The most common type of heat recovery ventilator is an "air-to-air heat exchanger." It consists of two fans and a heat exchanger core enclosed in a box that is usually attached to both supply and exhaust ductwork. One fan expels indoor air while the other draws in outside air. Inside the heat exchanger both air streams pass through a lattice of small air passages made of metal, plastic, or even treated paper. The air streams do not mix, but travel through adjacent passages.
In winter, the warmth of exhausted air is transferred to the cold incoming air, while in summer, the coolness of the air conditioned exhaust air reduces the temperature of the hot incoming air. Air-to-air heat exchangers are usually found in fairly tight houses. Most are used in northern climates for heat recovery in the winter where they recover as much as 80 percent of the heat from the outgoing air.