An apparatus for ventilation systems which include an element for the transfer
of heat from warm exhaust air (taken from inside a building) to cooler exterior
fresh air which is drawn into the building. During a defrost cycle, interior air
may circulate through both of the fresh air and exhaust air paths for delivery
back into the building, i.e. the warm interior air, used as defrost air, may be
able to circulate from the interior of the building into the ventilation apparatus
and back to the interior of the building. The apparatus can thus use interior air
as defrost air while diminishing or avoiding the creation of a negative air pressure
in the building.
