56 and the blade 47 is open to the flow of fresh air from the fresh air opening 34 through the opening 54 and to the evaporator 12 at the same time there is an open area to the left of the blade 48 wherein the return air may also flow through the evaporator coil 12 however the blade 48 does offer some blockage to the flow of the return air and it also acts to divert its flow to the right between the divider 51 and the condenser coil 13 this air then passes through the coil 13 and is caused by the fan 31 to exhaust through the opening 33 in this way as the return air becomes stale some of it may be exhausted out of the system and replaced with fresh air of course it will be understood that the flap 36 can be placed in any other desirable position between those described hereinabove so as to obtain the desired mixture of fresh air with the return air it is the intermediate positions of the flap 36 wherein the divider 51 comes into play for example in the fig6 c position the divider 51 forms a boundary between the flow of fresh air coming.