there.. but the house is ancient and swales and lvt worked fine.) the tyroc is purported to have an r value of 3.2 and as i know there were no drafts to start with, i suspect the moisture barrier (plastic sheeting), tyroc and then lvt acted in total as insulation by "sealing' us away from the slab. it really is warmer since we installed it. bottom line, for this project, i do not want to pour heat into the slab. i'd make the infloor heating pointless.

so here're my qs: sadly, we're going to have to yank the tyroc to pour self leveler. reuse of the tyroc may be an option... if we surgically remove it. perimeter insulation suggestions are needed and welcomed. this room has only two exterior walls and (what type of high r?) foamboard up the wall below grade is an option?

1. adhesion and self leveling.. many seem to be using a paint-on coating on existing linoleum prior to laying insulation to promote adhesion- your thoughts? so far ardex featherfinish seems the product to use atop lino for leveling, but we may need some sort of adhesion promoting primer.. your thoughts?

1. what would you suggest as the /insulating = first layer/ on the slab ? i would like to use some sort of affordable long lasting high r value insulation, and tyroc has so far been the highest rvalue for the lowest additional height, at only 1/2" thick. (i chose it as i did not want to use any wood- to cover potential future water issues- pipes bursting, etc.. as we have no moisture issues.

2. though you may prefer not to name brands, i'll still ask- which mfctr you might suggest for the cable system? the two products i've have been considering are tuffcable and zmesh https://www.heatizon.com/products/radiant-floor-heating/radiant-floor-heating-tile-stone
and i 'm happy to hear any comments or suggestions you may have.

3. based on your seemingly endless knowledge of products on the market, i wonder what your opinion of using latex additive thinsets and grouts over electric radiant heat (for indoor outdoor frost proof rated porcelain tile) are? brand suggestions?

4. considering my yen to use a more mobile thinset and grout.. do you think a crack isolation (schluter ditra etc) layer is necessary?
many thanks- ar in va
>

On 2017-10-24 by Tyler

I am adding a concrete floor addition to the back of our house in Kentucky. The slab will be several feet above grade on top of gravel. The foundation walls have been properly insulated. The contractor plans to insulate underneath the slab 2 feet in on the perimeter. I worry that they are not insulating beneath the entire slab. The contractor states this is okay since the slab is not on grade and we are in a more temperate area than up north. Any thoughts?

On 2017-05-06 by (mod) -

Keith,

In the radiant heat floor systems that I have seen the hallway is not usually an area of concern. However you could simply include the hallway in principle zone for that floor.

On 2017-05-04 by Keith Davis

I am currently renovating my home and plan to retro-fit my home with in-floor heat. As I work through the tubing design, one questions I have is if I need to run tubing down my upstairs interior hallway. There is heat loss through the ceiling, but it is small...about 70 btu/hr. Should I worry about this or not?

Any help is appreciated. Thank you.

On 2017/01/14 by (mod): The slab & foundation have no insulation anywhere.

I think so, mouse. Other studies of insulation placement in frame construction show that heat loss through the slab and foundation perimeter are significant.

On 2016-12-06 02:21:03.523861 by mouselb

It's been about 25 years ago that my builder step-dad (now deceased) built his retirement home. A 30' x 76' slab on grade inside a 42" frost footed stem wall (western WI) with a staggered stud (double wall) single story super structure with about 7" of insulation. He placed pex tube over fairly clean 3/4" gravel and under 4" of concrete.

The slab & foundation have no insulation anywhere. The unheated double garage juts into the floor plan for an integral (monolithic) edge 13' x 24'.

He had a medical condition where he needed ambient temperature of at least 78°F to be comfortable and had to add a 2nd 35k BTU propane fired source to the liquid manifold. Heating bills are astronomical!

Would placing 2" rigid insulation along perimiter stem walls down to the footings and cutting a few inches out from the garage perimerer gain any recoverable savings?

On 2016-10-20 by (mod) our plumber has not been able to get hot water to go through the tubing to heat the floor/room

If the system is flowing but losing an abnormal amount of heat passing through one area I worry that insulation was omitted or tubing was improperly located. Perhaps a home inspector who can make use of infra red scanning can survey the flooring to see if there is an obvious point of heat loss.

On 2016-10-20 18:26:20.065110 by Kimmy's Garden

We had new radiant heat installed under concrete and porcelain tile. So far, our plumber has not been able to get hot water to go through the tubing to heat the floor/room. He's installed additional pieces to help push/pull water, and it is going out hot (160 degrees or so?) but not returning hot (near 100 degrees or so?) He's flushed the system several times, and water flows through freely out of the hose out the back door. It's getting cold and our system hasn't worked yet, and he's run out of ideas. Help!!!

On 2016-09-26 13:32:35.095366 by Anonymous

Hi, we have electric, underfloor heating throughout our house. Recently, it stopped working in all four zones (though in one zone there is a tiny amount of heat). The four thermostats were checked by an electrician...and they all work. The power panel (and all four breakers) work. Is there anything that could explain this? We really don't want to have to rip up our floors when it seemed to be working perfectly, everywhere in the house!
Please help! Thanks! Dana

On 2016-09-04 17:49:36.268253 by Neil Meyer

I have an in-floor heating system under concrete in basement and is heated by a water heater. Every so often we hear water running into the water heater. Then after several fillings the water heater will run. It is summer and all the thermostats are not asking for heat. Could I have a leak or what could cause this? I can feel no water in the basement that is completely carpeted. Thanks for your response.

On 2016-06-01 08:41:08.985360 by (mod)

If you are discussing electric radiant heat panels it should be possible to test for open circuit or shorts. Screws can Pierce and short. Pull screws and see their length.

On 2016-05-22 21:49:17.501368 by Stephen Bruce

My new home under slab heating is not going and the switch board is triping,
2 electricians have looked for 2 hours at it and they carnt get it going.

The builders have fixed some chipboard to the floor in the lounge with screws going into the slab would this be the problem?
I asked the electricians if this could be the problem, they said no, only if they want down 150mm

On 2016-05-13 00:33:46.336515 by (mod)

Thanks CJ. I'd like to see photos of a "correct and successful" radiant heat slab. (Email is at page bottom CONTACT link)

You are dead right that the builder described in the article above (I call his company "Nightmareworks Construction" a variation of its actual name) didn't know radiant heat; worse his ego and pride got in his way - he'd never admit not knowing something and bullied his way through being questioned. He doesn't know that he doesn't know. Some guys just keep making the same mistake over and over. Me I don't repeat the mistakes, but my boss told me he thought I intended to make them all once.

On 2016-05-12 03:20:02.382922 by CJ

The above description of the installation of the radiant heating system by an uninformed contractor is typical. Contractors in general do not understand heat or the design of a heating system. I live in Wisconsin and built a 24 X 40 foot building with in floor radiant heat. I used 2 inch rigid insulation under the slab and 1 inch rigid insulation on the perimeter of the slab. I used 1/2 inch pex and a 35,000 BTU water heater for the heat source.

I designed and installed the system myself and it heats the building comfortable in below zero weather. I took pictures as I installed the system then wrote an installation guide I sell through Amazon.

I am a builder and a structural engineer so I was able to do all the necessary calculations for the design. I think radiant heating is great. It is quiet, efficient and simple.

On 2016-02-29 14:59:15.628971 by (mod)

Good question Concrete, though please avoid SCREAMING in caps.

I agree that a wire or fish line is not very reasonable given the number of bends and length of radiant floor tubing.

Borrow or rent a thermal scanner, infrared camera, or even a simple IR hand-held thermometer. With the heat on you should be able to see the tubing layout in the floor. Follow it and you'll see a sudden end of the hot tubing at the point of kink.

On 2016-02-29 14:57:06.423225 by THE CONCRETE MISTAKE

WHEN THE CONCRETE WAS INSTALLED THE MUST HAVE KINKED THE PEX PIPE HOW DO I FIND THE KINK
WE RAN A FISH IN BUT TO MANEY BENDS GO A IDEA

On 2016-02-17 21:10:55.420864 by (mod)

GeoDeo

Your comment that the author was also the customer is simply mistaken. The author, after 40 years of both training and field service in construction, is a journalist and a forensic investigator.

