An air treatment unit for an air-to-air heat pump plant used to heat the air of an inside environment, the air treatment unit (1) has an external heat exchanger (5), which normally works as an evaporator, at least one inlet opening (11, 12) to receive air from the inside environment (2) and/or from an outside environment, a delivery opening (14), fans (17, 18) to generate an air flow (22), which normally flows out of the delivery opening (14), and an internal heat exchanger (7), which normally works as a condenser to heat the air flow (22). The fan means (17, 18) can be controlled by electronic control means so as to reverse the direction of the air flow (28) during a temporary reversal of the refrigeration cycle of the heat pump of the plant. The air treatment unit (1) is provided with a closing device (26, 27), which can be controlled by the electronic control unit so as to close the inlet opening (11, 12) during the temporary reversal of the refrigeration cycle.
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