Multizone Systems
The multizone system is thermodynamically the same as the dual-duct system. They both involve mixing varying proportions of a hot-air stream with a cold-air stream to obtain the required supply temperature for that zone. In the dual-duct system, the mixing occurs close to the zone, in the dual-duct box. In the multizone system, as shown in Figure 7.11, the mixing occurs at the main air-handling unit. The basic multizone system has the fan blowing the mixed air over a heating coil and a cooling coil in parallel configuration. As you know, in the dual-duct system, the resulting hot and cold air is ducted through the building to dual-duct mixing boxes. In contrast, in the multizone system, the heating and cooling air-flows are mixed in the air-handling unit at the coils using pairs of dampers.
The hot deck coil is arranged above the cold deck coil and they are sectioned off into zones; just two sections are shown in the figure. Each section has a two-section damper that opens to the cold deck as it closes to the hot deck.
Each damper pair is driven by an actuator pushing the crank at the end of the damper shaft. The mixed air from each section is then ducted to a zone.
As in the dual-duct system, a certain amount of energy inefficiency occurs because the air is being both heated and cooled at the same time.