Abstract: A vertically situated head exchanger is mounted externally of a building and insulated relative to the ambient air. Fan units are situated within the building preferably between adjacent joists and in the basement and in alignment with the connections to the heat exchanger. One fan unit draws fresh air from outside the building, through the heat exchanger and into the building. Another fan unit draws exhaust air from the building and passes this through the heat exchanger thus giving up heat to the cool incoming air and is then exhausted externally of the building. A controlled duct may draw warm air from the building and pass it through the heat exchanger to defrost same when necessary.