The article above reports objective facts and truth about the difficulty of building radiant heat slab floors or for that matter anything else that a contractor may undertake for which she or he is not really properly informed. A professional puts aside ego and recognizes what he doesn't know, and consults someone who does.

On 2015-11-08 15:58:29.506864 by (mod)

Melissa:

As it may help other readers or prompt helpful suggestions I've included our radiant heat floor debugging discussion at the end of the article above, beginning at DIAGNOSE RADIANT HEAT FLOOR NOT WORKING - https://InspectAPedia.com/heat/Radiant-Slab-Heat-Mistakes.php#Diagnose

As I often discover in longer discussions with clients or readers, more clues emerge. The fact that your radiant floor heat worked properly at one time suggests that (barring something odd like a rising water table) the concerns of a innate design or installation errors such as tubing too deep or insulation left out are pretty much squashed.

So it sounds more like a control problem or even a tubing damage or leak or obstruction.

Before bringing in a plumber let's do some detective work: get a detailed thermographic scan of the floor and of the perimeter of the bottom of the building in cold weather with the heat on.

Send me the results.

See https://InspectAPedia.com/home_inspection/Thermography_Info.php for an intro to thermography and what to expect.

On 2015-11-08 02:29:08.882005 by Melissa

I completely agree. I am hoping that the new plumber will be able to troubleshoot and try and figure out possible causes of why the system won't heat the slab. It seems like people who take a look at our system are completely baffled as to why it won't work. When we first got the system it only worked for a few short months in the winter, but it did work at one time. I am just worried that the the plumber that I am bringing in won't be able to figure anything out for us, in which we will be left with a very expensive system that just won't work. Anyway the plumber is coming in a few days so we shall see...

Reader Question from Wenell: I would like to know what the persons that wrote and researched this article thinks about what Montana has on research. On their web page MONTANA SLAB EDGE INSULATION ANALYSIS FOR 2006 IECC ADOPTION. There seem to be so many theories on this.

One thing we have found that if the soil conditions are quite damp, there definitely needs to have some type of insulation under the slab.

Another theory I have read is that the heat as it goes down, which it will, some is that it radiates horizontally, which makes insulating the edge quite well. - Wendell Schubloom

Reply: thorough under-slab and perimeter insulation and proper tubing depth are critical for radiant heat floor slab designs

Wendell, there is not actually any contradiction between the Montana (DOE) research you cite above and radiant heat floor slab insulation requirements. The study you cite does not focus on radiant slab heating designs but or a more narrow question about the benefits of foundation/floor slab perimeter insulation. The DOE photo (below left) shows a typical Montana construction practice that gives a thermal break between a concrete floor slab (not yet poured) and the exterior foundation wall.

I've read quite a lot of supporting research on slab and slab perimeter insulation for radiant heat flooring, and I have some direct experience with installing radiant heat and more with inspecting radiant heat flooring problems.

Quoting from the conclusions of the Montana DOE-sponsored study you cite, [2] [photo at left showing interior foundtation insulation before the slab is poured, U.S. DOE, op cit.]

This study shows that insulating slab edges with R-10 insulation to 4-ft depth along the slab edge saves about 3% annual energy and reduces annual fuel cost by between 1 and 2%. The energy savings vary slightly depending on the insulation configuration and building type.

Although the current installation practice in Montana does not extend the interior footing insulation to the top of the slab, based on empirical data, this study concludes that irrespective of the insulation installation configuration, Montana buildings will save energy by insulating the slab edge with R-10 insulation to a depth of 4 ft. The payback period could vary from 4 years for small retail commercial buildings to 12 years in small office buildings.

This study, using eQUEST, Version 3.0 simulation modeling, compared full versus partial slab perimeter insulation schemes and found that there was useful energy cost savings even with partial insulation. The study data includes comparison with fully-insulated slabs too, but most important for our discussion, it does not address radiant-in-floor-slab heating designs that, without full insulation, can find an easier heat flow into the ground than into the building - not what we want to see nor pay for in heating bills. Quoting:

The local practice of insulating the slab footing on the interior allows heat loss along the slab perimeter and thus does not achieve the full savings that could be achieved with full edge insulation configurations, but the savings are still significant.

The risk in misinterpreting the Montana study conclusions above would be to apply them generally to radiant heat floor designs and that to improperly infer that complete under-radiant-heat-floor-slab insulation is not needed in cold climates. That study makes a general conclusion for all Montana buildings and by no means does the conclusion adequately address radiant in-slab heating system designs.

The fallacious concept held by the contractor in our horror story was that "once you heat up the earth below your building it will start "giving back" heat to the building and you'll be just fine. His theory was nonsense, as both expert advice and actual field experience proved.

The earth in a cold climate like Montana or Minnesota, is for practical and design purposes, an infinite heat sink. A radiant floor slab heating system will, if improperly designed, keep pumping heat into the ground as long as the heat is turned on. Forever. We saw this in astronomical heating bills and a cold building interior in the Minnesota home discussed above. Heat always flows, and continues to flow from a warmer material into a cooler material.

Heated the soil beneath a building where insulation was incomplete, inadequate, or omitted, will never reach some magic perimeter after which it stops sending heat into the surrounding soil any more than an ice cube placed into the sea will stop melting because it's "cooled down" the water around itself.

As the principal author of this material I relied largely on the concrete industry and the radiant flooring industry's radiant floor slab design specifications and advice [1] as they, above all, have a huge vested interest in their installations being successful.

There is no doubt that in virtually every radiant-heat-floor-slab design we need continuous insulation under the slab and at slab perimeter, though the appropriate insulation amount might vary depending on the local climate. The folks who seem to disagree have been people like the bully contractor who himself admitted he had never read instructions, attended a class, nor asked for expert advice.

As is often the case with small contractors in remote areas and without expertise, he was "winging it". Don't try mentioning "thermodymics" or "heat flow theory" to a bully.

Just how bad an uninsulated, under-insulated, or incompletely insulated floor slab will perform with radiant in-slab floor heating depends on some additional variables: climate, soil moisture (read thermal conductivity as you suggest), and critically, the depth of tubing in the slab. In ALL cases we want the insulation in place.

But in the horrible installation we describe in these articles, the contractor not only provided incomplete and no perimeter slab insulation, he also buried the tubing so deep in the concrete that heat moved much more down into the cold earth than upwards into the occupied space.

There was so much heat loss that we could not get the room temperature up even in cold but not bitter cold weather, and even though the same contractor had done a great job insulating the upper portions of the structure's roof and walls. (He was a framer/carpenter, and should not have attempted radiant slab installation nor tile work.) That's why we had to abandon the whole radiant floor installation.

If the floor slab had been very well insulated, the installation still would not have performed well because of the excessive tubing depth in the slab ( over 12" down in some sections ).

I appreciate the Montana reference and have added it to this article below at references [2].

Comment:

We are in the steel bldg business so we have alot of infloor heat done. with the experienced heating people we use, have had no problems. But the question I have is- in North and South Dakota there is a Cat dealer by the name of ButlerCat. they have built huge shops and I found out this spring what they do for floor hear. They place the foam down and put the pex directly to this and then place 4 to 6" of sand on top before pouring the floor. I ask why and was told if the have any floor problems they can remove any thing need to. They done this on I think four bldg's Waht are your thought's

Reply:

Wendell it's a fair question, and I welcome the disccussion. But I suspect this may be a case of intelligent people who think things up on their own, make up an explanation that sounds reasonable, but may not know the whole story.

The deeper you put radiant heating tubing in the slab the worse the heating system will perform in delivering heat to the interior. Furthermore, the thermal conductivity of sand is much below that of tubing directly in contact with the concrete slab itself.

