How radiant heat works, importance of radiant angle (C) Daniel Friedman

Radiant Heat Temperatures
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What is the optimal temperature for radiant heat piping
Radiant heat temperature settings under tile, hardwood, carpet, laminate floor choices
Radiant heat temperature settings at the boiler
Radiant heat temperature settings in the tubing
Maximum temperature for radiant-heated floor surfaces
Energy efficiencies of radiant floor heating
Heat loss rates into earth below radiant heated floor slabs
Operating temperature comparisons for types of radiant heating
Questions & answers about radiant heat system temperatures

This article discusses the maximum, minimum, & recommended operating temperatures for radiant heated floor systems, including typical temperatures used in different types of radiant-heated floors: tile, wood, laminate, carpeting, etc.

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What are the Normal Operating Temperatures for Radiant Heat Systems?

Our page top photo shows the output temperature of an electric boiler used to heat tubing in a radiant-heated tile floor set over concrete in a Minnesota home. Unfortunately that system never worked satisfactorily, as we explain at RADIANT HEAT Floor Mistakes to Avoid.

FLOOR, WOOD RADIANT HEAT - guide to installing wood floors over radiant heat systems
RADIANT HEAT - usage guide, Radiant Heat: how it works and when to use it - strategies for using radiant heat - this article is provided just below.
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
SLAB INSULATION, PASSIVE SOLAR - slab insulation & vapor barrier placement in heated floor slabs
WOOD FLOOR DAMAGE - from radiant tubing run too hot.

Question: what is "normal" and what is "too hot" for radiant heat water circulating in the piping?

I just inspected a complex combine forced air (hydronic) heating system combined with a several circulating radiant floor heating components. My question is what is normal and what is “too hot” for the water circulating through the piping distribution under ceramic tile floors installed over wood framed floor systems.

When I took the Watts® Regulator course they emphasized that temperatures should not exceed 130 degrees. During my inspection the water leaving the boiler was 167.8 degrees (going into the heating distribution system for hydronic radiant, hydronic forced and also potable (yeah, I know that part is a problem!). - Ron Wells (ASHI#515) Wells Inspection Services, Inc.

Reply: It depends ..

The high temperature effects of too-hot radiant heat under wood flooring, and the effect on finished wood flooring are cited at WOOD FLOOR DAMAGE, and some radiant heat design specs are at RADIANT HEAT Floor Mistakes to Avoid. Here we add details about different operating temperatures for radiant heating systems.

Typical Radiant Heat Floor System Operating Temperatures

Typical radiant floor systems operate at 85 - 125 degF water temperature entering the tubing, and put the floor surface temperature about 5 degrees above the room thermostat set temperature.

Similarly other sources such as the Oregon state energy conservation department gives the typical operating temperature range for radiant heating systems at 85-140 degF (30-60C) though we agree that 140 deg.F. is a bit higher than suggested by other sources.

A radiant heat system manufacturer, Radiant Floor Company, opines that 120 to 135 deg F is "ideal" and most sources we have found discuss typical radiant heat operating temperatures in the 115-135 degree range.

Where PEX tubing is used for radiant-heat under-floor tubing, 125 to 130 degF is typical operating temperature used for water entering the tubing.

Most radiant floor heating systems we've examined run at around 115F water temperature entering the tubing of the radiant section, and you'll see by the temperature gauge at the top of this article that 115 degF was the factory default set temperature on the control of an electric radiant heat boiler we installed. The control on that boiler's circuit board is shown just above.

But as we explain below, the optimum operating temperature for radiant heat floor systems will vary depending on the insulating or heat-conducting properties of the flooring material itself.

Typical Temperatures at the Upper Surface of Radiant-Heated Floors

Unless you have a black crayon and an infrared thermometer it may be a bit more difficult to measure the finished-floor surface temperature (what your bare feet would feel when walking on the floor) than it is to just look at a temperature gauge on the hot water heating pipes or heating boiler themselves, but looking at the finished floor surface is one important temperature to note.

The finished floor surface temperature is what the building occupants feel when walking on the floor, affects the rate of heat radiating into the air above the floor, and if too high, can also become a safety concern.

According to CMHC (Canadian Mortgage and Housing Corporation) the finished floor temperature should never exceed 85 F (29 C).

Our photo (above) shows our friend Steve and his dog explaining the radiant heat tubing layout in the new garage floor slab of a Minneapolis MN home.

Watch out: if you saw 167.8 F at the boiler, that sounds too hot for normal circumstances on a radiant heating system. Running a radiant heat system hotter than necessary can have a range of harmful effects including:

Shorter heating boiler life
Shorter radiant heat tubing life
Oxygen diffusion problems [discussed below]
Uncomfortably hot or even dangerously hot floor surfaces
Unnecessarily high heating system operation costs

It may be useful to ask why someone has set the temperature of this radiant heating system so high. It could be

Simply a mistake
A control not functioning properly
An improperly installed or designed radiant heat system that is not delivering adequate heat into the occupied space
A misunderstanding of the controls or a misread of the temperature at the wrong location. See "Where to Check the Temperature ..." at the end of this article.

