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This article explains how to avoid some really bad 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 had improperly placed radiant heat floor tubing and had omitted proper under-slab insulation which resulted in the need to abandon the entire heating system. Photos and details are given below.

See these articles on radiant heated floors

FLOOR, WOOD RADIANT HEAT - guide to installing wood floors over radiant heat systems - this article is provided just below
RADIANT HEAT - usage guide, strategies for using radiant heat
RADIANT HEAT TEMPERATURES - at the boiler, in tubing, under floors, floor surface, thermostat & control settings
RADIANT SLAB FLOORING CHOICES - wood vs ceramic tile over a radiant heated floor
RADIANT SLAB TUBING & FLUID CHOICES - tubing and fluid choices for radiant heating
RADIANT HEAT Floor Mistakes to Avoid - proper and improper radiant tubing depth in slabs & slab insulation needs - what can go very wrong when the building contractor substitutes brawn for brains - this article is provided just below.
SLAB INSULATION, PASSIVE SOLAR - slab insulation & vapor barrier placement in heated floor slabs
WOOD FLOOR DAMAGE - from radiant tubing run too hot

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How to Really Foul Up a Radiant Heat Concrete Floor Installation - Mistakes to Avoid

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 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 degF 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. See SLAB INSULATION, PASSIVE SOLAR for a discussion of proper insulation below a heated floor slab. 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:

Critical Design Details for a Radiant Heated Concrete Floor

• Insulate below the entire floor slab. Sketch at left courtesy of Carson Dunlop.
• Insulate the slab perimeter, making sure that the insulation design does not rely on foam placed against the slab perimeter and extending above grade up to siding where it will invite termites or carpenter ants into the structure
• Place the radiant heat tubing at the industry-recommended depth down from the surface of the slab. Typically the maximum depth that tubing should be placed in a concrete floor slab is 2" down from the finished floor surface.
• Supervise: If you cannot be present at the job site at critical stages in construction, find someone knowledgeable who can inspect for you before the work continues
• If your contractor is an opinionated bully like ours who ridicules standards for good workmanship and proper radiant heat floor design, find someone else to do the work. Most contractors are conscientious and are glad to hear about good design.

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:

I am doubtful that we can successfully and economically heat the cabin with radiant in floor heating as the current system is designed and installed, and it is unfortunately the case that the cost-to-cure is prohibitive as the slab would need to be completely replaced with one using proper insulation and tubing placement.

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:

• Forcing a faster hot water flow in the radiant tubing would only correct a boiler operating problem if the boiler internal temperature were running too high and causing a shutdown due to the thermal over temperature sensor - this is not what was happening (I measured input and output temperatures)
• The boiler was already operating at spec in that it was producing more than 20 degrees between the input line and output line - so the problem is not flow but rather the inability of the boiler to handle the heat loss through the slab due to the slab tubing placement and insulation design.
• Installing a boiler of higher capacity might permit delivery of more heat to the slab and raise the indoor temperatures and slab surface temp but at a higher heating cost
• The current heating costs if we turn on the radiant floor system run $300. to $440 a month which is 10 times what the contractor originally estimated - doubling these costs by adding a larger boiler or faster water flow is unreasonable.
• We didn't have the option of taking advantage of reduced electrical rates because the electrician did not install the electrical service to our specifications - leaving out a separate service and meter at the entry point.

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:

• Photographs of the slab and radiant tubing installation for the cabin show that the guidelines for radiant heat slab installations were not followed.
• Tubing is at a depth greater than 2" from the top of the slab and at some locations is considerably deeper than that, in some areas more than 12" deep in concrete.
• Insulation is incomplete around the slab perimeter and cannot be added outside due to 1. insect damage risk and 2. would not extend below and under the slab edges
• Insulation is incomplete within the slab where tubing installed at a lower level is stepped down from the upper slab level and heat transfer is permitted into the gravel fill below the main slab area.

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 degF. 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. (See INSULATION R-Values & Properties)

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

• Industry Expert Modeling of Effects of Tubing Depth in Radiant Floor Slabs: An excellent if somewhat techno article on the problems of putting tubing at the bottom of the slab is at http://findarticles.com/p/articles/mi_m0BPR/is_6_20/ai_102862289/pg_1
  
  
• The floor construction in that case is a 4-in, concrete slab sitting on 1-in. (R-5) polystyrene insulation, and covered by 3/8-in, oak flooring. The embedded tubing circuit consists of 1/2-in. PEX tubing spaced 12-in, on center. Several versions of this basic model were developed to simulate tubing at different depths with the slab.

