An air treatment system, comprising a heat exchanger for transferring heat from warm exhaust air, that flows into the system via an outlet air duct of said system and leaves it via an exhaust air fan thereof and there between passes an exhaust air section of the heat exchanger, to cold supply air, that flows into the system via a supply air duct of said system and leaves it via a supply air fan thereof and there between passes a supply air section of the heat exchanger, said supply air section being separate from said exhaust air section, a bypass, that communicates with the supply air duct upstream of said supply air section and the supply air fan downstream thereof and shows a flow resistance that corresponds to a normal through-flow resistance of the supply air section of the heat exchanger, and a damper device, that enables opening and closing of the bypass and of the supply air section on an upstream side the supply air section, characterized in that the damper device is divided into at least three individually controllable portions, of which a first portion is adapted to stop an inflow of supply air into the bypass, to allow a predetermined throttled inflow of supply air into the bypass or to allow an unthrottled inflow of supply air into the bypass, a second portion is adapted to stop an inflow of supply air into in a first half of the supply air section of the heat exchanger or to allow an unthrottled inflow of supply air into said first half, and a third portion is adapted to stop an inflow of supply air into a second half of the supply air section of the heat exchanger or to allow an unthrottled inflow of supply air into said second half. a heat exchanger for transferring heat from warm exhaust air, that flows into the system via an outlet air duct of said system and leaves it via an exhaust air fan thereof and there between passes an exhaust air section of the heat exchanger, to cold supply air, that flows into the system via a supply air duct of said system and leaves it via a supply air fan thereof and there between passes a supply air section of the heat exchanger, said supply air section being separate from said exhaust air section, a bypass, that communicates with the supply air duct upstream of said supply air section and the supply air fan downstream thereof and shows a flow resistance that corresponds to a normal through-flow resistance of the supply air section of the heat exchanger, and a damper device, that enables opening and closing of the bypass and of the supply air section on an upstream side the supply air section, characterized in that the damper device is divided into at least three individually controllable portions, of which a first portion is adapted to stop an inflow of supply air into the bypass, to allow a predetermined throttled inflow of supply air into the bypass or to allow an unthrottled inflow of supply air into the bypass, a second portion is adapted to stop an inflow of supply air into in a first half of the supply air section of the heat exchanger or to allow an unthrottled inflow of supply air into said first half, and a third portion is adapted to stop an inflow of supply air into a second half of the supply air section of the heat exchanger or to allow an unthrottled inflow of supply air into said second half.