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With Radiant Floor Heating Systems, Thermal Mass is not the Answer

Posted by Terry Alsberg, Guest Columnist

Jan 9, 2017 9:22:55 AM

High mass radiant is an outdated technology. The secret to performance, comfort and energy savings is low mass, high conductivity.

For decades, the term "thermal mass" has been used when describing radiant floor heating systems. Used in this context, it refers to the ability of a high mass radiant floor assembly to store heat.

This concept went mainstream in the 1960s and 70s when "passive solar homes" were growing in popularity. A dark colored, high mass slab made of Portland cement (or gypsum concrete), would sit under south-facing windows and soak up the heat from the sun all day long. When the sun went down, the slab would emit its stored heat to the habitable space for the next several hours.

While this method worked in some locations in the house, it was not an adequate heating method for an entire home. So tubing was installed and covered with concrete in other parts of the home. These homes were therefore a combination of radiant floor heating and passive solar. Because this combined system was common during this era, the term "thermal mass" and "radiant heat" are often thought to be synonymous, when in fact, they are not.

However, this same thermal mass which is so essential to a passive solar home is also the cause of the most common complaint with  radiant heat – it's slow. In every home, the heat load changes – up or down – can change quickly making it impossible for high mass systems to respond. It is common for these homes to be too cold in the morning and too hot in the afternoon (overshoot and undershoot). If the house has been unoccupied for a stretch of time, homeowners may have to wait over 24 hours for the system to adequately heat the home.

While the history of passive solar with radiant heat is interesting, given a blank sheet of paper, no one would design a heat system that acted primarily as a storage device. Any heating system, whether forced are or radiant floor heating, has the sole purpose of transferring heat into a specific space. Homes lose heat because heat flows from the warm interior to the cold exterior. A low mass radiant heating system with more conductivity allows heat to be produced at the same rate at which it is leaving the house, allowing the interior to maintain a constant level of comfort.

Conductivity, as we know, is the property of a material that allows heat to flow through it. Concrete is inherently a mediocre conductor, making it a poor choice for conducting heat. But aluminum is 240x more conductive than concrete. Because the amount of heat that must be supplied by a radiant panel is constantly changing due to weather changes, occupancy or just personal preference, the ideal heating system is able to quickly or decrease output in order to maintain optimum comfort.

With a high-mass system, the conditioned space is inconsistent and slow to respond to the needs of the occupant. On the other hand, highly-conductive, low mass radiant floor systems provide greater comfort through fast response.

Remember – Conductivity, not mass, is king.

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Keeping Glass Rooms Comfortable with Hydronic Heating and Cooling

Posted by Christina B. Farnsworth

Aug 5, 2015 9:28:06 AM

More natural light in commercial buildings improves worker productivity and decreases absenteeism and turnover. However, all that glass can be a challenge when it comes to keeping building comfortable.

In Seattle at Chihuly Garden and Glass, a low-temperature hydronic system has been combined with radiant floor heating to maintain a comfortable environment.

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How to Warm Your Home During a Power Outage

Posted by RESNET

Dec 30, 2014 1:15:00 PM

When the power goes out this winter, don’t panic. With a little planning, you can stay warm and keep pipes from freezing.

Fireplace or Wood Stove
A wood fireplace or wood stove is a great way of heating your home when you have no electricity, many people maximize their use of fireplaces during the colder months while minimizing the use of heaters and furnaces. This saves energy and money while providing effective heating. For an open hearth fireplace, however, make sure you install glass fireplace doors (Schott makes some) Otherwise that hearth will waste energy whenever the fireplace is not in use. If you go with a woodstove, get a cleaner burning model with a fresh air intake.

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Heating/Cooling: A Home's Biggest Energy Users

Posted by Green Builder Staff

Sep 5, 2014 4:37:00 PM

Know the Lingo

  • Annual Fuel Utilization Efficiency (AFUE): The percentage of a fuel’s potential energy that a furnace or boiler converts to usable heat. Government standards that take effect in 2015 require AFUE levels of 82% for gas boilers, 83% for oil boilers, 80% for gas furnaces and 82% for oil furnaces.
  • Air Handler: In a forced-air heating or cooling system, the air handler unit moves heated or cooled air through the home’s ductwork.
  • British Thermal Unit (BTU): The unit of measurement for heat, whether it’s the heat given off by burning fuel or extracted from a home for cooling. Technically, one BTU is the energy required to raise one pound of water one degree Fahrenheit.
  • Combustion Chamber: The part of a furnace or boiler where the fuel is burned.
  • Compressor: That part of the air conditioner or heat pump that compresses and pumps refrigerant.
  • Condenser Coil: The part of an air conditioner or heat pump that releases heat from the surrounding air in cooling mode and collects it in heating mode.
  • Distribution System: The network of air ducts or hot water pipes that delivers heat from a furnace, boiler or heat pump to the home’s rooms.
  • Evaporator Coil: The part of an air conditioner or heat pump that exchanges heat with the air in the home.
  • Heat Exchanger: Located in the furnace or boiler, it transfers heat from the combustion chamber to the air or water in the heat distribution system.
  • Heating Seasonal Performance Factor (HSPF): The heating efficiency of a heat pump. It’s a ratio of the heat it generates over the heating season, in BTUs, to the watt-hours of electricity it consumes. Heat pumps manufactured after 2006 have to have an HSPF of at least 7.7, but the best units have ratings as high as 10.
  • Seasonal Energy Efficiency Ratio (SEER): The cooling efficiency of an air conditioner or heat pump. It’s the ratio of cooling output to electricity used. The minimum SEER requirement for units manufactured beginning in 2006 is 13.
  • Zoning: A method of partitioning a home’s hydronic or forced-air distribution system into independently controlled comfort zones.