  THIS IS NOT OUR SLAB- which is has tubing at 6" deep and along one side where the tubing is deeper, 10-12" or more. We have better insulation but much deeper placement.

  Here is a quote from the last page of the article which reports an expert's study of the heat characteristics that change as tubing moves lower than 3/4" from the top of the slab:

  QUOTING except for [bracketed comments]
  "These results indicate that tube depth does have a nontrivial effect on the thermal performance of a heated floor slab. There is a performance penalty associated with leaving the tubing at the bottom of the slab vs. positioning it near mid-depth of the slab.

  The analysis performed was also based on steady state conditions. It doesn't predict the consequences of the longer response times associated with deeper tubing. These could be significant in situations where a building is recovering from a setback condition, or when heat flow from the slab needs to be reduced quickly to accommodate internal heat gains.

  Considering the tradeoffs, perhaps it is time we pay more attention to quality control procedures to ensure that performance is not compromised as concrete is poured over radiant tubing circuits.

  When future archeologists dig up the ruins of our buildings several centuries from now, will they ponder why we put the heating tubing at the bottom of the slab? Might they wonder if we didn't know any better? Would they conclude that some builders of the time were just too lazy to bother lifting the tubing? Thinking back to how ancient Romans used lead piping for water supplies, perhaps those archeologists will conclude that even after centuries of experience, we still had a hard time doing this pipe thing right.

  [FIGURE 4 OMITTED], [FIGURE 5 OMITTED]

  Table of heating water temperatures needed with radiant tubing at different depths in the concrete slab

  Table of Insulation Material Properties
  Average water temperatures needed for heat output of 15 and 30 Btuh/sq ft.
  Upward Heat Flux Requirement (Btuh/[ft.sup.2])	Tubing Depth 2" Below Slab Surface, Average Water Temp. Required degF.	Tubing Depth at Bottom of 4" Slab, Average Water Temp. Required degF.
  15 Btuh	95 degF	102 degF
  30 Btuh	120 degF	134 degF (1)

  NOTE: (1) regarding the "134 degF" in the bottom right of the above table: This is moving down just 2" deeper. We estimate maybe 168 degrees water temperature would be needed at 4" down and well over 200 deg heating water would be needed in tubing 6" down. In the slab in our construction project, the critical tubing, leaving the heating boiler, was placed more than 12" deep in poured concrete. Heating energy costs will increase consistent with the increase in heating water operating temperature requirements.

  John Siegenthaler, is a professional engineer specializing in radiant heat designs and heat transfer theory in buildings. Mr. Siegenthaler principal of Appropriate Design, a consulting engineering firm specializing in hydronic heating design. He is the author of Modern Hydronic Heating and Radiant Precision (available from the Radiant Panel Association (www.radiantpanelassociation.org, 800-660-7187).

  Siegenthaler explains in various articles that the rate at which a hydronic heating system can actually move "sensible heat" from the heating source (perhaps hot water in tubing in a radiant floor slab) into the occupied space (perhaps a room in a building over such a floor) can be calculated as q=(8.01 x D x c) x f x (deltaT). This formula is not as intimidating as it may seem.

• Even though our contractor said all of this theory was nonsense, q= the rate of heat flow in Btu/hr, D= density of the fluid (lb/cubic foot), c = the specific heat of the fluid (Btu/lb/degrees F), f= the fluid flow rate in the tubing (gpm), delta T= the temperature change of the heating fluid in deg F, and 8.01 is a units conversion factor.

• What is the R-value for earth, dirt, soil, backfill, or earth berms? found at INSULATION R-Values & Properties

Questions & Answers regarding this article

Questions & answers on radiant floor heating problems

Question: 2006 IECC: effectiveness of foundation perimeter insulation and insulation recommendations for radiant-heated floor slab designs

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

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

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

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

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

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

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

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

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

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

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

The risk in misinterpreting the Montana study conclusions above would be to apply them generally to radiant heat floor designs and that to improperly infer that complete under-radiant-heat-floor-slab insulation is not needed in cold climates. That study makes a general conclusion for all Montana buildings and by no means does the conclusion adequately address radiant in-slab heating system designs. The fallacious concept held by the contractor in our horror story was that "once you heat up the earth below your building it will start "giving back" heat to the building and you'll be just fine. His theory was nonsense, as both expert advice and actual field experience proved.

