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.

How do I isolate the heated from un-heated floor slab sections

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 2018-02-11 by Mike Hoezee

Reply 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.

How do I avoid slab cracking if my slab is thinner because I'm using foam below it?

@mr bliss / friedman

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 2018-01-12 by Ar

Reply 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.

Follow-up 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.

by Ar

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

Reply 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

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?

2. 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.

3. 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.

4. 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?

5. 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 2018-01-09 by Ar

 

Our floor slab & foundation have no insulation anywhere. Will insulating the perimeter help?

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 2016-12-06 by mouselb

by (mod): yes

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

Plumber has not been able to get hot water to go through the tubing to heat the floor/room

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

by (mod) - try an IR scan to locate the problem

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.

 

Use multiple loops of radiant tubing for even heating ?

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.

 

Can excessive radiant floor heat cause tile cracking?

have you ever had a problem with the boiler too hot and cousing the floor tile to have small cracks all over the house. (Mar 2, 2012) david

Reply:

Yes - see WOOD FLOOR RADIANT HEAT DAMAGE

But other mistakes can also cause tile cracks: anything that causes the slab to heave, settle, or crack.

 

What are the options for foam insulation at block foundation walls

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 (Aug 7, 2012) allan k said:

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.

 

Pex tubing size diameters - is bigger better?

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. (Aug 24, 2012) Ron

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.

 

OK to install wood flooring over a radiant heated slab?

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. v

Reply:

No, but

Watch out: if the wood floor has high moisture content you may find gaps when it is heated.

See WOOD FLOOR RADIANT HEAT DAMAGE

 

I need to bolt a safe to radiant-heated floor - how do I know where the tubing is located?

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? (Nov 7, 2012) Connie said:

Reply: see original plans or use a thermal scanner

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.

 

Problems with boiler output temperature for Radiant Floor Heat

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. (Dec 31, 2012) Mike 

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 losing 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

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.

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:

Mike:

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.

 

Explain Cracks in Radiant Heated Floor in Australia

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? (May 31, 2014) Leo Sadlek

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.

 

How to support the radiant tubing at 2" down when pouring the slab

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 (June 3, 2014) kamen

How do I maintain my PEX tubing at the suggested 2" depth in a 4" slab on grade new installation? (July 15, 2014) Mark King

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.

Other radiant heat slab contractors use PEX rails or tracks, tube organizers that also maintain tubing spacing, steel protectors for tubing.

Examples of radiant heat tubing supports

• Wire mesh (normal slab reinforcement) raised above the slab bottom by rocks, bricks, masonry scrap before the pour, with tubing placed atop the mesh and fastened to it, keeping the wire mesh 2" below tehe final upper surface of the slab.
• Foam board scraps atop the foam board insulation that will be under the bottom of the floor slab (sloppy in my OPINION)
• Heat Sheet insulation - styrofoam insulation with raised protrusions that permit routing the pex tubing - PROVIDED the tubing will be high enough in the slab
• Advanced Radiant Systems Reflectors & Brackets [Website]
• PEX Rails secured to the foam under-slab insulation PROVIDED the rails are raised to an adequate height above the slab bottom
• Wirco Radiant Tubing Supports [Website]

Watch out: 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.

 

Sand is installed over tubing in some radiant floors for thermal mass

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

Reply:

Thanks JD, good points.

 

Tell me how to shut off radiant heat completely without risking freeze damage

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. (Oct 31, 2014) Sylvain Soucis

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

 

Steps in debugging a 1950s radiant heat floor installation with a condensing boiler heat source - temperatures too high?

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? 11/20/2014 Susan 

Reply: no problem with boiler temperatures if you use normal radiant heat temperature controls (mixing valves etc)

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

See the article titled RADIANT HEAT TEMPERATURES for details on the usual temperatures and in the

And see RADIANT HEAT CONTROLS to, read how temperatures are controlled.

Bottom line: either your boiler's own temperature controls need to maintain the proper heat level for a radiant heated floor or you need to use conventional mixing valves found on radiant heat floor installations.

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.

By (mod)

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.

By Susan

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.

By (mod) - try a thermal scan to look for radiant tubing leaks

Susan in our conversation reported above at RADIANT HEAT we include a link to a PDF with data on the Weil McLain boiler you discussed.

Or see the complete set of boiler information

at WEIL McLAIN MANUALS, AGE, CONTACT 

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.

Susan

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.

...

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.

...

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.

By (mod)

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.

by Susan

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.)

(mod)

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.

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!

(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.

Susan said: Weil McLain says this boiler does not need a mixing valve for radiant heat & it was causing trouble

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.

 

Why has my floor grout turned white and grown efflorescence?

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

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.

 

Dirty Socks musty smell from radiant heated concrete floor

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? (Apr 20, 2015) Anonymous

Reply: If the odor isn't from antifreeze, find and fix the leak, then clean and remove the mold.

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.

See details at MOLD ODORS, MUSTY SMELLS 

 

Does the 2-inch maximum depth for radiant slab tubing apply for a supported slab or just slab on grade?

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. (Sept 30, 2015) John

Reply: yes keep your tubing high in all radiant heat slabs

Concrete is an excellent conductor of heat, John. If you don't keep the tubing in the to 2 inches of your supported slab you'll be sending more heat down into the mass of the concrete than up into the occupied space.

But I agree that the rate of heat loss under and around the slab - DEPENDING - on how it's insulated, could be less of an impact than the infinite heat loss if the same mistake is made in a slab on grade installation.

 

Pour the radiant heat slab before framing?

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

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.

 

Radiant floor has begun to heat unevenly - clogged pipes?

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? (Oct 16, 2015) Sue said:

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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