Abstract: Gas fired air heater for a central heating system comprising a heat exchanger in which are arranged a series of parallel rows of vertical heat exchange tubes, the lower ends of which are connected to a joint burner compartment and the upper ends of which are connected to a joint flue compartment. Two opposite lateral sides of the heat exchanger are connected to air supply and discharge compartments respectively for feeding the air to be heated transversely through the heat exchanger. The lateral air supply and discharge compartments have air inlet and outlet connections at their top and bottom walls only. One of said lateral air compartments is closed at one end and is connected to a fan at the opposite end, whereas at least one end of the other lateral air compartment is connected to a main duct for discharge of the heated air.