indoor environmental quality
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Warming Up - Banish cold feet with radiant heating
Copyright (c) 2009, Robert Bean, All rights reserved, originally published in K+BB, 12.01.2007

Few architectural systems serve the needs of designers and their clients as well as radiant under-floor heating. It is one of those rare elements of construction that is invisible to the eye yet greatly appreciated for how it makes people feel. Radiant is best described as a system that thermally conditions the body, as opposed to heating the building.

An ancient form of heating that can be traced back more than 6,000 years, it was used by early civilizations in the Far East and discussed among the monks as they made their pilgrimages around the globe. During the times of the Greeks and Romans, history records the European evolution of radiant heating known as "hypocaust": thermal labyrinths or heated channels underneath floors in bathhouses. And forms of radiant floor heating were used by General Sheridan to heat hospitals during the latter part of the Civil War. But it was in 19th-century France and later in England (1907) where heated water was first circulated through embedded tubes to heat and cool the occupants of buildings.

Energy Efficiency

Although both electric and hydronic, or water-based, radiant systems are equally capable of conditioning the occupants and the space, the former, by its very nature, is married exclusively to the power grid. How that power is generated (coal, gas, oil or hydro) and distributed determines its environmental impact. As noted by LEED engineer, Geoff McDonell, pe, "The average efficiency of the energy delivery system from a thermal plant is about 10 percent. This means that of the 100 units of coal, natural gas or oil burned to make electricity, only about 10 units of that energy actually gets to your plug in the wall on a good day."

Consequently, "green" designers wishing to use electric radiant should find out if the project is located in a hydropower-supplied region, which is ideal. Keep in mind, however, fuel and power are commodities whose prices change based on supply and demand. And electrical radiant can't be swapped out for other systems: Once electric radiant, always electric radiant. Typically, electric systems are favored where electricity may be more economical, more convenient or more suitable for the application, such as small retrofits or where spot heating may be required or hydronics is impractical.

Hydronic systems offer great versatility in that any utility (power and fuel) source can be used to heat the water, including electrical boilers; the choice is only limited by economics, availability and reliability. There are, in fact, dual-utility boilers or multi-type systems available that take advantage of the lowest-cost source.

Unlike traditional water heating systems, radiant hydronic floors typically use very low water temperatures, which improve the efficiency of condensing boilers and heat pumps. For example, a standard boiler operating at 180 degrees Fahrenheit may reach up to 84-percent combustion efficiency, but a condensing boiler operating at 100 degrees Fahrenheit will achieve 94 percent. This means that, of the gas purchased, only 6 percent of the energy is not captured by the condensing boiler. In the case of water-to-water heat pumps, the performance can be enhanced by as much as 25 percent. That's why heat pumps make sense for radiant heating and cooling applications in hydropower-supplied areas.

In addition to improving the combustion and electrical efficiency of heating and cooling systems, radiant systems can further enhance energy efficiency when installed (in high-performance homes) under masonry-type floor materials such as stone, tile and slate or stained concrete. This combination can move the combustion efficiency of condensing boilers into the 97-percent range. With such high efficiencies, nitrogen, carbon and particulate emissions are drastically reduced to a point that is difficult, if not impossible, to duplicate with other types of heating systems.

Going Pro

All radiant-based HVAC systems should be designed by professionals who are familiar with standards for comfort, ventilation, lighting, sound, health and safety. They should also have a knowledge of the building sciences. The reasons are many. For example, in a home with high-intensity lighting or a high-powered home theater system, both of which can actually create a cooling load instead of a heating load, adding radiant heating could lead to overheating—particularly if the thermostat is not in the same space as the thermal electrical load.

If your client prefers heavyweight carpet, the radiant system will need to operate at a higher fluid temperature, which reduces the combustion efficiency of the boiler or heat pump. This can occur in retrofits where floor coverings are changed without recognizing their comparative impacts on the radiant system. The effect will show up in the drop-in comfort and an increase in utility bills. Competent designers can inoculate against these unforeseen challenges by specifying more tubing, which improves heat transfer at lower temperatures. Keep in mind that radiant tubing and electrical conduit are the least accessible and lowest-cost components in a system, which is why their quantity and quality should never be compromised.

One of the more frequent misunderstandings is the claim that radiant heating causes hardwood floors to crack. Fortunately, wood science explains why shrinking and swelling occurs. In addition, it's important to point out that 100 percent of all hardwood floor cracking complaints occurred in homes that were heated exclusively with hot air and did not have radiant-heated floors to blame. The causes of cracking have more to do with changes in humidity, wood species, milling, conditioning and site acclimation. All are building science principles dealt with by the professional.

Many DIYers want to use domestic water heaters as a heat source. Some authorities permit this so long as the heater is used for both domestic and space heating. However, water heaters used for radiant space heating operate in the temperature range where bacteria, such as Legionella, develop. To prevent this, the water heater must be run continuously in the range of 140 degrees Fahrenheit or higher, and the radiant system must be cycled periodically, regardless of the need for heating. This operating temperature then forces the use of scald protection valves on the domestic side and mixing valves on the heating side. As is often the case, trying to make the systems more affordable and simpler actually results in more complications and hazards. Simply installing a heat exchanger between the water heater and the radiant system or just using a boiler designed to operate at low temperatures can solve this.

Skilled radiant designers are more than just "mechanics." In addition to protecting the health, comfort and safety of a home's occupants, they influence utility usage, and thus homeowner finances, and enhance the longevity, experience and appearances of floor coverings. To ensure project fluidity, radiant technicians should be part of the design team at the very outset of a project along with the client, architect, engineer and interior designer.

Air Quality & Comfort

An important concern in both kitchens and baths is ventilation and humidity control. In better buildings, the heating and cooling of occupants is treated separately from ventilation and conditioning of the air. Why? Because indoor-comfort quality is directly related to the exchange of radiant heating, which can be regulated by a radiant system. Meanwhile, air is very good at regulating indoor air quality.

In fact, the professional designer uses two different design standards when developing these human-based mechanical systems. Using a separate air system designated for ventilation, (de)humidification and air filtration promotes a healthier environment for occupants and better conditions for building materials, such as antique woods, used in furniture and musical instruments, or fabrics and paintings. Since these dedicated air systems do not have to heat and cool the occupant, they are quieter and smaller. By taking up less architectural space, they leave more room for additional comforts and conveniences, like heated towel warmers. Other wonderful applications include heated surfaces in the walls and ceilings of showers or steam baths, especially those rooms defined in part by the exterior envelope.

Radiant heating is an "invisible" solution that serves the needs of the designer by freeing up space for other equally appreciated architectural features. At the same time, radiant heating enhances the client's comfort and enjoyment of the indoor environment, while reducing fuel and power costs. To ensure thermal success with your next client, be sure to deal only with "blue chip" systems and controls producers, as well as professional installers—whether the radiant system is built with PEX tubing (hydronic) or electrical cable systems.

—Robert Bean, ret, is a registered engineering technologist in building construction, as well as director for, a not-for-profit educational resource site on indoor environmental quality. He is currently hosting a series of webinars on radiant heating topics for design professionals on behalf of Uponor, Inc.

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