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 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.
