We all know how nice it feels to be next to warm things in the winter, an oven, a radiator, the cat. Imagine how wonderful it would be to feel that warmth radiating from our floors, walls or our ceilings! Radiant energy transfer is caused by a warm surface giving up its heat to a cooler surface. Our human comfort relies just as much on radiant heat transfer as it does on air temperature, yet the majority of heating and air-conditioning professionals think only in terms of air temperature.
Skin surface temperature, about 85°F, is generally warmer than the surrounding surfaces. This makes us a radiant panel. Stand by a large picture window in midwinter and you will feel the heat leaving your body. If the rate at which we radiate heat is correct, we feel comfortable. When the temperature difference between our body and the surrounding cool surfaces becomes too great, we have to put on a sweater to slow down the rate at which we are radiating. When the sun beats down on us through the window, we receive heat instead, and off comes the sweater. Our normal state is to lose heat at a constant and regulated rate. We are also designed to lose heat in other ways as well. Air coming in contact with our skin conducts away heat. Our skin is moist and moving air also causes evaporative cooling. A truly comfortable environment is one designed to draw heat away from our bodies at precisely the correct rate.
This type of system heats water instead of heating air, the way most traditional systems do. Not only is it inherently comfortable and efficient, but the fact that it requires water temperatures as low as 90 degrees makes it a good match with lower-temperature heat sources like heat pumps and solar.
The warm water is then carried to surfaces in rooms that can best store and slowly give off the heat to you while you are in there, reading Gone With The Wind. In newly constructed buildings, this surface is usually the large concrete floor, a great material to hold and slowly radiate heat, much like the warm high school basketball courts after a sunny summer day. Other surfaces that can also be used include the room's walls, ceiling, and radiators, using much the same principles as the old cast iron ones in our grandparents homes.