The expert sources I found on this want tubing in the concrete and very close to the slab top surface, an inch or two at most down is best.

I agree that if there is enough insulation under the slab and it's well done and complete, in the design (foam, tubing, sand, concrete) you describe you will eventually probably warm the slab upper surface, but consider that there are heat flow rates through insulation too, it's not "heat proof".

With 6" of sand and say nominally 6" of concrete, your tubing is 12" down - way too deep, and furthermore, the first 6" of material (sand) between the tubing and the occupied space, does not quite the same level of thermal conductivity as tubing in contact with solid concrete.

The sources I cite at references below point out that there is heat flow resistance through concrete and sand as well. So while it may not be intuitively obvious, and while it's true that the thermal conductivity of concrete and even sand (which is not as good as concrete) is greater than insulation, if we have enough sand or concrete above the tubing, and little-enough insulation below the tubing, heat flow down through the insulation can still be significant. Think of it as "heat flow resistance" through various materials. You can have a more conductive material above the tubing, but if you have a lot of it, the total heat flow resistance can still be significant.

Finally, the supposition that "if they have floor problems they can remove anything they need to" sounds highly suspect to me - it's not thought out. In any case you'd have to chop entirely through the floor slab to get to the tubing below, and meanwhile you are paying in higher heating bills than necessary over the life of the building.

Question: do I insulate the footings in a monolithic slab?

Aug 8, 2011) bryce said:

im a contractor. do i insulate the footings on a monilific slab>>??

Reply:

Bryce it's good practice to provide a thermal break between the slab and the building exterior; the specific requirement depends in part on where your construction project is located, as clearly this is most critical in cold climates.

Question:

(Aug 11, 2011) wendell schubloom said:

I would like to know what the persons that wrote and researched this article thinks about what Montana has on research.

On their web page MONTANA SLAB EDGE INSULATION ANALYSIS FOR 2006 IECC ADOPTION

. There seem to be so many theorys on this. One thing we have found that if the soil conditions are quite damp, there definitely needs to have some type of insulation under the slab. Another theory I have read is that the heat as it goes down which it will some is that it radiates horizonally, which makes insulating the edge quite well.

Reply:

Wendell I've read quite a lot of supporting research on slab and slab perimeter insulation for radiant heat flooring, and I have some direct experience with installing radiant heat and more with inspecting radiant heat flooring problems. As the principal author of this material I relied largely on the radiant flooring industry's advice as they, above all, have a huge vested interest in their installations being successful.

There is no doubt that in virtually every case we need continuous insulation under the slab and at slab perimeter. The only folks who disagreed were people like the bully contractor who himself admitted he had never read instructions, attended a class, nor asked for expert advice. As is often the case with small contractors in remote areas and without expertise, he was "winging it".

Just how bad an uninsulated, under-insulated, or incompletely insulated floor slab will perform with radiant in-slab floor heating depends on some additional variables: climate, soil moisture (read thermal conductivity as you suggest), and critically, the depth of tubing in the slab

In ALL cases we want the insulation in place. But in the horrible installation we describe in these articles, the contractor not only provided incomplete and no perimeter slab insulation, he also buried the tubing so deep in the concrete that heat moved much more down into the cold earth than upwards into the occupied space.

I appreciate the Montana reference and have added it to this article below at references.

Reader follow-up:

(Aug 11, 2011) wendell schubloom said:

We are in the steel bldg business so we have alot of infloor heat done. with the experienced heating people we use, have had no problems. But the question I have is- in North and South Dakota there is a Cat dealer by the name of ButlerCat. they have built huge shops and I found out this spring what they do for floor hear.
They place the foam down and put the pex directly to this and then place 4 to 6" of sand on top before pouring the floor. I ask why and was told if the have any floor problems they can remove any thing need to. They done this on I think four bldg's Waht are your thought's

Reply:

Thanks for the clarification and comment. I'm not convinced the design you described meets industry recommendations and I'm concerned that your tubing ends up 12" down from the slab surface. I've posted detailed comments just above.

Question: even heating for radiant heat floors

Do you split up the floor area with multiple tubing in the same space? For example, in a 12x12 room, if you wound only one single tube through the floor, then the start point would be warmer than the end point.

If you use multiple tubes and manifolds (each tube would be the same length but flexed however it is needed), this would give a more even temperature through the floor. I don't know if this would keep the cooled water temp a little higher in the end? therefore not having to use as much energy to re-heat? Also, what are your thoughts on using solar evacuated tubes to pre-heat the water also, hopefully reducing the energy costs? what do you think?

Reply:

Typically we do not sub-zone within an individual floor area; more often several rooms will be on the same heating zone as with any other heating system design. However the radiant heat installer may choose different tubing routing options if un-even heat is a concer over a single large floor.

Question:

(Mar 2, 2012) david said:

have you ever had a problem with the boiler too hot and cousing the floor tile to have small cracks all over the house.

Reply:

Yes - see WOOD FLOOR RADIANT HEAT DAMAGE

Question: Electrical contractor reports on radiant heating system performance, insulation requirements

(May 3, 2012) marc stayduhar marcsayduhar@yaho said:

I have been in the electrical construction biz for over 30 years. Residential, commercial, and industrial. I do all my own HVAC work for myself, not for hire as I'm not certified. But, I have a very good working knowledge of how these systems opperate, and have installed the various electrics for so many HVAC systems in different types of buildings. Here in Southwestern PA, I have seen "in-floor" radiant heat installed many times.

It has been my experience that whether or not there is floor heat installed, the contractors I have whitnessed always installed 2" rigid foam insulation along the perimeter walls down to a depth of 4 feet, then the same insulation 4 feet in, towards the center of the building. Then, bar, mesh and chairs to hold the steel at a certain depth in the pour.

For radiant heat, the entire area is insulated with at least 2" foam. To me, this is a "no-brainer"!!!

If you have ever gone camping, and had to sleep on the ground, or floor of a tent without any insulation, then you should realize that you NEVER can get warmed up. Your body heat is conducted into the ground. I know first hand the level of incompetence some contractors have. I have gone rounds trying to explain things about my trade to "hard heads", the arrogant, bully mentality some so-called experts have.

Sad thing is that these people have the ability to convince the unexperienced owners that their way is the right way, and ultimately the owner gets screwed in the end. Trying to re-coup their money, or part of it in court proceedings is usually a waste of time. If I see something is wrong at a job by another trade, I will bring it to the owners attention. I know that if I were the owner I would want to know. Hopefully as more states adopt building codes, and require inspections, this type of thing won't happen.

But people are always looking for a deal. Cheap is never any good, and, if you don't know something, then get a second opinion, or third, or fourth. When in doubt, STOP the construction till ALL your questions are answered.

Question: radiant heat in Southern Spain

(May 8, 2012) John Jones said:

Hi
I live in southern Spain in what is a jhot climate and we generally have mild winters with temperatures seldom down to 0.

Typically the house is built with little thought to insulation and has a suspended concrete floor made up of reinforced concrete beams with hollow concrete blocks between the beams then a thin screed and then a pea shingle fill covered by anothe thin screed and then ceramic tiles. I am interested to know if it is practical to retrofit plex UFH tubes directly to the underneeth of the floor (it is accessable) and then cover this with say 150mm of insulation.

I intend to use solar heated water but will the results produce a good radiant heating effect?

Question: radiant heat contractor experience in Calgary

(Aug 23, 2012) Edward Wyatt said:

I have been a contractor for over 60 years and have had experience with in slab, on grade heating,n my ohouse which had a substantial walkout lower levelThis house was bult in
Calgary,Alberta, Canada and we can experience temperatures to the mid -30 degrees.

I have read your comments with much interest and agree with many of your recomendations however I think there are some hole in the procedures and while there is osme expense
entailed in implimenting them the pay back in savings is very short,I suspect 3-4 years.

My experience is thatthe foundation insulation on the exterior of the wall is the first line of defence in that it minimizes the thermal transfer through the foundation wall which then finds its way up to the wall plate on the exterior wall. I also insulated the inteior of the foundation wall to prevent thermal transfer to the slab and the subsequent cooling effect on the lines closest to the wall.

