Abstract: A ventilating unit has separate passages connecting internal and external environments, and a reversible heat pump with evaporator in one passage and condenser in the other passage. Separate reversible axial flow fans are located in the two passages. Two ventilating modes are provided, in each of which the fans are moving air in opposite directions through the passages. When the internal environment is relatively warm, the incoming air passes over the evaporator and the outgoing air over the condenser, and vice versa when the internal environment is relatively cool. Special fan design facilitates direction reversal, and also variable speed operation. The unit also has an additional inlet with a heater, and control means are adapted to provide a recirculation mode in which the recirculated air is heated thereby.