These are installed on board the unit and operate to exclude the fresh air intake and/or the room return air flow. This option is particularly useful in areas with very cold winter temperatures, where it is necessary to avoid dangerous self-induced cold air flows by the installation itself, during the stand-by period of the unit, with the risk of freezing the water contained in the water coils, if any. The dampers are controlled by On/Off actuators for the opening or the closing, or with return closing spring.