Why Ventilate

Whole House Ventilation

www.energy.gov

A whole-house ventilation system with dedicated ducting in a new energy-efficient home.

Energy-efficient homes — both new and existing — require mechanical ventilation to maintain indoor air quality. There are four basic mechanical whole-house ventilation systems — exhaust, supply, balanced, and energy recovery.

What Does This Mean For Me?

Exhaust Ventilation Systems

Exhaust ventilation systems work by depressurizing your home. The system exhausts air from the house while make-up air infiltrates through leaks in the building shell and through intentional, passive vents.

Exhaust ventilation systems are most appropriate for 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.

Exhaust ventilation systems are relatively simple and inexpensive to install. Typically, an exhaust ventilation system consists of a single fan connected to a centrally located, single exhaust point in the house. A better design is to connect the fan to ducts from several rooms, preferably rooms where pollutants are generated, such as bathrooms. Adjustable, passive vents through windows or walls can be installed in other rooms to introduce fresh air rather than rely on leaks in the building envelope. Passive vents may, however, require larger pressure differences than those induced by the ventilation fan to work properly.

Supply Ventilation Systems

Supply ventilation systems use a fan to pressurize your home, forcing 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 (if any exist).

Like exhaust ventilation systems, supply ventilation systems are relatively simple and inexpensive to install. A typical supply ventilation system has a fan and duct system that introduces fresh air into usually one — but preferably several — rooms that residents occupy most (e.g., bedrooms, living room). 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 exhaust ventilation systems do. By pressurizing the house, supply ventilation systems minimize outdoor pollutants in the living space and prevent backdrafting of combustion gases from fireplaces and appliances. Supply ventilation also allows outdoor air introduced into the house to be filtered to remove pollen and dust or dehumidified to provide humidity control

Supply ventilation systems work best in hot or mixed climates. Because they pressurize the house, these systems have the potential to cause moisture problems in cold climates. In winter, the supply ventilation system causes warm interior air to leak through random openings in the exterior wall and ceiling. If the interior air is humid enough, moisture may condense in the attic or cold outer parts of the exterior wall, resulting in mold, mildew, and decay.

Balanced Ventilation Systems

Balanced ventilation systems, if properly designed and installed, neither pressurize nor depressurize your home. Rather, they introduce and exhaust approximately equal quantities of fresh outside air and polluted inside air.

A balanced ventilation system usually has two fans and two duct systems. Fresh air supply and exhaust vents can be installed in every room, but a typical balanced ventilation system is designed to supply fresh air to bedrooms and living rooms where occupants spend the most time. It also exhausts air from rooms where moisture and pollutants are most often generated (kitchen, bathrooms, and perhaps the laundry room).

Some designs use a single-point exhaust. Because they directly supply outside air, balanced systems allow the use of filters to remove dust and pollen from outside air before introducing it into the house.

Balanced ventilation systems are appropriate for all climates. Because they require two duct and fan systems, however, balanced ventilation systems are usually more expensive to install and operate than supply or exhaust systems.

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