Since an energy-efficient house is tightly sealed, it needs to be ventilated in a controlled manner. Controlled, mechanical ventilation prevents health risks from indoor air pollution, promotes a more comfortable atmosphere, and reduces air moisture infiltration, thus reducing the likelihood of structural damage.
Furnaces, water heaters, clothes dryers, and bathroom and kitchen exhaust fans expel air from the house, making it easier to depressurize an airtight house if all else is ignored. But natural-draft appliances may be back-drafted by exhaust fans, which can lead to a lethal buildup of toxic gases in the house. For this reason, sealed combustion heating appliances, which use only outside air for combustion and vent combustion gases directly to the outdoors, are very important for ventilation energy efficiency and safety.
Heat recovery ventilators (HRV) or energy recovery ventilators (ERV) are growing in use for controlled ventilation in airtight homes. These ventilators can salvage about 70 percent of the energy from the stale exhaust air and transfer that energy to the fresh air entering by way of a heat exchanger inside the device. They can be attached to the central forced air system or may have their own duct system.
Other ventilation devices, such as through-the-wall or “trickle” vents, may be used in conjunction with an exhaust fan. They are, however, more expensive to operate and possibly more uncomfortable to use because they have no energy recovery features to precondition the incoming air. Uncomfortable incoming air can be a serious problem in northern climates and can create moisture problems in humid climates. Therefore, this ventilation strategy is only for arid climates. Other systems pull outside air in with a small outside duct on the return side of the furnace.