Abstract: A secondary heat exchanger is connected in the flow path of exhaust gases passing from the exhaust expansion chamber to the flue gas outlet of a pulse combustion system, so as to be heated by the hot exhaust gases flowing therein, and air to be heated is first passed over the exterior of the secondary heat exchanger and then over the exterior of the combustion chamber. The temperature at the outlet end of the secondary heat exchanger is below the condensation point of exhaust gases therein, and the temperature at the inlet end of the secondary heat exchanger is sufficiently low to permit use of a wider range of materials for the secondary heat exchanger. The secondary heat exchanger is made of a material of high thermal conductivity which is corrosion resistant, and increases the thermal efficiency of the system greatly while exerting a muffling action to reduce the noise in the system.