UFAD Constant Volume
UFAD Constant Volume
This system is similar to a Variable-Temperature, Constant-Volume (VTCV) system, except that:
1. the supply air is delivered via an underfloor plenum,
2. a return-air bypass arrangement is assumed in order to provide sufficient dehumidification without the need for reheat, and
3. space heating is supplied by a baseboard radiator or convector.
Because this system delivers air via an underfloor plenum, be sure to include a raised floor/underfloor plenum height when drawing the rooms that will be assigned to this system. At design cooling conditions, the supply air is delivered from floor diffusers at a warmer supply-air temperature (default = 65°F) and at a lower velocity than is typical for overhead delivery methods. Because the supply air is introduced from the pressurized floor plenum, a warmer supply-air temperature is necessary to avoid an uncomfortable temperature difference between ankle and head heights. As the room sensible cooling load decreases, the capacity of the main cooling coil will be reduced, resulting in a warmer supply-air temperature.
The main heating coil is combined with the preheat coil in the main air handler and can be used to provide morning warmup. If the zone thermostat calls for heat, the main heating coil will condition the supply air to the design room heating thermostat setpoint and the baseboard radiator or convector is activated to satisfy the space heating load. Since the main cooling and heating coils are located at the zone level, it is important to assign rooms that have similar thermal load characteristics to the same zone. For any underfloor air distribution system, the floor diffusers should be as close to the supply shaft as possible to minimize the heat gain as the cool supply air travels through the underfloor plenum.
In order to provide a certain amount of dehumidification without the need for reheat, a return air bypass arrangement is assumed, e.g., outside air is mixed with recirculated return air, then cooled to saturation at 53-54˚F in order to remove an adequate amount of moisture from the air stream. A portion of the return air is introduced downstream of the coil and mixed with the saturated mixture to achieve the higher discharge air temperature.