Abstract: Method and apparatus for preventing condensate from forming on the inside walls of the heat exchanger of a combination air conditioning and heating unit during the cooling mode of operation. The formation of condensate on the inside surfaces of the heat exchanger is reduced by bleeding a small but sufficient amount of air pressurized by the air blower and conditioned by the evaporator into the combustion chamber and then into the heat exchanger to displace ambient air with high moisture content from the interior of the heat exchanger. A hole or a number of holes is provided in the partition separating the discharge section from the burner control section which is open to atmosphere to allow a portion of conditioned air into the heat exchanger.