Abstract: In order to improve the efficiency of a heater for heating a fluid using hot products of combustion, a second heat exchanger is connected to a first heat exchanger, the first heat exchanger absorbing primarily radiant heat from the hot products of combustion and the second heat exchanger absorbing primarily convected heat from the hot products of combustion. The hot products of combustion either pass through an array of parallel tubes in the second heat exchanger, or the second heat exchanger is inserted into the end of the first heat exchanger.