Double-Duct Systems

A dual or double duct system is an air conditioning system in which dehumidified air is circulated throughout a building via two parallel ducts. Hot air flows within one duct, cold air within the other. A central plant supplies both cold and hot air. The proportion of hot and cold air delivered to each room within the building may be controlled by thermostatically operated dampers on the ducts outlet.

In dual-duct systems, the air handling unit has two coils, a continuously operating cooling coil and a continuously operating heating coil. The cooling coil feeds chilled air into a cold air duct. The heating coil feeds hot air into a hot air duct. The two ducts run in parallel throughout the building. At each space, air is tapped from the two ducts by a terminal unit. The terminal unit has a hot air damper and a cold air damper. When the space thermostat calls for heating, the hot air damper opens. When the thermostat calls for cooling, the cold air damper opens.

The two-fan double-duct variable-volume system shares many of the advantages of the variable-volume reheat system. Pressurization is actively controlled and hoods have adequate safety monitors. Temperature control is adequate, and average ventilation rates are higher than for other variable-volume alternatives. This system has the disadvantage of requiring more room for duct work above the ceiling, and controlling a double-duct system is more difficult.

A dual duct system was popular in the early days of air conditioning. It had several advantages, but the disadvantages, like excessive energy consumption, led to its downfall after the energy crisis of 1973. The dual duct concept was fairly simple. A fan discharged air in a blow-thru arrangement that could either be directed through the cooling coil or the heating coil. What determined which path the air would take? Actually it was a device separate from either the fan or the coil, a device called the dual duct mixing box.