AT THE HEART OF MOST HOMES' HEATING SYSTEMS is a furnace, a boiler or a heat pump. A furnace burns fossil fuel to heat air that’s forced by a blower fan through a series of ducts to the living spaces; a boiler heats water that’s then pumped to a hydronic, or water-based, distribution system. Most heat pumps run on electricity. They don’t create heat, but rather extract it from the air or the ground. Heat pumps are available for use with forced-air and hydronic distribution systems. If you want to minimize your fuel bill, an Energy Star rating is a minimum standard for these appliances.

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Heating Energy Assessment Tool by AREVS

Posted by RESNET

Jul 16, 2014 12:25:19 PM

Understanding home heating energy performance just got easier. The new Heating Energy Assessment Tool (HEAT) from AREVS is a RESNET Approved, easy-to-use web-based application that tells home energy professionals, homeowners, and renters whether or not a home is in need of energy retrofits or upgrades. Using patented algorithms that are normalized for house size and geographical location, and information from its utility bill, HEAT provides a heating energy audit for a home in less than 5 minutes. A simple A+ through F grade range gives instant understanding of home heating performance.

Advantages for Professionals:

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Storm Proofing; Come Inside and Out of the Weather

Posted by Jay Egg

Jun 4, 2014 11:47:30 AM

With geothermal heating and cooling systems, you can weather the storm and rest easy. The equipment is all inside, reducing premature wear and tear.  There is no equipment to encumber your landscaping or to interfere with children at play, and noise pollution is eliminated.  

On March 13, 1993, Floridians were awakened by what sounded like a passenger jet engine outside their homes.  It turned out to be 70+MPH winds from the fury of Florida’s no-name “Storm of the Century.”

Property damage was significant, and many were left without air conditioning when their outdoor condensers were destroyed by wind and water.  Clients with geothermal heat pumps fared better, sustaining no damage due to the "all-indoor" nature of a geothermal heating and cooling system.

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Duct Diligence

Posted by GBM Research

May 30, 2014 7:42:00 AM

The majority of heating systems are forced air, and distribution problems often lie in duct design and integration. What new technologies and research can make these air distribution systems more efficient?

EFFICIENT AIR DISTRIBUTION is critical in the delivery of high performance homes. Innovations—simple and cost effective to implement—have been developed and tested throughout the country. But the application of these strategies continues to be a challenge—especially ones that address less-than-optimal situations—such as flex duct and splitter box air distribution systems and ducts in unconditioned spaces. More focus needs to be put on dealing with these scenarios.

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Geothermal Heating and Cooling Systems; Great for the Environment!

Posted by Jay Egg

Apr 21, 2014 12:21:00 PM

Geothermal Heating and Cooling Systems; Great for the Environment!

An earth coupled HVAC system is packed with "Earth Friendly" benefits, especially when compared with any other types of air-conditioning and heating systems. As heating and cooling systems go, a ground sourced system comes closest to the way Mother Nature would heat and cool your home if given the opportunity.


That's because with geothermal, your house becomes coupled with the Earth beneath, sharing that temperature with your environmentally friendly geothermal heat pump, similar to the way the roots of trees share nutrients with their branches and leaves. Take a look at some of the environmental benefits you can enjoy with geothermal system:

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Get Your Spring-Time Energy Savings with Geothermal Pool Heating

Posted by Jay Egg

Apr 2, 2014 12:53:00 PM

SPRING IS HERE, and the cooling season is quickly approaching. Pools that have been out of commission for the winter are likely to stay that way unless we turn the heat on.  That can get expensive.


Stop for a moment and think of how many blow dryers, computers, cooking appliances, lights and people there are in your home that add to the cooling load.  These are all a potential source of energy that can provide domestic hot water and be used to heat spas and pools. With standard air source heat pumps or air conditioners, the heat generated inside your home typically goes out the return air ductwork and ultimately is exhausted through outside air exchange.

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After 5 Years of Stimulus, Home Energy Efficiency Emerges as Job One

Posted by Jay Egg

Mar 3, 2014 10:57:37 AM

Reviewing the five-year report to Congress on the economic impact of the American Recovery and Reinvestment Act, and resulting news reports and sound bites, I began to see through the rhetoric and partisanship.

Regardless of opinions, the fact is that there have been millions of jobs created in the green energy sector, and green building has arrived as the norm rather than the exception. But that is a broad brushstroke; and as the sun begins to set on the US stimulus package (2009-2016), the important thing is to look at what we have gained in the US, and where we're headed.

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Baby It’s Cold Outside

Posted by Christina B. Farnsworth

Feb 13, 2014 2:39:00 PM

Few consumers heat their homes directly with coal, though historic homes may still contain the coal bins and coal chutes needed for coal delivery and powering boilers.

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Geothermal Heating and Cooling Goes Mainstream in US

Posted by Jay Egg

Feb 12, 2014 1:25:00 PM

New reports show Geothermal to be, "One of the most mature and stable platforms of renewable energy usage…" High consumer satisfaction, green building growth, and programs to reduce first cost are rocketing geothermal heating and cooling into the US mainstream.


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Green Building Projected to Double: How about Geothermal?

Posted by Jay Egg

Feb 3, 2014 10:43:00 AM

Green movement number two is upon us, and for more reasons than cold weather and increasing natural gas prices.

Geothermal HVAC systems are the unexploited frontier of the green movement.  Geothermal HVAC systems don’t get much press.  You can’t see them (because equipment is all inside), you can’t hear them; the classic “out of sight–out of mind” scenario.  But can geothermal heating systems handle really cold weather? What about retrofits and high installation costs? Let’s take a lesson from our neighbors to the north; Canadians know cold weather-

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