Supply Ventilation Pressurizes the House
Bird's eye view
Fresh air for the whole house even when bath and kitchen fans aren't running
A supply ventilation system is the reverse of an exhaust ventilation system: Fresh outside air is pulled into the house with a fan, forcing inside air out through random openings in the building envelopeExterior components of a house that provide protection from colder (and warmer) outdoor temperatures and precipitation; includes the house foundation, framed exterior walls, roof or ceiling, and insulation, and air sealing materials.. A supply ventilation system pressurizes a house. In homes equipped with forced-air ductwork, supply ventilation systems are the least expensive way to provide whole-house mechanical ventilation.
Homes with supply ventilation systems still need spot exhaust fans in bathrooms and range hoods. However, a supply ventilation system can supply fresh air whether or not the home's exhaust fans are operating.
A motorized damper reduces energy losses
A central-fan-integrated supply ventilation system requires a special control called the FanCycler (www.fancycler.com) or AirCycler (www.aircycler.com). The device can be ordered from Energy Federation Inc. (www.efi.org), which also supplies motorized dampers.
A similar control is the Aprilaire model 8120 controller from Research Products Corp. (www.aprilaire.com).
One of the best-known models of stand-alone supply ventilation fans is the Multi-Air manufactured by Tamarack Technologies (www.tamtech.com).
Some builders worry that pressure imbalances resulting from the operation of a supply ventilation system might force warm, moist interior air into building cavities, leading to condensation and moisture problems. Fortunately, this worry is overblown. A properly designed supply ventilation system will work in any climate.
Residential ventilation requirements are relatively low — generally in the 40 cfm to 80 cfm range. This means that a supply ventilation system is no more likely to put some rooms of a house at positive pressure than other common forces acting on the house, including the stack effectAlso referred to as the chimney effect, this is one of three primary forces that drives air leakage in buildings. When warm air is in a column (such as a building), its buoyancy pulls colder air in low in buildings as the buoyant air exerts pressure to escape out the top. The pressure of stack effect is proportional to the height of the column of air and the temperature difference between the air in the column and ambient air. Stack effect is much stronger in cold climates during the heating season than in hot climates during the cooling season. and wind. The key to success: don't overventilate.
Control the amount of incoming air
There are three ways to control a supply ventilation system:
Keep intake vents away from pollutant sources
Ventilation of living space is covered in Section 303 of the 2006 IRCInternational Residential Code. The one- and two-family dwelling model building code copyrighted by the International Code Council. The IRC is meant to be a stand-alone code compatible with the three national building codes—the Building Officials and Code Administrators (BOCA) National code, the Southern Building Code Congress International (SBCCI) code and the International Conference of Building Officials (ICBO) code.. Outdoor-air intake openings should be at least 10 ft. from any potential source of contamination like exhaust vents, plumbing vents, streets, alleys, chimneys, parking lots, and loading docks. When a 10-ft. distance is impossible, the intake vent must be located at least 2 ft. below the source of contamination (303.4.1). Intake and exhaust openings must be protected with corrosion-resistant grilles, screens or louvers with a hole size between 1/4 in. and 1/2 in. (303.5).
ABOUT SUPPLY VENTILATION
Keeping Out Contaminants
Supply ventilation systems have two advantages over exhaust ventilation:
Existing Ductwork or Dedicated Ductwork?
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.
MORE ABOUT SUPPLY VENTILATION
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 62.2A standard for residential mechanical ventilation systems established by the American Society of Heating, Refrigerating, and Air-Conditioning Engineers. Among other requirements, the standard requires a home to have a mechanical ventilation system capable of ventilating at a rate of 1 cfm for every 100 square feet of occupiable space plus 7.5 cfm per occupant. requirements (1 cfm for every 100 square feet of occupiable space plus 7.5 cfm per occupant).
Compared to the typical exhaust-only ventilationMechanical ventilation system in which one or more fans are used to exhaust air from a house and make-up air is supplied passively. Exhaust-only ventilation creates slight depressurization of the home; its impact on vented gas appliances should be considered. 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 minimized by specifying a furnace with an efficient variable-speed ECM blower.
- Fine Homebuilding 178
- Dan Morrison/Fine Homebuilding