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

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

As the principal author of this material I relied largely on the concrete industry and the radiant flooring industry's radiant floor slab design specifications and advice [1] as they, above all, have a huge vested interest in their installations being successful. There is no doubt that in virtually every radiant-heat-floor-slab design we need continuous insulation under the slab and at slab perimeter, though the appropriate insulation amount might vary depending on the local climate. The folks who seem to disagree have been people like the bully contractor who himself admitted he had never read instructions, attended a class, nor asked for expert advice. As is often the case with small contractors in remote areas and without expertise, he was "winging it". Don't try mentioning "thermodymics" or "heat flow theory" to a bully.

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

But in the horrible installation we describe in these articles, the contractor not only provided incomplete and no perimeter slab insulation, he also buried the tubing so deep in the concrete that heat moved much more down into the cold earth than upwards into the occupied space. There was so much heat loss that we could not get the room temperature up even in cold but not bitter cold weather, and even though the same contractor had done a great job insulating the upper portions of the structure's roof and walls. (He was a framer/carpenter, and should not have attempted radiant slab installation nor tile work.) That's why we had to abandon the whole radiant floor installation.

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

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

Comment:

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

Reply:

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

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

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

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

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

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

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

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• Carson, Dunlop & Associates Ltd., 120 Carlton Street Suite 407, Toronto ON M5A 4K2. (416) 964-9415 1-800-268-7070 info@carsondunlop.com. Thanks to Alan Carson and Bob Dunlop, for permission to use illustrations from their publication, The Illustrated Home which illustrates construction details and building components. Carson Dunlop provides home inspection education including the ASHI-adopted Home Inspection Training Program (home study course), publications such as the Home Reference Book, report writing materials including the Horizon report writer, and home inspection services. Alan Carson is a past president of ASHI, the American Society of Home Inspectors.
• [1] Portland Cement Association: www.concretethinker.com/Papers.aspx?DocId=8 indicates that
  - tubing for radiant heat in a concrete slab is installed UP TO two inches below the surface of the slab
  - the slab is insulated from the ground at all sides to direct heat upwards to the living space [this is our preferred design for a cold northern climate]
  - "Radiant Heating with Concrete", Ingrid Mattson & Gary Fries, Concrete Technology Today, Portland Cement Association, Vol. 18, No. 1, April 1997
• [1] The Radiant Panel Association: www.radiantpanelassociation.org/i4a/pages/index.cfm?pageid=1 offers design guidelines at http://www.radiantpanelassociation.org/i4a/pages/index.cfm?pageid=115 including these insulation R-value and coverage details:
  Application#, Minimum R-Value, and Insulation Coverage
  The following insulation alternatives are given for Slab on Grade construction:
  Alternate #1 [(Ti-To)x0.125)=R-value, with coverage from perimeter to below frost line ["Ti-To" means we calculate the necessary R-value as (Ratio of indoor to outdoor temperature) x 0.125]
  Alternate #2 R-value=5, with coverage 4' horizontal or vertical at perimeter
  Alternate #3 R-value=5, with coverage under entire slab and slab edge [this is our preferred design for a cold northern climate]
  The Radiant Panel Association offers education and publications in radiant heat design. see radiantpanelassociation.org
• [2] Montana Slab Edge Insulation Analysis for 2006 IECC Adoption K. Gowri, Ph.D., March 2007, U.S. Department of Energy under Contract DE-AC05-76RL01830, web search 8/11/11, original source: U.S. DOE, http://www.energycodes.gov/publications/techassist/MTslab-edge_2006IECC.pdf
• Takagi radiant heat systems: Takagi offers pre-assembled radiant heating system installation packages including for do-it-yourself'ers, and including systems that combine radiant heat flooring with domestic hot water production using a gas-fired tankless water heater. See takagi.com for more information. "The T-KJr model (gas inputs up to 140,000 BTU per hour) is the smallest unit in the Takagi line-up. The T-KJr is perfect for light residential (i.e. small apartment units) and radiant heating applications." Also see Tankless Water Heaters.
• "Basic Hydronic underfloor - thermal storage 8 to 14 hours of control", this sketch, provided by OPTCO, is not a conventional radiant heating system design - and you'll see that the designer places the tubing too deep for efficient radiant heat delivery to the occupied space. However this design is intended for heat storage, such as in a solar heat storage system. See PASSIVE SOLAR DESIGN METHOD for more information.
• Thanks to James Darling, General Manager, Preferred Heating LLC, Eagle River, WI for discussing radiant heating installation design, problems, diagnosis (May 2010). As he is a professional in the field, we report below Mr. Darling's comments in full: [Also see RADIANT HEAT TEMPERATURES].

