A heater is disclosed that is installed in a window opening in a room for heating the room. The heater has a cabinet having an inner and an outer cabinet with an air gap therebetween, with a heating chamber in the inner cabinet. A burner is provided that is located on an outside wall of the heater. The burner has an inlet and an outlet with a combustion nozzle therebetween, with the burner outlet in communication with a respective serpentine heat exchange tube within said heating chamber where the heat exchange tube receives the hot products of combustion from its burner and discharges the products of combustion to the atmosphere. The heater has a blower that draws air from within the room and forces the air over and around the heat exchange tube(s) within the heating chamber discharges the heated air back into the room.