I agree with the depths of the piping at 2" but anything less than that could reult in cracking in the upper portion of the slab.
N additional measure i incorporated was tocover the exterior insulation with heavy gauge poly to a depth of 6 feet and carried it out 6 feet from the foundation.

This had a dual purpose in that it kept the moisture away from the foundation but also eliminated moisture from the surrounding soil which mitigated the freezing of the soil snd the subsequent cooling effect under the slab area
We were in thwe house for 10 years and never had the thermostat set above 15 degrees celsius in that area and it kept the area quite liveable.

My water souce was a25 gal electric water heater with an expansion tank and an in line tranfer pump. in the 10 years of use I did not experience any isues that require attention.PS I am c urrently designing a siliar sys

Question: options for foam insulation at block foundation walls

(Aug 7, 2012) allan k said:

i am building a home addition at this time, the dimensions are 28x28 its a attched garage with a bonus room above. my question is i put the 2 inch foam on the inside and outside of the block walls and im preparing the gravel for the foam board also. do i only install the foam on the perimeter or do i insulate the complete 28x 28 area. the reference of no more tahn 2" below the surface of the concrete. with only being 2" below i guess this elimantes the need to sawcut the concrete? thanks

Reply:

Allan, either option for foam will work; some like to put the foam on the outside because that gives the interior of the building the opportunity to use the block wall as a thermal mass.

Given modern energy costs and the decades of studies on where heat flows out of buildings, even though heat flows out fastest at the building perimeter, it will also flow out of the building concrete floor slab center - forever - if you don't insulate below the whole floor. Insulation is very very cheap compared with heating energy costs.

The 2" rule you cite is for the depth of radiant heat floor tubing within the concrete floor slab. If you put the tubing too deep you get too much resistance to heat flow out of the tubing, through the slab and into the occupied space.

Question: pex tubing diameters

(Aug 24, 2012) Ron said:

We had a system designed and they suggested 7/8" pex, 15" on center, instead of 1/2" pex 12" on center. Which do you think is better? This is in a 4" concrete slab, little less than 400 sq. ft.

Reply:

OPINION: we have had problems with under-sized tubing combined with a boiler and circulator pump near their operating limits delivering inadequate heat to an area that was not well insulated and had a high heat loss rate. I would always opt for larger diameter tubing for this application. The cost savings in smaller diameter tubing is not worth possible hassles later having to add a higher velocity circulator or to struggle with proper floor heat delivery temperatures.

Question: radiant heat over flooring

(Sept 2, 2012) pat said:

would laying 3/4 plywood and 3/4 oak flooring offer too much resistance to heat transfer from a concrete slab with radiant heat instalwould appreciate a reply....paddyd57@yahoo.com....thanks.

Reply:

No but if the wood floor has high moisture content you may find gaps when it is heated.

See WOOD FLOOR RADIANT HEAT DAMAGE

Question: need to bolt a safe to radiant-heated floor

(Nov 7, 2012) Connie said:

I have a small safe I need to bolt to a concrete floor. It's in a closet where there may or may not be tubing. Is there a way to determine where the in floor radiant tubing is running? I don't want to risk a puncture?

Reply:

Here are some options Connie:

Look for original specifications & plans on the tubing layout, including measurements

Use an IR scanner to map the warmest segments of the floor when heat has been turned on

Use the services of a thermographer who has more sophisticated thermal imaging equipment

More crudely, if the floor outside the closet getsmwarm and closet remains with a cold floor, tubing may have skipped that area.

Chip very carefully through the concrete where you want to insert anchors. I've done this to expose the actual tubing in a concrete slab. While drilling down risks puncturing tubing, chipping with care at the concrete usually can leave the tubing intact.

Question:

(Dec 23, 2012) Bob said:

Can a system like this cause your cold water to be tepid?

Reply:

Possibly if your boiler uses a tankless coil for making domestic hot wate AND the primary control on the aquastat on the boiler is not properly set.

See AQUASTAT HI LO DIFF SETTINGS -

Question: problems with boiler output temperature

(Dec 31, 2012) Mike said:

I bought a home this spring with slab on grade, radiant floor heat. It uses pex piping and was supposedly installed 3" deep according to the contractor.

Currently I can run my boiler for over an hour and not see a change in outlet temp. The boiler is trying to make 110 F but will only reach 101 F.

The outside temp is 33 F. This comes after running a wood stove during the day and raising the temp of the house 5-6 F above the thermostat setpoint of 68F.

Should I run my temp higher on the thermostat during the day to help heat the slab? My first month's bill with the floor heat on raised my KW usage by almost 4x. I went from around 3-400 Kwh a month to 1800. The next bill is really worrying me because I averaged the heater (20kw) ran for 2 hours a day last month. I'm sure I've more than doubled that this month.

Reply:

Anonymous said: Mike,

you are on the "roller coaster". You need slab sensors or you need to stop using the wood stove. The wood stove changes the ambient temp in the room with the thermostat that controls the infloor system allowing the pump to shut down and slab to cool. This is called loosing your stored energy...Heat in the slab is stored energy. If you loose it, even though the air temp is warm, once the thermostat calls for heat again, you have to heat that slab up again and that costs much more money than maintaining the temp in the slab.

Usually the slab sensors are installed in the slab before it is poured, but there might be a way to do that after the fact. Any heat source that affects the ambient air temp: wood stove, south sun through the windows, electric space heater, kitchen oven...If the source heats the space enough to turn off the thermostat, you start on the roller coaster of spending more money on energy.

Reader follow-up

(Feb 5, 2013) Mike said:

Anonymous:
I have confirmed there is no perimeter or underslab insulation, that is my problem. The guy who did the install and had the house built "researched" this together and didn't use insulation. Now I have to prove in court he knew about this for 7 years before selling me the home.

With my loops sending the hottest water to the perimeter of the home first, they get the largest temp drop. I'm unable to heat the slab above about 74 F. Without the wood stove or windows, I can't maintain the temperature in my home at 68 F.

(Feb 11, 2013) Anonymous said:

hey Mike, I wonder if you swap your tubes to reverse the flow perhaps it will heat the center before reaching the colder perimeter and heat the house better.

Reply:

(Feb 11, 2013) Anonymous said:

Anonymous:
I don't think that would work because there would be extremely cold water going back to the boiler due to the large losses. This in turn would limit the outlet temperature. The losses might be less due to the delta T going down, but either way, my floors will still be cold on the perimeter of my home. This is also a code requirement to prevent frost heaving.

My electric bill was well over $200 this month and I used almost 3000 KW compared to 450 KW in October without the heat. The average outdoor temp was only 37 F and I burned about 1/2 a cord of wood on top of that. The home will not stay heated with the floor heat alone, and I don't plan on stoking a fire 24/7.

While investigating this I found another latent defect in the home the seller "forgot" to disclose that he had knowledge of during the building of the house. At this point I'm getting lawyers involved.

Question: looking for radiant heat contractors in West Virginia

(Jan 1, 2013) William said:

Lookin for competent installers contractors in WV for heated concrete flooring

Question: Experienced in-floor heating installer in the U.K. gives advice

(Jan 6, 2013) Tony said:

I have been installing in-floor heating in the u.k. for over 25 years, and fixing poorly installed systems a few times a year.The most sucesfull way we found was a three stage process.

Stage 1. a concrete slab is poured leaving 4" from the eventual finnished surface.

Stage 2. 2" foil covered polystrene, laid on top foil side up, with an upstand at the edges. Mesh next, with 1/2"'Pex' oxygen barrier pipe tied to the mesh.

Stage 3. a 2"concrete screed.

The pipework is kept under pressure the whole time. On start up the temperature is kept low and increased daily until the design temperature is achieved.
The final floor covering is installed after 30 days.

