Various systems and apparati as well as devices and structures are provided which may be used or incorporated into air handling systems (e.g. (air) ventilation systems and apparati) for the manipulation or control of air in a enclosure such as a building, a room of a structure such as a residence, and the like. Such air handling systems (e.g. ventilation apparati or systems) may for example include an element for the transfer of heat from warm exhaust air (taken from inside an enclosure e.g. a building) to cooler exterior fresh air (drawn into the enclosure e.g. building). The present invention further relates to air handling systems (e.g. ventilation systems and apparati) which may not only exhaust stale interior air to the outside of an enclosure but as desired or necessary also intermingle a portion of such stale air with fresh air for delivery of the intermingled air back into the enclosure (during cold or warm weather). The present invention in particular relates to air handling systems (e.g. ventilation systems and apparati) which may exploit warm interior air as defrost air for defrosting the system during cool weather.