Summary
This chapter has introduced the various ways zoning can be achieved with all-air air-conditioning systems. They are all based on individually varying the air flow and/or temperature supplied to each zone.
The Reheat System
Reheat is the simplest system, known for both its reliability and the down side, its high energy wastage. Two induction variations were introduced: one that also provides some night time heating; and the other that accommodates very low supply-air temperatures.
Variable Air Volume, VAV System
More energy efficient than reheat, VAV is a very flexible system with many virtues. When there is a low load, however, it does offer challenges for main- taining adequate ventilation air and good room air distribution.
The Bypass System
A variation on the VAV system, the bypass system, is suitable for providing good control in smaller systems, and for constant flow over a direct-expansion cooling coil. Designers must be cautious to ensure that bypassed air goes straight back to the air conditioning unit, but it is generally a simple system to design.
The Dual-Duct System
The system provides full airflow when the system is on, but, like the reheat system, suffers from the energy penalty of simultaneous heating and cooling. A very attractive feature of the dual-duct system is that there are no reheat coils near the zones, so the problems of leaking hot water coils is avoided.
The Multizone System
A system thermodynamically similar to the dual-duct system, the multizone system features a different layout. The multizone system is not as energy effi- cient as the VAV system, and requires a separate duct to each zone. However, the multizone system has the advantage of requiring no maintenance outside the mechanical room, except for the zone temperature-sensors and associated cable.
Three-deck Multizone System
The more modern introduction of the third, neutral duct to the multizone system, avoids the conflict of concurrent heating and cooling.
Dual-Duct, Variable Air Volume System
A modification of the dual-duct system, this system uses variable volume dual-duct boxes to provide the thermal efficiency of the VAV system, while maintaining higher air flows, and thus better room air circulation when heating is required.
Dual Path Outside Air System
This system could be used to reduce the problem with excess moisture in the air that arises in warm/hot, humid climates.