This type of system is used in homes ,schools,hospitals, senior's homes. It was also used in London's British musuem, North American building where is is linked to hot and chilled water system's to keep exhibits a constant temperature.
One of the reason's it is used so much here is that health and safety is requiring radiators to be boxed in to prevent burns to the young and the elderly.
If I had the choice it would be in floor heating in my house every time.
Stay warm

Question: radiant heat experience

(Jan 7, 2013) john said:

I appreciate your site. This kind of discussion is very helpful.
I have been struggling for over 3 years with an incompetently installed hydronic system
My system pumps 180 degree water to concrete slap, under wood foors and radiant wall
heaters on three floors in a 106 year old wood house.

Question: radiant heat design in Canada

(Jan 7, 2013) Patrick Morin of Qc Canada said:

I am planning to build a garage with floor heating and this article really guides me well. What of the streght of the slab surface when tubing is only 2 inches or less from this surface? Would it weaken the structure? Ex: vehicle garage, tooling etc..

Reply:

A reinforced concrete slab or fibre reinforced slab with radiant heat tubing 2" from the slab surface has not been a source of reported structural issues that we've heard. Here is some research that's related.

• Al-Homoud, Dr, and S. Mohammad. "Performance characteristics and practical applications of common building thermal insulation materials." Building and environment 40, no. 3 (2005): 353-366.
• Behnood, Ali, and Masoud Ghandehari. "Comparison of compressive and splitting tensile strength of high-strength concrete with and without polypropylene fibers heated to high temperatures." Fire Safety Journal 44, no. 8 (2009): 1015-1022.
• Desoto, S., and L. A. Glenn. "An analog method for the solution of unsteady radiant heat-transfer problems with combined conduction and convection." Journal of Spacecraft and Rockets 3, no. 2 (1966): 224-230.
• Farid, M., and W. J. Kong. "Underfloor heating with latent heat storage." Proceedings of the Institution of Mechanical Engineers, Part A: Journal of Power and Energy 215, no. 5 (2001): 601-609.
• Han, Sang Heon, Daejun Chang, and Chang Young Kim. "A numerical analysis of slab heating characteristics in a walking beam type reheating furnace." International Journal of Heat and Mass Transfer 53, no. 19 (2010): 3855-3861.
• Mingqing, Sun, Mu Xinying, Wang Xiaoying, Hou Zuofu, and Li Zhuoqiu. "Experimental studies on the indoor electrical floor heating system with carbon black mortar slabs." Energy and Buildings 40, no. 6 (2008): 1094-1100.
• Sugiyama, S., M. Nishimura, and H. Watanabe. "Transient temperature response of composite slabs." International Journal of Heat and Mass Transfer 17, no. 8 (1974): 875-883.
• Yehia, Sherif, and Christopher Y. Tuan. "Conductive concrete overlay for bridge deck deicing." ACI Materials Journal 96, no. 3 (1999).
• Zhao, Hong-Ming, Song-Gen Wang, Zhi-Min Wu, and Guang-Jie Che. "Concrete slab installed with carbon fiber heating wire for bridge deck deicing." Journal of Transportation Engineering 136, no. 6 (2009): 500-509.

Question:

(Feb 4, 2013) steve said:

I came accross this site looking for systems to work as snow melting systems for 'my' snow piles I have to deal with in my parking lot. Can anyone steer me towards a site with some expertise and simplicity on this subject? I am not an engineer!

Question: cost vs quality of in floor heat?

(Jan 26, 2014) Sergei said:

PAY to contractor enough money and it will be done properly, according to that you pay. Do not look for the cheapest quote)!

Reply:

Really? Sergei, we agree that shopping for the lowest bidder does not mean that the job ends up with the lowest cost - since paying to fix mistakes is also a cost. Our article "How much should you pay " found by searching InspectApedia for that title, explains the problem of hidden costs.

But unfortunately the highest-priced contractor is also no guarantee that the job will be the best one. In fact in every field some vendors have a marketing strategy of charging the highest price precisely because it fools some consumers.

In the case of the job discussed above, the contractor used was in fact far more expensive than everyone else, promised the moon, and delivered a mix of nice work on the topics he was good at, and horrible work (and bullying) on the topics he really didn't understand - like radiant heat floors.

Question: radiant heat flooring & snow melting installer in Minneapolis comments

(Mar 15, 2014) Morgan Audetat said:

We design and install radiant floor and snow melting systems here in Minneapolis. Good people are out there, but you have to ask the right questions. There is simply no substitute for experience and design software. If your contractor can't produce a sample heat load, and pictures of similar installations, you need a new contractor.

As for tubing depth in a 4" slab. Much has been made of this interesting 'model', most especially by self-professed experts following John Siegenthaler's (my personal Hydronic Hero)interesting study. Unfortunately, this is a pour example of real hydronic radiant slab-on-ground installation as is this the this story of tubing installed in an un-insulated footer. This is not your slab or anybody's for that matter. Few put floor covering over new slab-on-ground radiant floors but those who do would likely not need 35 btu/sq ft. unless the they live here in Minnesota and left the windows open in January.

The first number then at 15 btuh/sq.ft. would be reasonable and then only only under "design" conditions, less than 2% of the heating season here in Minnesota. The rest of the season it simply doesn't matter. First, 2" of XPS is the current under-slab insulation of choice in most cold climates as it stands up to foot traffic and also meets the accepted standard of 5 times the heat flux to ground.

Second, the design water temperatures and tube spacing are completely arbitrary. The insulation below the slab has everything to do with the year round ground temperature, the indoor, outdoor design temperature, the design water temperature and the heat loads dictated by building construction and climate. Response time is also factored in and of little consequence when weather sensitive controls are employed.

Furthermore, Ziggy uses 12" O.C. spacing, which is common but by no means certain. If he used 8" O.C. the response time would be decreased along with the average water temperature, downward heat flux and consequent cost of this current standard. The reason PEX tubing is placed at the bottom of nearly every residential and light commercial slab in the country is simple. It is much harder (more expensive) and dangerous (easy to hit with a saw or fastener) to raise by pulling or set PEX on chairs. Folks in South Dakota bury their PEX to "charge" theirs slabs with cheap off-peak electricity

Reply:

Thanks Morgan, I agree;
The horrible radiant heat intallation we describe was in fact in a Minnesota home. But let's keep in mind that a significant number of homeowners in MN do not live near a large city where experts are available, and at least some contractors in MN make the same mistake as that of contractors all over the world, being unable to recognize the limits of their own ability and expertise. The SNAFU contractor did beautiful carpentry work but was frankly both incompetent and a bit of a bully when it came to other areas outside his expertise. The customer doesn't even understand that more expertise is needed - until too late.

Question:

(Apr 25, 2014) anonymous said:

My son installed his floor system, Pex in concrete, and lives in Arkansas. He has high heat bills and I told him to set his thermostat closer to the floor at 68-70 and leave it on starting in October. He has it placed up on the wall with the regular thermostat, heat pump, and leaves it set at 70. He also is away for several days at time and will shut down the system to "save electric" which I have told him not to do.

The house is 1600 sq ft in northern Arkansas and has insulated slab and was built with Styrofoam blocks filled with concrete, 12 inches thick. Cooling cost is minimal, but heating is in neighborhood of double or triple. I have "harped", as parents do, to not turn it off, start it earlier, he sometimes waits till December, and move the thermostat to sense the warm floor and not the room temperature as the room will follow the floor temperature eventually. Maybe not move the thermostat down, but lower the room temperature setting so you get the same effect, but am I right or wrong on other counts?

Reply:

Anon,

Previously I agreed with you thinking that the cost to re-heat cold stuff pushed up the heating total cost but more recent studies argue the opposite.

I think some diagnostics might be helpful. If we knew more details about how the slab was built, how it was insulated from the earth, and how deep the tubing is in the slab we could form an opinion. If his builder, like mine, was a love when it came to radiant slab basics, then we might have to make some changes to obtain economical heat.

