A central unit provides heated and cooled air at a constant temperature in a dual-duct system. Two ducts go to a mixing box at each zone. There, hot and cold air are mixed by a set of dampers to deliver the air temperature needed to satisfy the load conditions of the space served. There are several ways to reduce the energy use by such a system:
1. Lower the temperature of the air-handler hot deck and raise the temperature of the cold deck.
2. Reduce the airflow to all the terminal boxes to the lowest volume possible.
3. The cold ducts can be dampered off, and the cooling system shut down, when there is no cooling needed by any zone; likewise with the hot deck, when no heating loads are present. Operating the dual duct system in this way, as a single duct system, delivers half as much air to each space—with the expected energy savings at the fan, due both to reduced volume and reduced static pressure— but also results in fewer air changes and less outside air being delivered to each occupied zone. This might be a problem in some installations.
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Excerpt from Energy Conservation Projects.