Interior Duct System
Perhaps the most innovative design feature in the PV home is its interior mounted duct system. In conventional houses (including our standard home) the ducts are located in the hot attic (Figure 1). Some houses even have the air handler located in this space (Figure 2). Previous research at FSEC has shown that not only does the attic sometimes reach 130oF in Florida's summers, but heat transfer to the duct system can rob the air conditioner of upto a third of its cooling capacity during the hottest hours. The reason is simple: R-5 flex ducts contains the coldest air in the home (~60oF) while being exposed to the hottest temperatures; the area of ducts in a typical home is a third of the floor area (Q= UA dT). At 130oF, and a 2,400 square foot home, this equates to nearly a ton of air conditioning lost before it ever reaches the registers!
To avoid this problem, and to allow down-sizing of the PVRES air conditioning system, we designed the duct system so that it fits inside the conditioned space. Whatever heat is gained by the duct system is the heat that is removed from the conditioned space. To hide the ducts, false dropped ceilings, lower cathedral sections and chases were used throughout the interior (Figure 3). To avoid problems with leakage, the duct system was carefully sealed with mastic and tested. Finally, we oversized the duct system, so that that air flow resistance would be minimized. This not only provides critically important air flow across the evaporator , it also reduces air handler fan power and improves system efficiency and reduces noise.
In Figure 4, Project Builder Rick Strawbridge looks on as personnel from Ward's air conditioning seal the interior air handler plenum in the PVRES home.