Question: saw cuts in concrete slabs

(Apr 25, 2014) Anonymous said:

If you recomend to bury the pipe 2 in. Below finish in concret, how deep can I make my cuts.
the whole reason im pouring new crete is my floor is saging and all cracked up. So I plan on busting out old crete, dig outside edge down 11.5 in. 8 in for center

Then I would lay a plastic sheet on dirt, then add 2 in. Waterprof insulation. Then pipe. But if I bring pipe up 4 in. Or 2in.down that wont let me cut 3 in. Now im confused.i was going to put pipe on top of insulation..

Reply:

Anon I don't understand the question "how deep can I make my cuts" - what "cuts"?

The reason for the 2" recommendation is that deeper tubing won't transfer heat well to the occupied space.

I understand that it's mechanically easier, faster, cheaper to just staple the radiant heat tubing to the foam insulating board, then pour the slab, but that puts the tubing in the wrong location. That's what the contractor did in the article above. The result was a totally un-usable system, made moreso because the insulation under the slab was inconsistent and incomplete and moreso because he ran tubing fastened to insulation that at some locations (the perimeter of the floating slab) was beneath even more concrete thickness.

Question: Radiant heat floor in Australia

(May 31, 2014) Leo Sadlek said:

I am looking at buying a house in Melbourne Australia with hydronic heating in the concrete slab floor. I noticed some cracks in the surface of the concrete floor, perhaps 1mm (1/16") wide and at least a metre (3 feet) in length in some places.

Two questions: 1, is this common to see cracks in concrete floors with hydronic heating installed; 2, can there be some compromise of the heating conduit in the slab as a result of these cracks?

Reply:

Leo,

Often cracks are caused by shrinkage or settlement - you'll want to diagnose their cause to understand whether or not they're important.

See
inspectapedia.com/structure/FloorCracks.htm

for details about cracks in concrete slab floors.

The location, pattern, direction, as well as the width you cite all go into the diagnosis.

If the heating conduit is rubber tubing and the cracks are not associated with settlement or more significant dislocation in the slab I'm doubtful that the cracks will cause leaks in the tubing.

That'd be particularly so if the cracks are due to concrete shrinkage.

Use our CONTACT link if you want to send me some photos for comment.

Question: how to support the radiant tubing when pouring the slab

(June 3, 2014) kamen said:

Great article. I am doing UFH in an apartment on a higher floor. I plan to have about 1" XPS over the concrete slab and 2" screed. As per your recomendation I'd like to put the pipes in the middle of the screed but how can I do that? If we put a steel mesh below the pipes it will get into the XPS while we are working on the floor doing the pipework or pouring the screed.

Another option would be two layers of screed but this way the layers should only be 1/2" so they will easily crack (esp the 1st layer). I try to avoid thicker screeds as the rooms will become very confined. Thanks in advance for the help

(July 15, 2014) Mark King said:

How do I maintain my PEX tubing at the suggested 2" depth in a 4" slab on grade new installation?

Reply:

Ah the intrusion of real world problems. The manufacturers typically recommend additional supports to hold the tubing up where you want it. I've seen contractors support tubing atop the reinforcing wire mesh which may be acceptable (check with your engineer) provided the mesh itself is supported high enough during the slab pour.

I've also seen radiant floor heat contractors use metal stands and clips to support tubing.

What I do not want to see is tubing at the bottom of the slab.

Mark,

Agree that it's more trouble - the manufacturer recommends and some sell additional supports placed below the tubing to keep it high in the slab. I've seen installers fiddle with lifting tubing during the pour - a less reliable and messier approach.

Question: sand for thermal mass

(Aug 3, 2014) j.d. overmyer said:

The practice of putting additional sand over the tubing is for thermal mass. The power companies (at least some in Minnesota) have something called "off peak power".

The rate is 1/2 the cost but it will only operate between 11 pm and 7am. The floor heats up during these hrs and then gives off heat the rest of the day, then charges again at night. If theres not enough mass then you runn out of heat during the day.

Reply:

Thanks JD, good points.

Question: how to shut off radiant heat completely without risking freeze damage

(Oct 31, 2014) Sylvain Soucis said:

I have a concrete floor heated (slab) by boiler (water heating) inside my floor is PEX tubes. This system heats my dinner and living room. I have to quit my house for the winter time. I'm live in British-Colombia, Canada. The average lower temperature in the winter time at my place is 25F in the night and about 35F in the day time.

I want to shut the system down for the winter time (the water heat system) and let only electrical baseboard working around 50F for the house. My decision is because if something happen (leak or pump failure ) Nobody will be there o fix the problem and I don't want a flood in my house. My concern is about the slab and the water inside the slab. Do I have to purge all water inside the floor; Does the slab has a chance to crack because it doesn't heated anymore and it is directly on the ground even I will keep the room temperature at 50F.

Thank in advance for your suggestion.

Reply: use an antifreeze mix in the radiant floor tubing

Sylvan

Indeed leaving all heat off risks frost damage and freezing damage to various building components.

But instead of draining the slab heating system, why not install boiler antifreeze to protect it?

See ANTIFREEZE for BOILERS - inspectapedia.com/heat/Boiler_Antifreeze.php

and to be more thorough in avoiding damage see

WINTERIZE - HEAT OFF PROCEDURE - inspectapedia.com/plumbing/Winterize_Heat_Off.php

Question: debugging radiant heat floor installation with a condensing boiler heat source

11/20/2014 Susan said:

We have a radiant heat system installed at house construction in 1952. It was installed with steel pipe (*not* copper). About 1/3 of the house is on slab with the heat embedded there; there is also heat in all ceilings. It is not under-floor in those parts of the house with wood floors.

The system has worked wonderfully well for all of these years. However the *burner* part of the old American Standard boiler has failed, and no-one seems able to repair it or replace it. The *boiler* itself has no leaks, nor does any of the radiant heat loops. It's just the burner.

We've had several contractors through; none can repair the old burner, all suggest replacing it; and we have a quote on replacing the whole American Standard unit with a new Weil-McClain Ultra 230. He also proposes to combine the hot water heater with the boiler via an indirect heater.

This makes me concerned because I believe the operating temp of the old system would have been around 110-120 degrees. (We can't fire it up to check the exact temp.) In fact that's why I think the Ultra 230 was specified as it can be run at the lower temps, as the old system did.

So how can a domestic hot water heater also be fed off this system and provide usable hot water at fixtures? Current standalone HWH temp is about 145 to get about 120-130 at the fixtures; this does have to be seasonally adjusted.

If the contractor wants to up the temp of the boiler to make the indirect HWH work, I think that will bring the temp way too high for the older steel pipe system -- do you all agree?

Reply:

Susan, your contractors suggestions are excellent ones. The heating boiler on both old and new system would operate at around 180 -200 deg. f. A separate mixing control regulates the radiant heating system temperature

At the Continue Reading Link above see the article
titled RADIANT HEAT TEMPERATURES for details on the usual temperatures and in the

More Reading links above
see RADIANT HEAT CONTROLS to, read how temperatures are controlled.

Reader follow-up:

The old system is an odd duck - was unique when it was put in in 52, let alone now! - and has *no* outside mixing control (direct from boiler to the pipes), nor does the contractor propose to put in a mixing or tempering setup. That's why we are concerned.

We did speak to Weil-McLain because we understand most boilers run much hotter -- that's why the Ultra 230 was spec'd, because it has an aluminum tank and can run at the lower temperatures.

Thank you for your references to other pages, I spent an hour or so on your web site before posting. Great reading.

Reply:

You are right to be concerned. You need a contractor who knows radiant heat. Without appropriate temperature control, usually done with a simple tempering valve that mises water returning from the radiant heat system I with outgoing to get the proper temperature, the floors may be too hot or even may suffer damage.

I add that you can run a boiler at lower temps but typically also that means lower efficiency, and you may find that the indirect water heater performance is ... Well ... Not so hot.

