A double-duct air-handling system provides a hot air supply duct in parallel with a cold air supply duct. These ducts extend throughout the building. At each zone of control a "mixing box" is provided, with hot and cold duct dampers to mix the air streams to satisfy the air-conditioning load of the zone.

 This system was very popular in the 1950s and 1960s when energy was cheap. The advent of the "energy crisis" in 1973 made the design unpopular, since the reheat principle and high fan work requirements consumed much extra energy.

A good way to salvage an old DD system is to retrofit it to a variable air volume (VAV) arrangement.

The two-fan double-duct system, incorporating VAV, is an excellent method of obtaining the control advantages of DD while including many energy conservation procedures.

Units: Dual