Designs of heat exchangers for heat recovery from humid air are described. A mathematical model for heat transfer with simultaneous mass transfer is constructed, based on heat and mass balances. Methods for solution are shown for two cases: 1. The water on the exhaust side of the heating surface is only condensate, whose heat content may be neglected. Also, the case of only partially condensing heat transfer surface is discussed. 2. The heating surface is flushed with abundant water. - Each case is illuminated with a calculation example.