UFAD VAV Fan Assisted Reheat
UFAD VAV with Fan Assisted Reheat
This system is similar to a VAV Reheat (VRH) 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 an underfloor terminal that uses a small fan (Secondary Fan) to draw air from the floor through a heating coil.
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.
If the zone thermostat calls for heat, the Secondary Fan activates and draw air from the floor plenum through the terminal heating coil. The static pressure and energy rate for this terminal fan are entered as the Secondary Fan on the Create Systems – Fan Overrides screen. To delete this terminal fan, select “Shutoff VAV” from the VAV Minimum Type drop-down list (Create Rooms – Airflows screen). 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.
This is an illustration of a fan terminal discharging into an insulated discharge duct feeding several floor outlets that condition the perimeter. Perimeter space heating may be incorporated in the form of reheat coils within these terminals or may utilize a separate fin tube coil located in the discharge plenum. As this terminal has no provision for the induction of room air, it will likely incorporate a variable speed motor to adjust the airflow delivery in response to space thermostat requirements.