Abstract: In flat heat exchangers for ventilating dwellings, swimming pools, public premises, etc., which are used for air entering and leaving, problems arise when the air entering has low temperature. This results in a cold corner (A) appearing in the heat exchanger and its efficiency thus being reduced. The object of the present invention is to reduce the effect of the cold corner by introducing throttling means (9) along a number of the channels (3) for air leaving. The throttling means (9) are of equal size along one and the same channel, but different in the different channels (3), the channel (3) with the smallest throttling means (9) being located closest to the inlet for the air.