The rate of heat emission from a ceiling panel constructed as shown is about 0.71 Btu/hr./ft.2 for each 1°F the average water temperature exceeds room air temperature. Thus, if the ceiling tubing operated with an average water temperature of 110°F in a room with 70°F air temperature, each square foot of ceiling would release about 28.4 Btu/hr./ft.2 [0.71 × (110°F - 70°F)]. Although not as high as the radiant wall due to lower convection, this performance is still very acceptable for use with most renewable heat sources.
Generously sized panel radiators can also provide good low-temperature performance. The suggested guideline for sizing panels to deliver the maximum heat output is to use a supply water temperature no higher than 120°F.
Manufacturers provide output ratings for their panel radiators in tables or graphs. Most list heat output for high water temperatures such as 180°F. Correction factors are provided to determine heat output at lower water temperatures. As an approximation, a panel radiator operating with an average water temperature of 110°F in a room maintained at 68°F provides about 27% of the heat output it would yield at 180°F. Larger panels increase surface area to compensate for lower operating temperatures.
None of these hydronic heat systems will deliver as expected without a well-planned distribution system. Although there are several piping layouts that may serve your purpose, the simplest, easiest to install, and most flexible approach is a “home run” distribution system, which starts with a manifold station—the same kind as used for radiant panel heating.
Two lengths of 1/2-inch PEX or PEX-AL-PEX tubing provide the supply and return from the manifold station to each heat emitter. The flexible tubing allows routing through framing cavities much like an electrical cable. This is particularly helpful in a retrofit situation, where the use of rigid tubing would be difficult.
A variable-speed, pressure-regulated circulator pump provides flow through the home run distribution system. Available from Grundfos, Taco, Wilo, and Bell & Gossett, these circulators can operate over a wide range of speeds, and in different control modes, depending on the application. For a home run distribution system, the circulator is set to “constant differential pressure” mode. Its responsibility is to maintain a constant (installer-set) pressure differential between its inlet and outlet ports. It does this by varying speed in response to changes in the distribution system’s hydraulic resistance.
At full speed, the motors in these “intelligent” circulators operate on approximately 50% of the electric power required by standard hydronic circulators of equal capacity. This characteristic, in combination with speed control, delivers annual electrical energy savings of 60% to 80% relative to standard hydronic circulators.
A thermostatic radiator valve (TRV) is used on each panel radiator. Each TRV constantly monitors the room’s air temperature. As that temperature drops slightly below the TRV’s setting, the valve slowly increases water flow through that panel. This causes a very slight drop in the distribution system’s hydraulic resistance, a change that’s quickly detected by the pressure-regulated circulator which responds by increasing pump speed to restore the set differential pressure. This yields a slight flow increase through the panel that needs it, and virtually no flow changes in the other panels—a “cruise control” for system flow.