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Air-Water Central Systems

General

All-Air Central Systems supply the latent and sensible cooling capacity with cold air ducted to the conditioned space. Heating is accomplished by the same airstream, either in the central system or in each zone controlled by a thermostat. The simplest all-air central system in a single duct, single zone system.

Some of the all-air central system advantages are:

* Maintenance is performed in unoccupied areas, since major equipment is centrally located.
* No drain piping or power wiring or compressors are located in occupied areas.
* Systems can include options such as air-side economizer, heat recovery, winter humidification, and large outside air volumes, where required.
* Good choice where close zone temperature and humidity control is required.
* Simple seasonal changeover
* Simultaneous cooling and heating in various zones.

Some of the all-air central system's disadvantages are:

* Duct space may add to the building height and vertical air shafts detract from usable space.
* Large systems may be difficult to air balance.
* May encounter perimeter zone cold spots in cold climates where only air is used to heat.
* Close coordination is needed between designers and installers to assure accessibility to terminal units.

All-air systems fall into two general categories - constant volume and variable air volume (VAV). Constant air volume systems accomplish cooling and heating by varying the supply air temperature and keeping the air volume constant.

VAV systems keep the air temperature constant and vary the air supply volume. VAV systems are easy to control, are energy efficient, and allow fairly good room control. A drawback is possible poor ventilation under low load conditions, and humidity control is difficult under widely varying latent loads.

There are a number of VAV variations:

* Simple VAV applies to cooling only with no requirement for simultaneous heating and cooling in different zones, such as the interior zone of an office building. Air volume can be varied by fan volume control, fan bypass, or diverting excess air into the return air ceiling plenum.
* VAV Reheat or Dual Duct sequences reheat or blending in each zone after throttling the zone air supply. It can be used for both perimeter and interior zones.
* VAV with independent perimeter system typically supplies VAV to the interior zones and supplies ventilation air to the perimeter in conjunction with a constant volume perimeter system. This perimeter system is indoor/outdoor temperature scheduled to offset the skin loss. In some buildings a hydronic or electric perimeter system provides heating only to offset the winter transmission losses; the VAV system handles the cooling load in all zones year-round.
* VAV with Constant Zone Volume uses fan-powered terminals to maintain minimum or constant air volume to the zone while the supply air to the zone boxes is varied. This system is useful in zones with a large internal load variation, such as conference rooms, and ensures air circulation in occupied spaces during reduced load periods. It is sometimes combined with terminal reheat.
* VAV with Economizer Cycle reduces chiller power requirements by using outside air when its enthalpy is lower than that of the return air and dumps the return air. While this design increases first cost and requires larger outside air intake and exhaust ducts, it reduces energy requirements, except in humid areas where such favorable conditions rarely occur, such as the southeastern U.S.

Links to Related Topics

Ventilation
Indoor Air Quality
Filtration
Outside Air Control
Controls
Central Plant System
Central (or Built-Up) System
All-Air Central Systems
All-Air Central Reheat Systems
All-Air Central Dual Duct
All-Air Central Multizone
All-Air Rooftop
All Water Central Systems
Two Pipe System
Three Pipe System
Four Pipe System