Frequently Asked Questions
1.How Is It Different From Conventional "Radiation"?
2.Why Are The Panels In the Ceiling?
3.Is this a Proven Technology?
4.What About Cold Feet... Warm Head?
5.How Does Radiant Cooling Work?
6.What About Condensation?
7.Is Radiant Cooling Appropriate for Residential Applications?

1. How Is It Different From Conventional "Radiation"?

Heat may be transferred by conduction (warmth transferred by objects that are touching each other), or convection (heat that is carried from a warmer to a cooler body by an intermediate fluid such as air) or by radiation. Most conventional tube and fin "radiators" are in fact, convectors. They heat a space by warming the air that passes by the heated fins. This air, now warmer than the surrounding air, rises due to buoyancy. This is known as free convection. Where the air is moved past the fins by a blower or fan it is called forced convection. In either case the idea is the same, to compensate for perimeter heat losses by circulating warm air within the space.

By contrast, between 75 and 90 per cent of the energy from a radiant panel is propagated by thermal radiation. Thermal radiation is an electromagnetic radiation propagated due to a difference in temperature. Therefore, the heat is transferred to all bodies "seen" by the panel that are at a lower absolute temperature than the panel itself.

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2. Why Are The Panels In the Ceiling?

As It is often asked "Why put the panels in the ceiling? Heat rises, doesn't it?" The answer is no, heat does not rise. Warm air rises. The panels are most effective in the ceiling because from there, they are best able to "see" the objects in the room. Lights are placed in the ceiling for maximum effect. This is because from above, it is not obstructed by room objects and its light can be most effectively cast on the surroundings. Light is a type of electromagnetic radiation which has a shorter wave length than thermal radiation. Therefore, as with a light, the optimal location for a radiant panel is the ceiling.

Another reason for placing the panels in the ceiling is so that higher surface temperatures can be used. The rate of radiant heat transfer from the panel is governed by the Stefan-Boltzmann Law, q = S Ts4 , which demonstrates that the radiative heat transfer from a body increases dramatically as its temperature is increased. Radiant ceiling panels are typically operated at 79-85C (175-185F). On the other hand, were the radiant panels mounted on the walls or near the floor, safety code dictates that the panels could not be operated at high temperatures. Thus by placing the panels in the ceiling, the panels can be made to produce more output per area by simply increasing the water temperature.

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3. Is this a Proven Technology?

Radiant panel technology is not new to the HVAC (Heating, Ventilating and Air Conditioning) industry. It is a mature, proven technology that has been in use in Europe for over 100 years. Over the last 35 years, the Canadian and United States HVAC industry has employed radiant panels in many commercial and institutional settings. In fact, since the much publicized outbreak of Legionnaire's disease (spread by contaminated central air systems), radiant heating systems have been the choice for U.S. Veteran's Administration Hospitals and, in the last 10 years, over 80% of the new hospitals constructed in the provinces of Western Canada have employed radiant panel ceiling heating systems.

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4. What About Cold Feet... Warm Head?

Cold feet? Hardly. Tests have shown that the floors in a room heated by a radiant panel ceiling exhibit temperatures 1 to 2C (3 to 4F) above the ambient air temperature and actually provide a source of re-radiated heat themselves. In fact, where down drafts from cold walls or glazing present design challenges with respect to occupant comfort, radiant panels provide a solution. The ceiling panels warm the wall or window surfaces by direct transfer of radiant energy, significantly increasing the temperature of each. It has been found that even under extreme cold wall conditions (27F, -2.7C), the air velocities are non-draft in nature (less than 50 fpm or 0.25 m/s).

Typically, radiant panel surface temperatures are 170 to 185F (about 82C). This can cause the directional mean radiant temperature (DMRT) to be 16 to 20C (30 to 40F) above the ambient air temperature. By contrast, on a sunny day, the DMRT outside is 30 to 40C (50 to 70F) higher than the ambient air temperature. So, if you enjoy being out in the sunshine, you will enjoy the indoor warmth of a radiant panel ceiling.

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5. How Does Radiant Cooling Work?

Radiant cooling follows the same principles as radiant heating. The heat transfer occurs between the space and the panels through a temperature differential. However, unlike radiant heating, the colder ceiling absorbs the thermal energy radiating from people and their surroundings. The major difference between cooled ceilings and air cooling is the heat transport mechanism. Air cooling uses convection only, whereas cooled ceilings use a combination of radiation and convection. This amount of radiative heat transfer can be as high as fifty five per cent, while convection accounts for the remainder. With cold ceilings, the radiative heat transfer occurs through a net emission of electromagnetic waves from the warm occupants and their surroundings to the cool ceiling. On the other hand, convection first cools the room air due to contact with the cold ceiling, creating convection currents within the space which transfers the heat from its source to the ceiling where it is absorbed.

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6. What About Condensation?

Condensation on the surface of the panels is not a problem with radiant cooling. Since condensation of water occurs when the dew point temperature is reached, proper water temperature control will help avoid any condensation. To prevent the formation of condensation, a sensor monitoring the dew point temperature of the room is used in conjunction with a controller which modulates the inlet water temperature accordingly. Therefore, if a risk of condensation is present, the water temperature is raised or the water flow is shut off. However, since the lower the panel's inlet temperature is, the more work the panels do, the inlet temperature should be determined to be as close as possible to the room's dew point temperature. Consequently, the cooling capacity of a radiant cooling system is generally limited by the minimum allowable temperature of the inlet water relative to the dew point temperature of the room air.

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7. Is Radiant Cooling Appropriate for Residential Applications?

Unfortunately radiant cooling is not a good choice for residential houses. Radiant panels require the air in the space to be dehumidified and controlled very carefully. In a residential home, the ability to open windows or leave doors ajar, would allow humid outside air to enter the space. This could raise the dew point temperature to a point where to avoid condensation, the radiant panels would have to operate at relatively high temperatures. Since the panel's cooling capacity is dependent on the temperature differential between the panels and the room temperature, having to raise the panel temperature to avoid condensation would severely hamper the panel's cooling ability.

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