Abstract: In a heat exchanger of the kind used for the transfer of heat from exhaust air, e.g. from rooms, to fresh air to be led into rooms, or for the transfer of heat the opposite way, means (9) for the cooled exhaust air are arranged after the outlet (4) with a view to transferring heat from the exhaust air to a heat reservoir (12), i.e. a heat pump circuit (10, 11). The thermodynamic efficiency is dependent on the difference in temperature, of course, and by transferring heat from exhaust air to for instance a water tank for hot water, the temperature being in some places close to the flow temperature of tap water, it will be possible to make use of much more of the latent heat of the exhaust air.