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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. © Copyright 2008 Daniel Friedman, All Rights Reserved. Information Accuracy & Bias Pledge is at below-left. Use the links at page left to navigate this document or to go to Other Website Topics. Green links at left show where you are in our document & website.

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. Our contractor for a small cabin in the North combined being opinionated and a bully with an awful ignorance of how to construct a properly insulated radiant floor slab. Not only did we have to battle with our bully to put insulation under the entire slab (he thought that Mother Earth would be warming our home from ground heat (which is below 40 degF in winter there)). We 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, our bully-contractor also pushed the radiant heat tubing so deep into the concrete (ranging from 6" deep to nearly 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. Don't permit your contractor to make the (many) mistakes ours 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
• 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.
• 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 who rejects standards for good workmanship and proper radiant heat floor design, find someone else to do the work.

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 our 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 flow 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 run $300. to $440 a month which is 10 times what the contractor originally estimated - doubling this 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.

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
• 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.

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 this 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.

• 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]
• 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 Coveratge
  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]

Technical Reviewers

Particular thanks are due to experts and also consumers who read these articles and suggest corrections, changes, and additions to the material. Content suggestions, technical corrections and content critique are invited for any of the content at our website.

• Daniel Friedman - principal author/editor of the InspectAPedia ^© Website
• The Radiant Panel Association offers education and publications in radiant heat design. see radiantpanelassociation.org
• Critique, contributions wanted: Contact Us to suggest text changes and additions and, if you wish, to receive online listing and credit for that contribution.

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More Information on Building Diagnostic Inspections and Repairs

How to Calculate Heat Loss in a Building
Priorities for building energy efficiency
Radiant Heat Floor Mistakes to Avoid
Design Temperature for Buildings
Definition of "degree day"
Table of Insulating Materials

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