Commercial Energy Systems

All-Air Central Dual Duct


One cold air duct, and a parallel warm air duct, distributes air from the central air handler to the conditioned spaces. A room thermostat controlled mixing damper in each zone mixes the air to the proper proportions to satisfy the space load. Return air is through a single duct system.

single fan dual duct system

The single-fan dual duct reheat system controls the relative humidity high-limit throughout the full range of operation. All the air is first cooled and dehumidified, then divided into two ducts with hot deck reheat. Because of the continual demand for reheat, the operating cost is high.

The advantages include:

  • Good temperature and humidity control
  • Accommodates a variety of zone loads
  • Ease of adding or subdividing zones in the future
  • Adaptable to either constant volume or VAV systems

The disadvantages include:

  • High first cost and space requirements for two sets of ducts throughout the building
  • High energy consumption
  • Large number of mixing boxes to maintain
  • Difficult to use economizer cycle

They are no longer popular due to their high energy consumption, although they are sometimes used in hospitals where VAV is inappropriate. Use of VAV control on the main air supply fan to reduce air flow during low loads can improve their efficiency somewhat.

When the humidity must be kept below a specified value under all operating conditions and the dual duct system cannot accomplish this, these two alternative systems can be considered.

dual fan dual duct system

1. The two-fan dual duct blow-through system has each fan sized to handle 50% of the total system air flow. The two bypass connections do not require any control as air flow will be determined by the space loads and the temperatures maintained in each duct.

2. An alternative is where the bypass on the supply fan discharge side is omitted and both fans are sized for the total system air flow. This arrangement eliminates the possibility of excessive air stratification in the cold and warm chambers.

These systems' disadvantages include:

  • Increased cost and power requirements of supply fans
  • Increased control complexity to control static pressures in each duct
  • Increased apparatus room cost.

Links to Related Topics

Indoor Air Quality
Outside Air Control
Central Plant System
Central (or Built-Up) System
All-Air Central Systems
All-Air Central Reheat Systems
All-Air Central Multizone
All-Air Rooftop
Air-Water Central Systems
All Water Central Systems
Two Pipe System
Three Pipe System
Four Pipe System