Radiant heating, or panel heating as it is sometimes referred to, consists of a warm pipe coil embedded in the floor, ceiling, or walls. The most common arrangement is to circulate warm water through the pipe under the floor. Some installations with warm-air ducts, steam pipes, or electric-heating elements have been installed.
Warm-air ducts for radiant heating are not very common. A modified system normally called the perimeter warm-air heating system circulates the warm air around the perimeter of the structure before discharging the air into the premises via grilles.
Pipe coils embedded in concrete floor slabs or plaster ceilings and walls should not be threaded. Ferrous pipe should be welded, while joints in nonferrous metal pipe should be soldered. Return bends should be made with a pipe bender instead of fittings to avoid joints. All piping should be subjected to a hydrostatic test of at least 3 times the working pressure, with a minimum of 150 psig. Inasmuch as repairs are costly after construction is completed, it is advisable to adhere to the above recommendations.
Construction details for ceiling-embedded coils are shown in Figs. 13.19 to 13.22. Floor-embedded coil construction is shown in Fig. 13.23. Wall-panel coils may be installed as in ceiling panels.
Electrically heated panels are usually prefabricated and should be installed in accordance with manufacturers recommendations and local electrical codes.
The piping and circuiting of a hot-water radiant heating system are similar to hot-water heating systems with radiators or convectors, except that cooler water is used. However, a 20F water-temperature drop is usually assumed. Therefore, charts used for the design of hot-water piping systems may be used for radiant heating, too. (See ASHRAE Handbook Fundamentals.) In radiant heating,
the warmer supply water may be routed to the perimeter or exposed walls and the cooler return water brought to the interior zones.
Panel and Room Temperatures. Heat from the embedded pipes is transmitted to the panel, which in turn supplies heat to the room by two methods: (1) convection and (2) radiation. The amount of heat supplied by convection depends on the temperature difference between the panel and the air. The amount of heat supplied by radiation depends on the difference between the fourth powers of the absolute temperatures of panel and occupants. Thus, as panel temperature is increased, persons in the room receive a greater percentage of heat by radiation than by convection.
Inasmuch as high panel temperatures are uncomfortable, it is advisable to keep floor panel temperatures about 85F or lower and ceiling panel temperatures 100F or lower. The percentage of radiant heat supplied by a panel at 85F is about 56% and by one at 100F about 70%.
Most advocates of panel heating claim that a lower than the usual design inside temperature may be maintained because of the large radiant surface comforting the individual; i.e., a dwelling normally maintained at 70F may be kept at an air temperature of about 65F. The low air temperature makes possible a reduction in heat losses through walls, glass, etc., and thus cuts down the heating load. However, during periods when the heating controller is satisfied and the water circulation stops, the radiant-heat source diminishes, creating an uncomfortable condition due to the below-normal room air temperature. It is thus considered good practice to design the system for standard room temperatures (Table 13.10) and the heating plant for the total capacity required.
Design of Panel Heating. Panel output, Btu per hour per square foot, should be estimated to determine panel-heating area required. Panel capacity is determined by pipe spacing, water temperature, area of exposed walls and windows, infiltration air, insulation value of structural and architectural material between coil and occupied space, and insulation value of structural material preventing heat loss from the reverse side and edge of the panel. It is best to leave design of panel heating to a specialist.
(ASHRAE Handbook Fundamentals, American Society of Heating, Refrigerating, and Air-Conditioning Engineers, 1791 Tully Circle, N.E., Atlanta, GA 30329.)