At the core of an ERV is the heat transfer module. Both the exhaust and
outdoor air streams pass through the module and the heat from the exhaust
air is used to pre-heat the outdoor air stream. Only the heat is
transferred; the two air streams remain physically separate. Typically, an
ERV is able to recover upto 85 percent of the heat from the exhaust air and
transfer it to the incoming air. This dramatically reduces the energy needed
to heat outdoor air to a comfortable temperature.

An installed energy recovery ventilator -- "ERV" -- sometimes called an
"air-to-air heat exchanger"

The air in re-circulated space will quickly become stale and unhealthy.
Odors, excessive moisture, and carbon dioxide from the occupants breathing
are all forms of indoor air pollution. So in a tightly-built space it's
important to provide good ventilation.

It's also important to do this in an energy efficient way. An energy
recovery ventilator is the answer. These simple units consist of a box with
fan and a special "core" element inside.