Keeping Out Contaminants
Supply ventilation systems have two advantages over exhaust ventilation:
The most common type of supply ventilation system uses a home’s existing forced-air ductwork to distribute fresh ventilation air. This system, called a central-fan-integrated supply ventilation system, requires a passive duct to bring fresh outdoor air to the furnace’s return air plenum. (To learn more about central-fan-integrated supply ventilation systems, see “More About Supply Ventilation,” below.)
More rarely, a supply ventilation system has its own dedicated ductwork. In that case, a remote-mounted supply fan is ducted to grilles in the living room and bedrooms.
Central-fan-integrated supply ventilation systems
A simple whole-house ventilation system requires supply air to be ducted from the outdoors to the return plenum of a furnace. According to the Building Science Corporation, this is “the simplest, most effective and most economical way to introduce fresh air” to a house. These systems, called central-fan-integrated supply ventilation systems, are controlled by a patented device called the FanCycler.
The FanCycler controls the furnace fan as well as a motorized damper installed on the 6-in. or 8-in. passive fresh air intake duct, preventing both overventilation and underventilation. When the furnace fan operates for long periods of time, the FanCycler closes the motorized damper. When the furnace fan is idle for too long, the FanCycler energizes the fan to assure adequate ventilation.
Most installers choose to program a FanCycler so that the system provides ventilation meeting ASHRAE 62.2 requirements (1 cfm for every 100 square feet of occupiable space plus 7.5 cfm per occupant).
Compared to the typical exhaust-only ventilation system, a central-fan-integrated supply ventilation system provides more even distribution of fresh air to remote bedrooms. The main disadvantage of the systems is the high cost required to run the furnace blower during the shoulder seasons (spring and fall). This disadvantage is…