the present invention relates to an apparatus for defrostable ventilation systems which include an element for the transfer of heat from warm exhaust air to cooler exterior fresh air which is drawn into the building , the system using warm interior air as defrost air for defrosting the system during cool weather . the present invention in particular provides an apparatus whereby , during a defrost cycle , the interior defrost 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 may be confined 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 .