I read this article while researching a project I'm working on for a customer. I found it interesting and informative, but at the same time somewhat misleading.

The description of the project noted a few interesting assumptions. One was that the contractor had told the homeowner their heating bill would be $20/month. That amount seemed to get carried throughout the article like it was a benchmark to find the problems with the installation of the radiant system in the first place. I put forth that the belief, even in the beginning phases of the project, that the home when finished would heat for such a cheap amount was nothing more than wishful thinking on the part of the owner. Even with today's energy efficiency requirements for new construction it wouldn't take a genius to figure out that at the going rate for KW's and the "average" btu's/hr of heat loss at design conditions it would more than likely be a very very small house that would heat for $20/month in any place in the United States much beyond the central part of the country. Then if you factor in the air exchange requirements even for the tightest of homes, that amount is going to be even more unbelievable.

To the point of where the tubing should or shouldn't be placed within the thickness of the slab most of John's article is focused on response time. And the insulation values being used for the model are barely half of what is today's standards. Here in Wisconsin the code requires an R-10 beneath the slab and R-15 on the perimeter of new construction with radiant heat in the slab. Granted..... there is still going to be more loss to the earth with the tubing closer to the bottom of the slab and there is going to be a slower response with the tubing in that position. In residential applications and even most commercial ones the ability with radiant slab heat to get any real savings out of a setback routine is negligible. That is unless you have no floor covering, float the tubing to as close as possible to the surface and have a substantial heat loss. Even slabs with the tubing right near the top will "store" energy in them of sufficient quantities that the home will loose very little if any temperature over the average daytime setback due to outdoor radiant gain and warmer temperatures and the losses during a night time setback are maybe 2-3°F over the average night except at design temperatures.

Having been installing radiant heating in homes and commercial buildings since 1988 I would suspect the issues with the home in the article were from multiple issues. The large energy costs in dollars seemed to be mentioned and then not specifically addressed. The fact that the dual fuel price meter set wasn't installed and they were paying premium for any electricity they used was almost mentioned, but not factored in.

The worst thing you can do with any radiant system is to not have the correct temperature drop across each and every one of your loops in the slab. What the drop is across the boiler is fine when figuring out if the boiler is getting the correct flow across it..... but only then if it is a single stage non-modulating type. If the boiler itself sensed lack of load on it it would either stage down or modulate according to its design and the temperature rise across it would stay fairly steady independent on the flow through it. But, back to the flow through the individual loops. Say the circulator(s) weren't sized right and there was a 40°F drop across all or some of the tubing in the slab. Now you have a radiant panel that isn't performing to design and the space won't heat properly and the thermostat will call for heat continuously. What if the loops near the perimeter are the only ones working properly? Now your largest heat loss area of your slab is getting the majority of the heat being put out by the boiler. The room still isn't heating properly, but the boiler is running constantly trying to heat the room via say...... half the tubing/slab.

I was confused and couldn't figure out how a 4" thick slab with insulation installed under it ended up with tubing at depths of 10-12". Unless the floating slab had modified footings on the perimeter and the pictures or information the homeowner saw led them to believe that tubing was installed at the bottoms of these footings. Or did they see pictures that showed potable water lines buried under the slab and figured those were radiant heat tubing. Either way....... the picture on this part of the description was vague at best.

What the article does bring to light is that no one should be doing radiant heating that doesn't have a full full knowledge and the experience needed in design, installation and the ability to control all factors involved in the construction process. "Rules of Thumb" do not work well in this type of installation/system and the ability most times to go back and modify or fix things is extremely limited.

I was just called in on a 3 year old system that at first glance looks absolutely beautiful when you first look at the boiler room. $80,000.00 radiant heating job. The system hasn't worked since the day it was installed. The basement slab zones heat but the rooms are uncomfortable in spots. The upper 2 floors of radiant heating (hardwood floors with staple up) run away and overheat the spaces. Does any of the problems have to do with where the tubing was buried in the slab? No. Have any jobs I've worked on in over 25 years have anything to do with where the tubing is buried in the slab? No.

I am not saying your article is without merit. I'm just suggesting that of all the things that can go wrong when people don't pay attention to all of the details of radiant heat design and installation the placement of the tubing is pretty much on the bottom of the list in my experience.

Books & Articles on Building & Environmental Inspection, Testing, Diagnosis, & Repair

• Our recommended books about building & mechanical systems design, inspection, problem diagnosis, and repair, and about indoor environment and IAQ testing, diagnosis, and cleanup are at the InspectAPedia Bookstore. Also see our Book Reviews - InspectAPedia.

