A system for controlling the indoor climate in a building, wherein the system at least comprises - a first air duct which is adapted for introducing outdoor air into the system and for discharging outdoor air treated in the system as supply air into the interior of the building, - a second air duct which is adapted for introducing exhaust air from the interior of the building into the system and for discharging exhaust air treated in the system into the outdoor air outside the interior of 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 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 which is arranged in connection with said water system for selective transfer of heat between said water circuits (106, 107), a control unit which is adapted for controlling the flows of air (A1, A1', A5, A2, A3, A6, A4) and the flows of fluid (W1, W2, W3, W4, W5, W6) in the system, whereby at least one first fan device (111, 111') is arranged in connection with said first and/or second air duct for circulating ventilation air (A2, A3) to and/or from the interior of the building, that the system has a passage for admitting a partial flow (A1') of outdoor air upstream of said water-exhaust air heat exchanger but downstream of said exhaust-supply air heat exchanger, that at least one second fan device is arranged in connection with said second air duct for sucking or blowing said partial flow (A1') of outdoor air, together with exiting circulated ventilation air, through said water-exhaust air heat exchanger, wherein said at least one second fan device is dimensioned to be capable of providing a maximum air flow rate which is at least 1,7 times greater than the air flow rate provided by said at least one first fan device (111, 111') when the system is in heating operation. - a first air duct which is adapted for introducing outdoor air into the system and for discharging outdoor air treated in the system as supply air into the interior of the building, - a second air duct which is adapted for introducing exhaust air from the interior of the building into the system and for discharging exhaust air treated in the system into the outdoor air outside the interior of 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 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 which is arranged in connection with said water system for selective transfer of heat between said water circuits (106, 107), a control unit which is adapted for controlling the flows of air (A1, A1', A5, A2, A3, A6, A4) and the flows of fluid (W1, W2, W3, W4, W5, W6) in the system, whereby at least one first fan device (111, 111') is arranged in connection with said first and/or second air duct for circulating ventilation air (A2, A3) to and/or from the interior of the building, that the system has a passage for admitting a partial flow (A1') of outdoor air upstream of said water-exhaust air heat exchanger but downstream of said exhaust-supply air heat exchanger, that at least one second fan device is arranged in connection with said second air duct for sucking or blowing said partial flow (A1') of outdoor air, together with exiting circulated ventilation air, through said water-exhaust air heat exchanger, wherein said at least one second fan device is dimensioned to be capable of providing a maximum air flow rate which is at least 1,7 times greater than the air flow rate provided by said at least one first fan device (111, 111') when the system is in heating operation.