Typical Radiant Heat Temperatures Under Ceramic Tile

Under ceramic tile a higher temperature might be allowed without risking floor damage (we don't have wood shrinkage for example) and/or might have been set by a homeowner who likes a really warm bathroom floor. But since ceramic tile is a *better* heat conductor than wood flooring, if anything, one would expect the temperature setting for that floor to be lower, not higher than otherwise.

Radiant under ceramic tile or vinyl tile, lower end of the temp;

Typical Radiant Heat Temperatures Under Finished Wood Flooring

Under wood floors nailed right to the subfloor and tubing below, radiant heat temperatures are typically at the middle of our 85-135 degF. temperature range, or about 105-115 degF.

The radiant heated wood floor shown in our photo (left) was installed in a New York home.

Attempting to "drive out" tubing odors the installer pushed the floor temperature up a bit too high, taking the blame for gaps that appeared in the flooring as a result discussed at at WOOD FLOOR DAMAGE.

Radiant Heat Temperatures Under Laminate Flooring

Under laminate type finish flooring, keep the temperature under 82 degF or risk floor damage (per http://www.shawfloors.com/)

Radiant Heat Floor Temperatures Under Carpeting

Under carpet over subfloor, radiant heat will need to operate at the higher end of its temperature range to overcome the insulating effect of the carpeting and carpet padding.

Oxygen Diffusion in Too-Hot Radiant Heated Floors

Radiant Floor Company, a radiant heating system manufacturer, raises another interesting point, indirectly: at temperatures over 140 degF can cause an oxygen diffusion problem and require special "oxygen barrier" tubing in some cases (depending on the boiler design) - otherwise system life may be reduced or the boiler damaged.

Where to Check the Temperature of A Radiant Heated Floor System - Mixed Radiant & Hydronic Baseboard or Radiator Heating Systems

Watch out: often there are mixing and flow controls on the radiant system that limit the actual temperature in the radiant tubing, typically by only introducing new hot water into the loop when temperature requires it - otherwise recirculating most of the loop water without returning it to the boiler.

You will particularly see this if the boiler is serving a mixed-design system, including some heating baseboards and other radiant floor heated sections.

That's because we want the hotter temperatures in the baseboards (hotter is more efficient heat transfer), and cooler temps in the radiant flooring.

So: was this a mixed design system with some baseboards and some radiant? and were there mixing controls on the radiant loop section ?

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FLOOR, WOOD RADIANT HEAT - guide to installing wood floors over radiant heat systems
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
SLAB INSULATION, PASSIVE SOLAR - slab insulation & vapor barrier placement in heated floor slabs
WOOD FLOOR DAMAGE - from radiant tubing run too hot

  FLOOR, LAMINATE PLASTIC
  FLOOR, RESILIENT VINYL or CORK
  FLOOR, WOOD ENGINEERED & LAMINATED
  FLOOR, WOOD SOLID STRIP, PLANK
  FLOOR, WOOD ENGINEERED, LAMINATE, INSTALL
  FLOOR, WOOD FINISHES
  FLOOR, WOOD INSTALLATION GUIDE
  FLOOR, WOOD MOISTURE
  FLOOR, WOOD RADIANT HEAT