We don't know the depth nor spacing nor under-slab insulation nor original temperatures nor flow rates for your system nor the remaining in slab pipe life. But you can see typical radiant heat temperatures in these articles. It would be a bummer to put in a special low temp boiler just because the contractor didn't know about radiant controls (if that's the case)

And worse If next year you decide to abandon leaky in slab tubing to go to baseboard but then wish for a different boiler.

Reader follow-up:

I'm glad to hear a pro confirming our thoughts! I personally think we should just give up on the indirect hot water heater - why complicate an already complicated situation - and if code truly won't allow us to keep both the existing independent gas HWH and a new furnace on the same chimney, just give in and put in an independent electric HWH. That way furnace debugging and HWH debugging are unrelated.

We've been told that the Weil-McLain Ultra 230 could be adapted to other kinds of hot water heat if the old systems piping leaks at some point -- so it seems a worthwhile investment. We'd probably stay with radiant, looking at a ceiling installation in the parts of the house that are on slab.

It's been suggested that the old system *might* have had some kind of tempering valve buried in it's internals; unfortunately we don't have specs on that unit. I tried to get them from American Standard but they sold their boiler business in the...70's, I think it was?...and neither they nor the new company kept any of the old manuals or specifications. Which is too bad, because at this point I'd rather just replace the burner and keep the old boiler going through the winter, instead of having this crazy rush to get heat fixed under the gun of Old Man Winter.

I'll keep you updated as we hear more.

Reply: advantages of an indirect-fired water heater

Susan

Well if you install a normal boiler with proper temperature controls for the radiant heat, an indirect-water heater is one of the most satisfying, efficient ways to make hot water. An independent electric water heater is simpler, cheaper, and more expensive to operate except in areas where electric rates are low.

See INDIRECT FIRED WATER HEATERS for information about this approach to providing domestic hot water.

The Weil McLain Ultra 230 gas fired boiler is a sophisticated design that should be impressively economical to operate.

You can obtain specifications and a nice WEIL MCLAIN ULTRA 230 BOILER [PDF] specification sheet for this boiler from www.weil-mclain.com or if you can't find it ask me by email and we can send you the product PDF file.

Reader follow-up: 40 days to resolve radiant heat floor slab temperature controls

We've sent several questions and concerns to the contractor and will see what happens -- I expect it will take a few days for him to get back to us.

We have had a lot of people look at it over the last few weeks....most of them say either WTF or WTFF and won't even bid the job. Some of the radiant heat contractors won't even come look at it since it's got metal piping (and is older than many of their companies are). The folks who installed it had been around since 1907 but went out of business in the 1990's.

Thanks again for the great site! I will let you know what happens.

(Nov 23, 2014) (mod) said:

Thanks for the details, Susan. Keep us posted - it will help other readers.

Oxygen may not be much of a problem: oxygen in heating system water is more of a concern with steam heat in which new water is constantly being introduced into the system. Hydronic systems, including radiant floor heating, keep the same water in the piping - one re-load of water is not going to be enough to explain corrosion. YOu can also discuss adding an antifreeze and corrosion protective chemical to the water if necessary.

(Nov 22, 2014) Susan said:

Contractor will be giving us a revised quote. He didn't go into enough detail on the first one, apparently. Your info will help us read it properly!

We agree that the potential for leaking is a big concern, and is why we had never upgraded the boiler/burner to a more efficient one -- didn't want to open the system and expose it to oxygen, etc.
It's been closed and stable for many decades -- no pressure drop, no need to add water.

I'd done crude mapping of the system with an infrared thermometer in the past and seen no noticable cold spots; we don't think we can get it hot enough now to get a good pic with a thermal camera (rentable in our area for $80 a day).

And we are accepting language in the quote to not hold the contractor responsible for leaks in the existing radiant loops where he has done no work.

The original installers were *very* thorough; there are 16 valves that can cut off individual parts of the system (labeled for each area like "kitchen floor" "Hall bathroom" etc). So if the slab pipe in the kitchen leaks, we can still heat the bedrooms and bathrooms.

(Nov 21, 2014) (mod) said:

Susan in our conversation reported above at the "Click to Show or Hide FAQs" link in this article we include a link to a PDF with data on the Weil McLain boiler you discussed.

On old in-slab tubing, I cited the leak worry earlier - which would make most contractors nervous. A thermal scan of the slab can tell you if there are already leaks as can noticing a pressure drop in the system.

(Nov 21, 2014) Susan said:

We've sent several questions and concerns to the contractor and will see what happens -- I expect it will take a few days for him to get back to us.

We have had a lot of people look at it over the last few weeks....most of them say either WTF or WTFF and won't even bid the job. Some of the radiant heat contractors won't even come look at it since it's got metal piping (and is older than many of their companies are). The folks who installed it had been around since 1907 but went out of business in the 1990's.

Thanks again for the great site! I will let you know what happens.

(Dec 20, 2014) Susan said:

Arrgh! We're two days into the install...and the contractor thought he was close to done...and we've found a problem. He has the mixing valve installed with the "cold" side on the *municipal water feed* (for volume make-up, also goes to the bladder expansion tank). Since this is a closed system, that's not going to work!

Weil-Mclane (on page 17 of the manual you sent me) doesn't mention using a mixing valve at all, just to run the system in condensing mode with their controller.

I don't know what's going to happen next...but now we have no hot water as well as no heater.

(And the tempering valve seems pretty...unsophisticated? A valve that we can manually adjust to allow more cold or more hot water, with a thermometer a bit downstream from it that lets us see the temp. No automatic adjustment, we do it by hand.)

PS: as the old boiler was removed, water was clean, no visible corrosion, and the excess steel pipes removed were unobstructed; x100 was put into the system as a protectant. My concern now is the temperatures. There are several tile floors involved -- we really want the temperature going through the system to err on the side of being LOW not high.

(Dec 21, 2014) Susan said:

Update: The contractor has decided that we'll ignore the mixing valve, we are using the Ultra controls only, and he will return tomorrow with the "optional" overtemp shutdown sensor. Meantime, we will stay home and watch the floor temps and manually shut down if they get too warm.

The system has been running for 14 hours now...*and the house is not warming up*. The pipes are warm where they go into walls or floors, but the house is now in the low 50's (where the space heaters were holding it in the high 50's or sometimes low 60's). We've turned off most of the space heaters, and the radiant system might be slowing the heat loss, but it ain't heating.

With the old setup, once you changed the temperature, you could *feel* warmth from the ceiling areas that have heat within, oh, 20 minutes at the most. (Yes, the slab floors took longer; the tile floors were fairly quick as well).

Before he left the contractor showed us how to adjust the temp settings on the boiler control panel, and we've bumped it up to 135. It was at 125 and water was returning at 120 (per the furnace) which leads me to wonder if the issue is the circulator pumps? They took out our old ones and put in Grundfos ALPHA units, which are set to the highest fixed speed setting. Should they be in constant pressure mode instead? Or are they the wrong pumps for the system? (They are on the return side, not the outgoing side, and this is the way the old setup was too, but the old pumps looked bigger).

And silly note, they insisted on new thermostats...programmable...but in a radiant heat setup that seems silly to me. At least we get a rebate on the for energy efficiency. Right at the moment the thermostats are all set for 74 degrees, all the time, as we try to get things warm.

(Dec 21, 2014) (mod) said:

Susan I can't diagnose this system by e-text, but the snafus that you have cited make one worry that there may be others. This sounds really like a mess that requires an experienced onsite radiant heat flooring installer who can look at your controls, piping, valves, etc. If you have photos of the installation as it went in those may be of immense help. That was how I learned why the radiant heat floor in the article above was ruined - during installation tubing was improperly located and sub-slab insulation was incomplete. YOu'd never seen those big mistakes when looking at the finished floor.

(Dec 21, 2014) Susan said:

Hi, Dan. We don't have photos of the original installation process, or of the exact pipe depths in the slab (but given that it worked OK for 60 years, I assume it was done right!), but I can get you photos of the current setup - tell me what should be focused on.

