and leave the heat exchanger at a temperature in a range of approximately 20 u00b0 40 u00b0 c the heat exchanger 10 of this embodiment essentially comprises longitudinally extending channels 26 that are disposed on both sides and are disposed one upon the other without a space between them those longitudinal channels 26 disposed in a given plane are interconnected via a plurality of transversely extending channels 27 in order to be able to convey fresh air back and forth in the transverse channels 27 which are disposed one above the other the longitudinal channels 26 are alternately connected on opposite sides via openings 28 that are distributed over the length of the longitudinal channels 26 the exhaust gases are also conveyed back and and forth and in particular are conveyed in channels 29 that extend parallel to the longitudinal channels 26 at the top and bottom a given channel 29 is delimited by the transverse channels 27 which are disposed one after the other without being joined the channels 29 are delimited to both sides by the longitudinal channels 26 the remaining delimiting walls can be formed by the walls 30 of the apparatus the channels 26 27 are formed by joined together sections 31 32 e g deep drawn sections the edges or rims of which are welded together at 33 longitudinal channels 26 disposed above one another positively engage one another via a longitudinally extending profiling in addition projections 34 are distributed over the length of the channels and engage in corresponding recesses 35 of the adjacent channel this positive connection is provided on those sides of the longitudinal channels that do not communicate with one another in contrast in the.