A system for heating supply air, with simultaneous defrosting of a water-exhaust air heat exchanger, when controlling the indoor climate in a building, wherein the system (101; 401; 501) at least comprises - a first air duct having first input means for introducing outdoor air into the system and first output means (104, 104') for discharging outdoor air treated in the system as supply air into the interior of the building, - a second air duct having second input means for introducing exhaust air from the interior of the building into the system and second output means (107, 107', 107") for discharging exhaust air treated in the system into the outdoor air outside the building, - an exhaust-supply air heat exchanger, which is arranged in connection with said first and second air ducts for selective transfer of heat from said exhaust air being treated in the system to said outdoor air being treated in the system or from said outdoor air being treated in the system to said exhaust air being treated in the system, - a water system (109; 209; 309) having a hot water circuit and a cold water circuit, - a water-supply air heat exchanger, which is arranged in connection with said first air duct for selective connection to said hot water circuit or said cold water circuit for transfer of heat from said hot water circuit to said outdoor air being treated in the system or from said outdoor air being treated in the system to said cold water circuit, - a water-exhaust air heat exchanger, which is arranged in connection with said second air duct for selective connection to said hot water circuit or said cold water circuit for transfer of heat from said hot water circuit to said exhaust air being treated in the system or from said exhaust air being treated in the system to said cold water circuit, - a heat pump (114; 214; 314; 414; 514), which is arranged in connection with said water system for selective transfer of heat from the hot water circuit to the cold water circuit or from the cold water circuit to the hot water circuit, whereby the exhaust-supply air heat exchanger is designed as a rotary heat exchanger, the rotary heat exchanger, with respect to the flow direction in the second air duct, is positioned upstream of the water-exhaust air heat exchanger, and the rotary heat exchanger has an efficiency correlating to rotational speed, characterized in that the rotary heat exchanger is provided with rotational speed control means for enabling defrosting of the water-exhaust air heat exchanger by means of a reduction of said rotational speed (R RED) in order to obtain an elevated temperature of the exhaust air passing through the water-exhaust air heat exchanger in combination with the hot water circuit simultaneously being connected to the water-exhaust air heat exchanger, wherein the rotational speed control means comprise an electric drive motor for driving the rotation of the rotary heat exchanger, wherein the electric drive motor is adapted to be capable of being controlled to a normal rotational speed during normal operation, in order to provide the highest possible heat transfer capacity of the rotary heat exchanger, and to be capable of being controlled to said reduced rotational speed (R RED) during a defrost cycle, in order to provide the lowest possible heat transfer capacity of the rotary heat exchanger that is appropriate taking into account other prevailing system conditions at the moment, and thereby the highest possible temperature of the exhaust air passing through the water-exhaust air heat exchanger, wherein the hot water circuit provides hot water for use in the water-supply air heat exchanger in said heating of supply air and in the water-exhaust air heat exchanger in the defrosting thereof. - a first air duct having first input means for introducing outdoor air into the system and first output means (104, 104') for discharging outdoor air treated in the system as supply air into the interior of the building, - a second air duct having second input means for introducing exhaust air from the interior of the building into the system and second output means (107, 107', 107") for discharging exhaust air treated in the system into the outdoor air outside the building, - an exhaust-supply air heat exchanger, which is arranged in connection with said first and second air ducts for selective transfer of heat from said exhaust air being treated in the system to said outdoor air being treated in the system or from said outdoor air being treated in the system to said exhaust air being treated in the system, - a water system (109; 209; 309) having a hot water circuit and a cold water circuit, - a water-supply air heat exchanger, which is arranged in connection with said first air duct for selective connection to said hot water circuit or said cold water circuit for transfer of heat from said hot water circuit to said outdoor air being treated in the system or from said outdoor air being treated in the system to said cold water circuit, - a water-exhaust air heat exchanger, which is arranged in connection with said second air duct for selective connection to said hot water circuit or said cold water circuit for transfer of heat from said hot water circuit to said exhaust air being treated in the system or from said exhaust air being treated in the system to said cold water circuit, - a heat pump (114; 214; 314; 414; 514), which is arranged in connection with said water system for selective transfer of heat from the hot water circuit to the cold water circuit or from the cold water circuit to the hot water circuit,