Radiantly Heated Ceilings

Ceiling installations are usually preferred over floors systems. Ceiling constructions have less thermal capacity than floors, and therefore respond faster. They can also be heated to higher temperatures. The system is concealed except for thermostats and balancing valves.

The wiring for electric resistance heating can be installed in the ceiling. It is acceptable for ceilings to get hotter than walls or floors, since they are not usually touched. However, downward convection is poor and the hot air stays just below the ceiling. When the ceiling is at its warmest, the room may feel uncomfortable. Overall efficiency suffers, and cooler air may stratify at floor level. Tables and desks block heat from above, resulting in cold feet and legs.

Hidden wires in radiant ceiling systems can be punctured during renovations or repairs. Even though a plaster ceiling may have to be torn down for system repairs, the expense is less than tearing up a concrete floor. Some systems use snap-together metal components for easy maintenance.

Preassembled electric radiant heating panels (Fig. 24-8) are also available. They can be installed in a modular suspended ceiling system, or surface mounted to heat specific areas. Radiant heating panels can be installed at the edges of a space to provide additional heat with variable air volume systems. Applications include office building entryways and enclosed walkways. They are useful in hospital nurseries, and in hydrotherapy, burn, and trauma areas. Residential uses include bathrooms, above full height windows, and in other cold spots. Factory silicone sealed panels are available for use in high-moisture areas. Some panels can be silk-screened to provide an architectural blend with acoustical tiles. Custom colors are also available. Radiant heating panels operate at 66°C to 77°C (150°F-170°F).

Research has found that heating a home with ceiling-mounted radiant panels produced energy savings of 33 percent compared to a heat pump and 52 percent compared to baseboard heaters. The research project, completed in May 1994, was sponsored by the U.S. DOE, the National Association of Home Builders (NAHB) Research Center, and Solid State Heating Corporation, Inc. (SSHC), the maker of the panels used in the tests. These panels differ from other types of radiant heaters in several ways. They mount to the ceiling surface, not behind or inside gypsum board. Their light-

Figure 24-8 Surface-mounted radiant ceiling panel.

weight construction has little thermal mass that must come up to temperature, and the textured surface adheres directly to the heating element. These characteristics make the panels able to reach operating temperature in only three to five minutes. Because the panels respond quickly, people can turn the heat on and off as they would the lights. The panels operate quietly and without air movement.

Most of the heat from radiant heating panels flows directly beneath the panel and falls off gradually with greater distance, dropping by about 5°F over the first 6 feet. This may seem like a disadvantage, but some occupants like to find a spot that is relatively cooler or warmer within the room. Proper placement of panels must be coordinated with ceiling fans, sprinkler heads, and other obstructions, which can be a problem when installing them in an existing building.

Manufactured gypsum board heating panels use an electrical heating element in 16-mm (f-in.) fire-rated gypsum wallboard. They are 122 cm (4 ft) wide and 183, 244, 305, or 366 cm (6, 8, 10, or 12 ft) long. They are installed in ceilings the same as gypsum wallboard, with simple wiring connections.

Radiant panels avoid some of the problems inherent with forced-air systems, such as heat loss from ducts, air leakage, energy use by furnace blowers, and inability to respond to local zone conditions. Installation costs for energy-efficient radiant panels are considerably less than the cost for a forced-air system, but radiant panels can't provide cooling, as a forced-air system can.

Embedded radiant heating systems went out of favor in the 1970s due to the expense of the large quantity of piping and ductwork, and high electrical energy costs. Malfunctions were difficult and expensive to correct. Systems were slow to react to changing room thermal demands, due to the thermal inertia of concrete slabs, so they were slow to warm up after being set back for the night.

Radiant devices are also used to melt snow on driveways, walks, and airport runways. They circulate an antifreeze solution or use electric cables. Newer products use flexible plastic piping that operates continually at around 49°C (120°F) or higher, and have a 30-year expected lifetime.

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