In some cases, it may be desirable to design a whole-building ventilation system that operates intermittently. The most common example of intermittent ventilation is when outside air is ducted to the return plenum of the central heating/cooling system, and thus the central heating/cooling system fan is used to distribute the ventilation air to the rooms in the building. This type of ventilation is permitted as long as the ventilation airflow is increased to respond to the fewer hours of fan operation. The increased flow depends on the fraction of time the fans operate.
Intermittent ventilation systems have to be automatically controlled by a timer or other device that assures that they will operate the minimum amount of time needed to meet the ventilation requirement. The automatic controls shall make sure that the fan operates at least 1 hour in twelve. The California Energy Commission Title 24 Residential Compliance Manual contains more information on this option.
(Excerpted from the CEC Title 24 Residential Compliance Manual)