Exhaust and supply systems

Exhaust ventilation systems work by depressurizing the building. By reducing the inside air pressure below the outdoor air pressure, they extract indoor air from a house while make-up air infiltrates through leaks in the building shell and through intentional, passive vents.

Exhaust ventilation systems are most applicable in cold climates. In climates with warm, humid summers, depressurization can draw moist air into building wall cavities, where it may condense and cause moisture damage. The systems are relatively simple and inexpensive to install. Typically, an exhaust ventilation system is composed of a single fan connected to a centrally located, single exhaust point in the house.

A preferable option is to connect the fan to ducts from several rooms (especially rooms where pollutants tend to be generated, such as bathrooms). Adjustable, passive vents through windows or walls can be installed to introduce fresh air rather than rely on leaks in the building envelope. However, passive vents may be ineffective because larger pressure differences than those induced by the ventilation fan may be needed for them to work properly.

Spot ventilation exhaust fans installed in the bathroom but operated continuously represent an exhaust ventilation system in its simplest form.

Supply ventilation systems work by pressurizing the building. They use a fan to force outside air into the building while air leaks out of the building through holes in the shell, bath- and range-fan ducts, and intentional vents.

As with exhaust ventilation systems, supply ventilation systems are relatively simple and inexpensive to install. A typical system has a fan and duct system that introduces fresh air into usually one—but preferably several—rooms that residents occupy most (for example, bedrooms, living room, kitchen). This system may include adjustable window or wall vents in other rooms.

Supply ventilation systems allow better control of the air that enters the house than do exhaust ventilation systems. By pressurizing the house, these systems discourage the entry of pollutants from outside and prevent backdrafting of combustion gases from fireplaces and appliances. They also allow air introduced into the house to be filtered to remove pollen and dust or to be dehumidified.

Like exhaust ventilation systems, supply ventilation systems do not temper or remove moisture from the air before it enters the house. Thus, they may contribute to higher heating and cooling costs compared with energy recovery ventilation systems. Because air is introduced in the house at discrete locations, outdoor air may need to be mixed with indoor air before delivery to avoid cold air drafts in winter.