A Heat Recovery Ventilator (HRV) provides controlled fresh air ventilation while recovering most of the energy from the exhaust air stream. It uses a heat exchanger core where outgoing stale air and incoming fresh air flow in opposite directions, with heat transferring between them without the air streams mixing.
In winter, an HRV recovers 70-85% of the heat from exhaust air, pre-warming incoming fresh air and dramatically reducing the energy penalty of ventilation. In summer, it pre-cools incoming air using the cooler exhaust air. HRVs are most effective in cold, dry climates.
HRVs are distinct from ERVs (Energy Recovery Ventilators). HRVs only transfer sensible heat (temperature). ERVs transfer both sensible heat and latent heat (moisture), making them better suited for humid climates where controlling indoor moisture is important. Both devices are required or recommended by ASHRAE 62.2 and many building codes for tightly constructed homes.