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A heat-recovery ventilator (HRV), sometimes called an air-to-air heat exchanger, differs from conventional vents and fans. With standard ventilation, air circulates through static, open vents or is expelled by fans, such as those used in bathrooms, kitchens and attics. When room air escapes or is expelled, the energy that was used to heat or cool it is wasted. An HRV can save 75% or more of that wasted energy. As it pushes out stale air, it pulls in fresh air, and--with little or no mixing of the two air streams--it transfers the heat or chill from the outgoing air to the incoming supply. The fresh air arrives pre-heated or pre-cooled and, with some units, pre-humidified or dehumidified. Some HRVs mount like a room air conditioner in a window or wall opening; these are meant to handle individual rooms that have ventilation problems: bathrooms, laundry rooms, artist studios and darkrooms, for example. Larger, whole-house HRVs provide fresh air for all rooms. They often require routing ductwork to several places: to rooms where you want to exhaust stale air, particularly bathrooms, laundry rooms and kitchens; to the outdoors; and often to the central heating and air-conditioning system's return air supply. Though whole-house systems are installed primarily in new houses, they can be retrofitted into some houses that have good access for ductwork, particularly those with unfinished basements. To simplify installation, some systems can take advantage of existing ductwork--they're piped into the return-air system.
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