Consider Figure , which shows a section through a building. The prevailing wind infiltrates one side of the building, where the space heating appliances must be sized to raise the temperature of the incoming outdoor air. The heated air moves across the building to the leeward side, where it is exfiltrated. Here the heating appliances are not required to treat the air. Clearly, on another day the wind direction will have changed, and therefore all heating appliances serving the building perimeter must be sized to offset the infiltration loss Qv as well as the structural heat loss.
