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Radiant heating system design or installation mistakes that must be avoided. This article explains how to avoid some fatal mistakes when installing radiant heat in a concrete floor slab by describing an incompetent radiant heat floor installation along with an explanation of why things went wrong and how to avoid these errors.
The workers in the photograph at page top, where our concrete slab was being poured, were not guilty of a thing. But the contractor who prepared the forms and under-slab insulation placed radiant heat floor tubing too deep in the slab and he omitted proper under-slab insulation. The result: the owners ultimately had to abandon the entire radiant heated floor system.
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Original Research Identified Heat Loss Rates Through a Concrete Slab on Grade with Various Insulation Schemes
The nonsensical view that one can heat up the soil below an building slab on grade and that the soil would magically stay warm forever was put to the test experts when the author was five years old and the contractor was not even a gleam in his daddy's eye.
During February and March 1948, using a specially built, instrumented structure, Harlan Bareither and other experts and students at the University of Illinois Department of Mechanical Engineering conducted careful tests of various slab on grade floor and insulation designs to map heat loss, temperature, and moisture permeation characteristics of nine types of concrete slab subfloor constructions laid on the ground. 
Previously, the US National Bureau of Standards had already indicated that the heat loss of a concrete slab (floor) on grade (on the ground) is proportional to the perimeter of the building. Bareither et als. referred to that work, but because the original testing was in warmer conditions (outside temperature had not been below 35°F. for more than three successive days), they recognized the need to test slab-on-grade floors in still colder conditions - in a climate where the ground is frozen during much of the heating season.
The 1948 heat loss research was important in part because it recognized that the rate of heat transfer from the heated building to the outside (earth and surrounding air) would be greater in proportion to the temperature difference between the heated space and the surrounding soils.
This research on floor slab heat loss rates confirmed that
Nightmare-Construction's Insulation Scheme & Radiant Tubing Location Details
One exception to the general order of priorities of where to insulate in buildings concerns homes built with slab-on-grade construction, particularly homes which have used radiant heat in the floor slab.
The contractor (Nightmare-Works Construction) for a small cabin in the North combined being opinionated and a bully with dismal ignorance of how to construct a properly insulated radiant floor slab. (See Slab Log Cabin Siding for cabin photos and other comments).
Not only did the owners have to battle with the bully to put insulation under the entire slab (he thought that Mother Earth would be warming the home from "ground heat" (which is below 40 °F in winter there).
Owners also lost a battle to have the contractor install proper insulation around the slab perimeter with a frost wall before the floor was poured (he insisted on a floating slab with no inside-perimeter insulation plan).
Worst of all, the contractor also pushed the radiant heat tubing so deep into the concrete (ranging from 7" deep to more than 18" deep) that the entire radiant heat system was not useable at all. Running the radiant heat pushed heat faster into the ground than it did up into the building, even with foam insulation under the slab. We had to abandon the (expensive) radiant floor system and install alternate heating.
The Results of Combined Incomplete Floor Slab Insulation near the Perimeter and "Too-Deep" Radiant Tubing Placement were an Abandoned Heating System
Details about this radiant floor slab heat failure and and research on its cause are provided below. Also
Critical Design Details for a Radiant Heated Concrete Floor
Don't permit your contractor to make the (many) mistakes this one did. Insist that radiant heating in a poured concrete slab have these attributes:
Radiant Heat Floor Slab Design Specification Details
After reviewing photographs taken during installation of the radiant heat floor slab described above, here's what we wrote to the owner and to the contractor:
The bully contractor, who originally estimated the monthly heating cost for this small and otherwise well-insulated building, had said the owners would face winter heating bills of about $30./month based on his prior experience. Stunning heating bills arrived, exceeding $400./month or more than ten times the estimated amount. That's when we began digging into the installation details of this project. The floor slab and radiant heat tubing had been placed by the contractor while we were unable to attend the jobsite.
When the heating bills were excessive and when the heat, running 24-hours a day for weeks, was unable to raise the interior temperatures above 60 deg .F., the contractor offered to "correct" the problem by installing larger capacity circulator pumps.
The "option" of adding larger pumps for this radiant heat floor was not a proper solution for several reasons:
The most economical fall back is to install electric baseboard heating or possibly hydronic heating using the existing electric boiler which was installed to pump heated water through the radiant tubing in the concrete floor.
Meanwhile we shut down this unfortunate radiant slab heat system, installed a few portable electric heaters, and given the tight, well-insulated construction, we found we can keep the little cabin comfortable for a fraction of the cost of heating the earth underneath our floor with the contractor's heating installation.
Here are the details of the errors visible in photographs taken during installation of the radiant floor:
Discussion of the Above Radiant Slab Heat Performance Case Study
James Darling, General Manager of Preferred Heating LLC, in Eagle River, WI commented on this article that the contractor's promise of heating the building for $20. a month was an unreasonable promise not to be relied on - one that could make the article above misleading. We agreed that the description of the failure of this installation needed some clarification, and added the following information that should be considered:
Actual Heating Costs for the Building Described Above
Keep in mind that this was a small new structure (624 sq.ft.) whose construction details, methods, materials were unusually well documented as a project. So the insulation, air tightness, materials, heating details were known.
The building was super insulated, tiny, airtight, with double-glazing throughout, leading to an expected low heating cost. If the owner's actual heating bills for the structure had been even five times what was promised for this building that was occupied only part-time, the owners would have been happy. Heating bills weren't the arm-waving promise of $20 per month, they were not $200. per month. They were more.
