Handbook of air conditioning and refrigeration / Shan K

21.56 CHAPTER TWENTY-ONE
● During the unoccupied period in winter, the AHU or PU can be shut off while the fan-powered
boxes provide the necessary heating.
● Fan-powered boxes installed inside the ceiling plenum must meet local safety code requirements.
21.6 COMPARISON BETWEEN VARIOUS VAV SYSTEMS
REFERENCES
VAV systems are used in applications where space load is varied in order to maintain a required indoor
environment and to save fan energy. Single-zone VAV systems are mainly used for single-zone
conditioned spaces such as arenas, indoor stadiums, assembly halls, and many factories.
VAV reheat systems using reheating VAV boxes are simple and effective for perimeter heating in
winter and with VAV boxes provide cooling only in the interior zone in a building. Simultaneous
cooling and heating may occur in reheat systems at low zone cooling loads. The VAV reheat system
becomes energy-inefficient for locations where the winter heating load linear density exceeds 120
Btu/h�ft (115 W/m).
Perimeter heating VAV systems are used for buildings that need heating for their perimeter
zones in locations with cold and long winters.
Dual-fan dual-duct VAV systems provide satisfactory indoor environmental control for multizone
high-rise buildings and health care facilities with flexible operating characteristics. If the shutoff
air leakage in the mixing VAV box can be minimized, it is often an appropriate choice for more
demanding applications.
Fan-powered VAV systems provide low cooling load compensation and winter heating from a
two-stage sequence operation: intake warm plenum air and winter heating. The small centrifugal
fan in the fan-powered box also provides the mixing of cold primary air with the plenum air. Fanpowered
VAV systems are widely used in buildings where cold air distribution is needed, such as
office buildings and schools.
Alley, R. L., Selecting and Sizing Outside and Return Air Dampers for VAV Economizer Systems, ASHRAE
Transactions, 1988, Part I, pp. 1457–1466.
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ASHRAE, ASHRAE Handbook 1999, HVAC Applications, Atlanta, GA, 1999.
ASHRAE/IESNA Standard 90.1-1999, Energy Standard for Buildings Except New Low-Rise Residential Buildings,
ASHRAE Inc., Atlanta, GA, 1999.
Avery, G., The Myth of Pressure-Independent VAV Terminals, ASHRAE Journal, no. 8, 1989, pp. 28–30.
Avery, G., The Instability of VAV Systems, Heating/Piping/Air Conditioning, February 1992, pp. 47–50.
Avery, G., Selecting and Controlling Economizer Dampers, HPAC, no. 8, 1996, pp. 73–78.
Brothers, P. W., and Warren, M. L., Fan Energy Use in Variable Air Volume Systems, ASHRAE Transactions,
1986, Part II B, pp. 19–29.
Cappellin, T. E., VAV Systems—What Makes Them Succeed? What Makes Them Fail? ASHRAE Transactions,
1997, Part II, pp. 814–822.
Chen, S., and Demster, S., Variable Air Volume Systems for Environmental Quality, McGraw-Hill, New York,
1996.
Coggan, D., Mixed Air Control with DDC, Heating/Piping/Air Conditioning, no. 5, 1986, pp. 113–115.
Cox, R. L., Retrofit for a College Campus, 1996 ASHRAE Technology Award Case Study, ASHRAE Journal,
no. 11 1996, pp. 59–64.