We've been doing some careful measurements and keeping records, and we think the problem is *flow* rate - the circulator pumps, on max, only report a flow of 1GPM, and the separate setup for the DHW is reporting *5* GPM when it runs. We found one of the old pumps still in the garage and it's specs are 33GPM with no head, the new one is 22GPM with no head (but we're not getting even 2GPM).

We also think that there are some places where the new copper return pipes are smaller than the old steel ones (but we need to know ID not OD, so we're going to have to ask the contractor when he comes back to be sure of this).

Having the new boiler isn't going to help at all if the heated water doesn't get where it needs to go!!!

The lesson I hope folks take from our experience is...don't let your heating system get too old. IF things had been upgraded before the old system was so obsolete, we might have had more choice in contractors. (Most of the radiant folks in our area won't *touch* a system with metal pipes at this point; finding someone who would take the job was massively difficult.)

(Dec 21, 2014) (mod) said:

Susan I didn't know that the in-slab radiant tubing was an old installation. IN that case I agree with your focus. (Except for looking out for leaks in the tubing system).

The gpm flow rate depends on more than the pump: a partly closed valve, a blob of solder in a bad spot, and for some models pump settings impact the flow rate as does also length and diameter of piping and number of elbows, and as you noted lift or head that the pump has to overcome.

But I also agree that upping the flow rate is another way to increase the heat output of the system.

It's worth noting that in a building where we're trying to improve water flow rate in any system, increasing the diameter of even a portion of the system will help.

Keep me posted

(Dec 21, 2014) (mod) said:

Susan I didn't know that the in-slab radiant tubing was an old installation. IN that case I agree with your focus. (Except for looking out for leaks in the tubing system).

The gpm flow rate depends on more than the pump: a partly closed valve, a blob of solder in a bad spot, and for some models pump settings impact the flow rate as does also length and diameter of piping and number of elbows, and as you noted lift or head that the pump has to overcome.

But I also agree that upping the flow rate is another way to increase the heat output of the system.

It's worth noting that in a building where we're trying to improve water flow rate in any system, increasing the diameter of even a portion of the system will help.

Keep me posted

(Dec 22, 2014) Susan said:

It's better but not right yet. The mixing valve is a big part of the problem; the supplier insisted on it but it turns out to not be needed, it's now on full open and it's secondary line has been turned off, and we're getting much better flow through the system. We still think the pumps are undersized but they are now doing 2-3 GPM. The house is slowly warming.

The contractor is going to eliminate some bends in the line and increase the diameter a bit when he *pulls* the un-needed mixing valve - that won't happen right away, but will happen soon.

The crew was actually pleased that we had learned enough about the system to contribute to problem solving. Great site you run here!

(Dec 24, 2014) (mod) said:
Susan

Usually a mixing valve is needed to temper hot water cycling through a radiant heated floor slab. Indeed one could simply run the heating boiler at a lower temperature instead, but that approach is usually significantly less efficient - thus increases heating costs.

Sounds like you're doing well at getting along with the contractor. Kudos.

2 January 2015 Susan said:

Finally! The mixing valve was removed completely (Weil-Mclain says this is a condensing boiler and it does all the modulation, the mixing valve is not needed and was actually inhibiting flow); the pipes have been straightened out and several 1" lines are now 1.5"; and there's a proper overtemp cutoff in place on the floor loops.

The existing loops really do seem to be fine -- a sample of water was drawn for a repeat test of concentration for the X100 anti-corrosion agent, and the water was clean and clear.

And we can now run both pumps at once with flow of 5-7 GPM. The house is *evenly* warm now.

I think we're finally done! Just in time, 5-10 degree temps in the forecast.

Thank you for your time, Dan. Sites like this can help a *lot* - we were educated enough to understand what was going on, ask reasonable questions and help with the debugging process, and had a contractor who would work *with* us. As a partnership we got this resolved better and faster.

Reply: condensing boiler regulating radiant heat slab temperatures without a separate mixing valve

Thanks for the helpful feedback, Susan. The notion that a condensing boiler is also moderating temperature to a radiant heat floor system is useful to learn about but a bit uncertain at least for cases in which a heating boiler is being used both for radiant heat flooring and other applications such as heating conventional radiators or baseboard (those want to be at a higher temperature than radiant floor tubing) or heating a tankless coil for domestic hot water.

Kudos to you and your contractor.

Question: efflorescence as a floor problem clue

(Mar 9, 2015) chris said:
the floor grout has turned white and effereciant, could it be from the poured screed?

Reply:

Chris if this is a new condition in an old floor - years old - then I suspect there is a leak below the floor or rising ground-water or roof spillage leaking under the floor.

Question: musty smell from radiant heated concrete floor

(Apr 20, 2015) Anonymous said:
I have a musty smell coming from my radiantly heated concrete floor. It was originally plumbed to flow thru my water heater, but was subsequently connected to a closed system using a gas fired, on demand, water heater. Ever since, the basement has smelled like dirty socks. An ideas as to the cause?

Reply:

Anon

There are in fact some genera / species of mold that smell like "dirty socks" -so perhaps there was a leak in the piping system. Some anti-freeze mixes may also give off an odor if there's such a leak.

Question: enforcing the 2-inch maximum depth for radiant slab tubing?

(Sept 30, 2015) John said:
I have a radiant flooring system in a supported concrete floor slab, would the 2" maximum tubing installation depth apply in this scenario? It appears to me this applies to slab on grade conditions.

Reply:

Yes

Question: radiant slab pour before framing?

Oct 11, 2015) Dan said:
Should I have the concrete for radiant heat pored before we frame the building

Reply:

Interesting design question: if your framing is atop a conventional foundation your architect may be planning to later pour the slab inside the foundation walls: that's ok but be sure that the insulation inculdes a break at the walls.

If you pour the floor before framing and the wall sills sit atop the wall, be sure that no one drives power-nails through concrete into the tubing of the radiant heat layout. That means careful measuring and marking safe-nailing areas for partition wall or exterior wall sills.

Question: radiant floor has begun to heat unevenaly

(Oct 16, 2015) Sue said:
The floor heats unevenly. Some spots are cold even though other spots are toasty warm. This just started last winter. Can the pipes plug up?

Reply:

Yes individual circuits or loops can clog, particularly if they were run as separate loops off of a main. Also see DIAGNOSE RADIANT HEAT FLOOR NOT WORKING

...

Continue reading at RADIANT HEAT TEMPERATURES or select a topic from the closely-related articles below, or see the complete ARTICLE INDEX.

Or see these

Radiant Heating System Articles

• RADIANT HEAT - home
  • CARPETING IMPACT on RADIANT HEAT FLOOR
  • DRIVEWAY & WALK DE-ICING SYSTEMS
  • RADIANT HEAT AIR BOUND
  • RADIANT HEAT CONTROLS
  • RADIANT HEAT DEFECTS LIST
  • RADIANT HEAT DESIGN & INSTALLATION MANUALS
  • RADIANT HEAT INSTALLATION
  • RADIANT HEAT MISTAKES
  • RADIANT HEAT TEMPERATURES
  • RADIANT HEAT TEMPERATURE CONTROL / MIXING VALVES
  • RADIANT HEAT TEMPERATURE NIGHT SETBACK
  • RADIANT SLAB FLOORING CHOICES
  • RADIANT SLAB TROUBLESHOOTING
  • RADIANT SLAB TUBING & FLUID CHOICES
  • RADIANT SLAB TUBING LEAKS
  • SLAB INSULATION, RADIANT / PASSIVE SOLAR
  • THIN FILM RADIANT HEATING SYSTEMS for Ceilings & Floors
  • WOOD FLOOR RADIANT HEAT
  • WOOD FLOOR RADIANT HEAT DAMAGE

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RADIANT HEAT DESIGN FAQs at InspectApedia.com - online encyclopedia of building & environmental inspection, testing, diagnosis, repair, & problem prevention advice.

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INDEX to RELATED ARTICLES: ARTICLE INDEX to RADIANT HEAT

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