In fact, the utility cost to heat this tiny cabin resulted in bills that more than doubled the corresponding costs of the nearby 1960's vintage two story large old, comparatively poorly-insulated house on the same property, exposed to the same conditions. And the exploding heating costs were observed when heating the building well before the coldest part of the heating season.
Heating Capability Limitations of an Improperly-Installed Radiant Floor Slab
The effects of putting the tubing deep into the slab created a problem of heat transfer losses to the ground, not just a matter of longer response time to warm the building. Even if money had been no object, the system simply could not heat the building to an acceptable temperature.
The problem with very deep radiant-heat tubing, combined with incomplete insulation, is that even with just 12 to 18" of concrete above the tubing, heat flowed enough into the ground below the building that even with the thermostat set to maximum, and running heat continuously for a week solid, in moderately cold weather (in the 40's in Northern Minnesota where in winter it can drop to 20 deg F below zero) we never ever could get the indoor temperature above 59 to 60 °F. And this was in a new, small, airtight one-story well-insulated building.
even if we had continuous solid foam insulation under the slab, say R-10 for simplicity, if we have enough inches of concrete above, even though the "R" of concrete is much lower than the insulation, it's the total heat resistance by the total inches that comes into play.
If we have enough thickness of concrete above the tubing (Where 1" to 2" tubing depth is the best design and 6" is considered a lot, in this building we are looking at 18" or more at least in many areas, maybe 24"). With radiant tubing at those depths, the concrete begins to offer not just a lag time in heating (Mr. Darling's point) but also an actual resistance to heat transfer until we begin losing at least some heat into the ground.
The contractor and others tried to improve the system's performance by changing the boiler settings from those set by the manufacturer on its integrated circuit control board, upping the circulator size and capacity, checking flow rate through the system, checking the thermostat controls.
What Caused the Failure of This Radiant Floor Heating System?
Our photo (left) shows where we found the radiant heat floor tubing when we later broke open a section of the floor slab. Radiant tubing was at the bottom of the slab, in this area more than seven inches down in the concrete, and set atop the foam sub-slab insulation.
Our photo above on this page shows that tubing was in some sections more than 18" deep, and adjacent to a large area where sub-slab insulation was simply omitted by the contractor.
We also measured floor temperatures in different areas of the building, mapping clearly where the radiant heat tubing dropped to the bottom of the footing-portion of the monolithic-slab footings! That deep run, probably combined with the incomplete insulation at the level drop between slab bottom and the integrated footings, were almost certainly the prime cause of the failure of this system to heat the building.
As our reference document(s) below show by calculation and model, ultimately, the heat flow into the ground for tubing really too deep in the slab can be significant, even if there is insulation below all or part of the slab. In the structure described here, not only was some tubing 12 to 18" or even more below the slab top, the insulation below the slab was incomplete, inviting ready heat flow into surrounding soils.
Despite varying opinion by some radiant floor installers, consumers, and installers as well should be wary of ignoring the advice of the radiant heating design experts and heat transfer engineers about tubing depth in radiant floor slabs shown just below.
Worse than too-deep radiant floor heating tubing, in this case, because the contractor put NO insulation at the area of soil where he stepped the slab down to the depth of the monolithic integrated footings, we have heat transfer from some of the tubing through concrete right into the cold soil, not just through concrete up into the room through the ceramic tile floor.
In this egregious error, even worse than putting radiant heat tubing too deep in the slab, insulation was simply omitted where the floating-slab monolithic footings were poured. The R-value of concrete is roughly .08/inch (US DOE). The builder located sections of the radiant tubing so that there was about 6" or less of concrete (in the 12" footing section") between the tubing and the cold soil, giving us a heat transmission path (tubing to soil) of R 0.24 or less. This is a likely area of heat loss at all four sides of the building: where the slab dropped down to form footings.
As an aside the ceramic tile on the finished floor slab was set in mastic - leaving some air spaces and mastic that is a poor conductor compared with tile set in concrete (optimal) - but we doubt that's nearly as important in the system failure in this case.
References for proper radiant heat concrete slab design
Our photo (left) illustrates a successful radiant heat system installation in Minneapolis, MN - a climate simliar to that where we had trouble with the Two Harbors system above.
2006 IECC: effectiveness of foundation perimeter insulation and insulation recommendations for radiant-heated floor slab designs
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,  [photo at left showing interior foundtation insulation before the slab is poured, U.S. DOE, op cit.]
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 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.
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  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.
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 ).
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
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.
Continue reading at RADIANT HEAT TEMPERATURES or select a topic from the More Reading links shown below.
Suggested citation for this web page
Green link shows where you are in this article series.
Frequently Asked Questions (FAQs)
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>>??
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.
(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.
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.
(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.
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?
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.
(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.
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:
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
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.
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
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
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.
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....email@example.com....thanks.
No but if the wood floor has high moisture content you may find gaps when it is heated.
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?
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.
(Dec 23, 2012) Bob said:
Can a system like this cause your cold water to be tepid?
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.
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.
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.
(Feb 5, 2013) Mike said:
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.
(Feb 11, 2013) Anonymous said:
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.
Question: radiant heat experience
(Jan 7, 2013) john said:
I appreciate your site. This kind of discussion is very helpful.
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..
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.
(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)!
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
Thanks Morgan, I agree;
(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?
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.
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?
Often cracks are caused by shrinkage or settlement - you'll want to diagnose their cause to understand whether or not they're important.
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?
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.
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.
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
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.htm
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 useable 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?
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
More Reading links above
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.
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.
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
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.
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.
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