• Carbon Monoxide Gas Toxicity, exposure limits, poisoning symptoms, and inspecting buildings for CO hazards
• DUST CONTAMINATION FROM HVAC? An Investigation of Indoor Dust Debris Blamed on a Heating/Cooling System Reveals Carpet Dust
• Fuel Oil & Oil Heating Magazine, 3621 Hill Rd., Parsippany, NJ 07054, 973-331-9545
• Goodman Furnace High Temperature Plastic Vent HTPV safety recall US CPSC notice
• Home Heating System Should Be Checked [for proper venting and for CO Carbon Monoxide Hazards - DJF]
• Inspection Procedures for Oil-Fired Heating Systems Detailed step by step approaches for inspecting complex systems]
• Lennox Pulse Furnace Safety Inspection/Warranty Program: Carbon Monoxide Warning
• Oil Tanks - The Oil Storage Tank Information Website: Buried or Above Ground Oil Tank Inspection, Testing, Cleanup, Abandonment of Oil Tanks
• Oil Tanks Above Ground, UL Standards, guidance for home owners, buyers, and inspectors
• Plastic Heating Vent Pipe & Other Heating Safety Recall Notices
• Weil McLain Model GV Gas Boiler/gas valve CPSC recall/repair
• Domestic and Commercial Oil Burners, Charles H. Burkhardt, McGraw Hill Book Company, New York 3rd Ed 1969.
• National Fuel Gas Code (Z223.1) $16.00 and National Fuel Gas Code Handbook (Z223.2) $47.00 American Gas Association (A.G.A.), 1515 Wilson Boulevard, Arlington, VA 22209 also available from National Fire Protection Association, Batterymarch Park, Quincy, MA 02269. Fundamentals of Gas Appliance Venting and Ventilation, 1985, American Gas Association Laboratories, Engineering Services Department. American Gas Association, 1515 Wilson Boulevard, Arlington, VA 22209. Catalog #XHO585. Reprinted 1989.
• The Steam Book, 1984, Training and Education Department, Fluid Handling Division, ITT [probably out of print, possibly available from several home inspection supply companies] Fuel Oil and Oil Heat Magazine, October 1990, offers an update,
• Principles of Steam Heating, $13.25 includes postage. Fuel oil & Oil Heat Magazine, 389 Passaic Ave., Fairfield, NJ 07004.
• The Lost Art of Steam Heating, Dan Holohan, 516-579-3046 FAX
• Principles of Steam Heating, Dan Holohan, technical editor of Fuel Oil and Oil Heat magazine, 389 Passaic Ave., Fairfield, NJ 07004 ($12.+1.25 postage/handling).
• "Residential Steam Heating Systems", Instructional Technologies Institute, Inc., 145 "D" Grassy Plain St., Bethel, CT 06801 800/227-1663 [home inspection training material] 1987
• "Residential Hydronic (circulating hot water) Heating Systems", Instructional Technologies Institute, Inc., 145 "D" Grassy Plain St., Bethel, CT 06801 800/227-1663 [home inspection training material] 1987
• "Warm Air Heating Systems". Instructional Technologies Institute, Inc., 145 "D" Grassy Plain St., Bethel, CT 06801 800/227-1663 [home inspection training material] 1987
• Heating, Ventilating, and Air Conditioning Volume I, Heating Fundamentals,
• Boilers, Boiler Conversions, James E. Brumbaugh, ISBN 0-672-23389-4 (v. 1) Volume II, Oil, Gas, and Coal Burners, Controls, Ducts, Piping, Valves, James E. Brumbaugh, ISBN 0-672-23390-7 (v. 2) Volume III, Radiant Heating, Water Heaters, Ventilation, Air Conditioning, Heat Pumps, Air Cleaners, James E. Brumbaugh, ISBN 0-672-23383-5 (v. 3) or ISBN 0-672-23380-0 (set) Special Sales Director, Macmillan Publishing Co., 866 Third Ave., New York, NY 10022. Macmillan Publishing Co., NY
• Installation Guide for Residential Hydronic Heating Systems
• Installation Guide #200, The Hydronics Institute, 35 Russo Place, Berkeley Heights, NJ 07922
• The ABC's of Retention Head Oil Burners, National Association of Oil Heat Service Managers, TM 115, National Old Timers' Association of the Energy Industry, PO Box 168, Mineola, NY 11501. (Excellent tips on spotting problems on oil-fired heating equipment. Booklet.)
• Links to our list of additional information on heating system inspection, repair, maintenance
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