Air handlers with dual ducts, or multi-zone units, have adjacent heating and cooling decks that produce hot and cold air to the dual duct system, before being delivered to the mixing box and then to the zone by a single duct. This system spends a lot of energy on reheat in the cooling season; sometimes—such as at peak demand for the building—all but the zone with the highest cooling load requires the air to be reheated. Even this zone can need reheat, depending on the temperature settings of the coils. The deck temperatures are typically fixed at 95 and 55°F. These temperatures can be automatically reset so that no reheat is required for the zone calling for the most cooling. In the heating season, the zone with the highest heating requirement is selected, and the hot deck temperature is adjusted accordingly. In either case, the temperature difference between the hot and cold deck surfaces is minimized and
reheating, and “recooling” are reduced to the absolute minimum.
These same controls can be incorporated into a single-duct variable air volume system in which the VAV boxes have reheat coils. Frequently, the cooling coils are adjusted to provide 55°F air at all times, so many zones may require reheating. By resetting the temperature of the discharge air until the zone with the largest cooling load closes its reheat coil valve, the base amount of reheat can be minimized. This controls scheme is called discharge air temperature reset.
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Excerpt from Energy Conservation Projects.