12.5 TEMPERATURE CONTROL AND ZONING

Please refer to Chapter 6, "Controls, Operation, and Maintenance," for a more detailed discussion of control issues. There are several approaches to address zones with significantly different thermal loads, including:

• plenum partitioning with ducted VAV devices supplying air to each zone;

• plenum partitioning with fan-powered terminal devices supplying air to each zone;

• thermostatically controlled VAV diffusers, which may be used in both partitioned and open plenums;

• local fan-driven supply outlets, which may be used in both partitioned and open plenums;

• open plenums with mixing boxes and ducted outlets.

Partitioning and any other obstructions in the underfloor plenum should be kept to the minimum necessary to optimize system performance and efficiency, as this helps to maintain the plenum for its intended purpose to serve as a highly flexible and accessible service plenum.

Although not a requirement, some designers recommend limiting the size of underfloor zones (partitioned or otherwise) that are served by a single ducted primary air inlet from the air handler (see below). This ensures the system's ability to avoid unacceptable variations in supply air temperature (due to heat gain from or loss to the concrete slab and raised floor structure).

In some system designs, using multiple medium- or small-sized (floor-by-floor) AHUs can minimize or totally eliminate ductwork and improve zone control when AHU capacities correspond to the specific requirements of each plenum zone.