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Zoning Systems for Residential and Light Commercial Applications

Originally published: 09.01.11 by Tom Edwards


When to use complete electronic bypass variable air volume or constant-volume duct systems.

Zoning systems for these types of applications come in two primary flavors — complete electronic bypass variable air volume systems (BPVAV); and constant-volume duct systems, in which 20% or less of the system airflow is subject to complete shutoff without bypass.

BPVAV zoning systems consist of a shutoff damper and thermostat for each zone with a bypass damper located in the duct connecting the main return and supply of the rooftop or air handler/furnace. As the zone dampers close, pressure building in the supply duct will cause the bypass damper to open and redirect this air back into the unit. In the cooling mode, this cooler temperature is sensed at the unit, and the compressor shuts down, saving energy. The same happens during the winter months. In the heating mode, higher return air temperatures are sensed, and the heating source shuts down, saving energy.

Rooftop and split system manufacturers have variable-speed airside systems available that may preclude the need for the bypass loop. Every manufacturer has recommendations on their variable speed fan units for zoning applications.

Some applications, such as a retail space with only

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a back office or two, may require 80% (or more) of the capacity to be serviced by one thermostat and 20% to be serviced by a thermostat in the separate office(s). In these cases, a single zone damper and thermostat will meet the comfort needs without the need for bypass control in a constant-volume system.


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Zoning Systems for Residential and Light Commercial Applications

Tom Edwards explains the difference between electronic bypass variable air volume (BPVAV) and constant-volume duct systems—and when to use each application.
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Zone control in the light commercial and residential market has become the norm rather than the exception. It is imperative that every HVAC contractor and system-design professional understands the application. It is a key strategy for success on both new construction as well as improving overall comfort on existing jobs. The steps identified should help identify additional thoughts to be included in a successful zone-control strategy.
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