Thanks to reader Ron Wells, Wells Inspection Services, Inc., Chapel Hill, NC 27515, Tel: 919-968-4981, Cell: 919-302-2652 License Number: 75 - Mr. Wells is a professional home inspector and a member of ASHI, the American Society of Home Inspectors. He can be reached at Ron@weelsinspections.com
CMHC (Canadian Mortgage and Housing Corporation), web search 10/11/2010, original source: http://www.cmhc-schl.gc.ca/en/co/renoho/refash/refash_010.cfm
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.
Oregon state energy conservation department gives typical radiant heat floor operating temperature at 85-140 degF (30-60C). Web search 10/11/2010, original source: http://www.oregon.gov/ENERGY/CONS/RES/tax/Radiant.shtml - but it appears that the actual author of this info is: Radiant Panel Association - Radiant Professionals Alliance 8512 Oswego Road Suite 180 Baldwinsville, New York 13027 Phone (315) 303-4735 Fax (315) 303-5559 http://www.radiantpanelassociation.org/ Here's their page on hydronic floors: http://www.radiantpanelassociation.org/i4a/pages/index.cfm?pageid=99 where you'll see some vague "source temperature required" graphs that give relative but not absolute temperatures.
Passive Solar Design Handbook Volume I, the Passive Solar Handbook Introduction to Passive Solar Concepts, in a version used by the U.S. Air Force - online version available at this link and from the USAF also at wbdg.org/ccb/AF/AFH/pshbk_v1.pdf
Passive Solar Design Handbook Volume II, the Passive Solar Handbook Comprehensive Planning Guide, in a version used by the U.S. Air Force - online version available at this link and from the USAF also at wbdg.org/ccb/AF/AFH/pshbk_v2.pdf [This is a large PDF file that can take a while to load]
Passive Solar Handbook Volume III, the Passive Solar Handbook Programming Guide, in a version used by the U.S. Air Force - online version available at this link and from the USAF also at wbdg.org/ccb/AF/AFH/pshbk_v3.pdf
The Passive Solar Design and Construction Handbook, Steven Winter Associates (Author), Michael J. Crosbie (Editor), Wiley & Sons, ISBN 978-047118382 or 0471183083 is available at Amazon.com and via the The Passive Solar Design and Construction Handbook, Steven Winter Associates (Author), Michael J. Crosbie (Editor), Wiley & Sons, ISBN 978-047118382 or 0471183083 is available at Amazon.com and via the InspectAPedia Bookstore
"Passive Solar Home Design", U.S. Department of Energy, describes using a home's windows, walls, and floors to collect and store solar energy for winter heating and also rejecting solar heat in warm weather.
Radiant Floor Company, Barton, Vermont USA 1-866-Warm-Toes (1-866 927-6863) info@radiantcompany.com web search 10/11/2010 original source: http://www.radiantcompany.com/faq/
"Solar Water Heaters", U.S. Department of Energy article on solar domestic water heaters to generate domestic hot water in buildings, explains how solar water heaters work. Solar heat for swimming pools is also discussed.
"Heat Exchangers for Solar Water Heating Systems", U.S. DOE describes the types of solar water heater heat exchange methods between the sun and the building's hot water supply
"Heat-Transfer Fluids for Solar Water Heating Systems", U.S. DOE, describes the types of fluids selected to transfer heat between the solar collector and the hot water in storage tanks in a building. These include air, water, water with glycol antifreeze mixtures (needed when using solar hot water systems in freezing climates), hydrocarbon oils, and refrigerants or silicones for heat transfer.
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
The Radiant Panel Association: Radiant Panel Association - Radiant Professionals Alliance 8512 Oswego Road Suite 180 Baldwinsville, New York 13027 Phone (315) 303-4735 Fax (315) 303-5559 http://www.radiantpanelassociation.org/
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
Here's their page on hydronic floors: http://www.radiantpanelassociation.org/i4a/pages/index.cfm?pageid=99 where you'll see some vague "source temperature required" graphs that give relative but not absolute temperatures.
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.
The US DOE page on Radiant heating : web search 10/11/2010, original source: http://www.energysavers.gov/your_home
/space_heating_cooling/index.cfm/mytopic=12590 is, unfortunately short on details.
"Solar Water Heating System Maintenance and Repair", U.S. DOE
"Solar Water Heating System Freeze Protection", U.S. DOE,using antifreeze mixture in solar water heaters (or other freeze-resistant heat transfer fluids), as well as piping to permit draining the solar collector and piping system.
"Scaling and Corrosion in Solar Water Heating Systems", U.S. DOE
www.energysavers.gov/your_home/water_heating/index.cfm/mytopic=12850 is the base U.S. DOE website for these articles
"Active Solar Heating Systems", U.S. Department of Energy, including
"Radiant Heating Systems" U.S. DOE
"Absorption Heat Pumps & Coolers", U.S. DOE
"Solar Air Heating" U.S. DOE also referred to as "Ventilation Preheating" in which solar systems use air for absorbing and transferring solar energy or heat to a building
"Solar Liquid Heating" U.S. DOE, systems using liquid (typically water) in flat plate solar collectors to collect solar energy in the form of heat for transfer into a building for space heating or hot water heating. The term "solar liquid" is used for accuracy, rather than "solar water" because the water may contain an antifreeze or other chemicals.

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What are the Normal Operating Temperatures for Radiant Heat Systems?

Question: what is "normal" and what is "too hot" for radiant heat water circulating in the piping?

Reply: It depends ..

Typical Radiant Heat Floor System Operating Temperatures

Typical Temperatures at the Upper Surface of Radiant-Heated Floors

Typical Radiant Heat Temperatures Under Ceramic Tile

Typical Radiant Heat Temperatures Under Finished Wood Flooring

Radiant Heat Temperatures Under Laminate Flooring

Radiant Heat Floor Temperatures Under Carpeting

Oxygen Diffusion in Too-Hot Radiant Heated Floors

Where to Check the Temperature of A Radiant Heated Floor System - Mixed Radiant & Hydronic Baseboard or Radiator Heating Systems

Questions & Answers regarding this article

Ask a Question or Search InspectAPedia

Technical Reviewers & References

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

Radiant Heat Temperatures
InspectAPedia^® -