UFAD Parallel Fan Powered VAV
UFAD Parallel Fan Powered VAV
Cool air (from the pressurized supply plenum) is modulated by dampers within the diffuser terminals (on the inlet and discharge side) according to the space cooling demand, while the fan terminal remains off. Upon a further drop in space temperature, the dampers close and the integral fan and heating coil is energized. The diffusers on the inlet side of the fan become return inlets while those on the discharge supply a constant volume of reheated air to the space. Room air conditions are slightly stratified, but mixed conditions exist to approximately 6 foot level during design cooling and heating operation, negating most displacement advantages.
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.
This perimeter system option consists of essentially an overhead VAV system placed under the floor in the raised-floor plenum. In general, this approach does not take advantage of the plenum as a low-pressure air distribution pathway, and the large amount of equipment and ducts placed in the plenum severely limit the flexible use of the plenum space. This system option is usually employed with conventional 55°F (13°C) supply air temperature and can be necessary if envelope loads are high, particularly solar loads. In the case of high envelope loads, the 63-65°F (17-18°C) supply air temperature typical of UFAD systems may be too warm to effectively remove the loads. This underfloor conventional VAV system using 55°F (13°C) air or colder can deal with high loads. The system efficiency and cost of this option is comparable to standard OH systems, though taken together with the cost of the raised floor it can be an expensive choice.
