Abstract: A gas-fired booster water heater for providing water at the sanitizing temperature to a commercial dishwashing machine includes an outer housing which is sized to be installed below a counter and contains a burner comprising a burner box and combustion chamber separated by an infrared burner combustion surface facing into the combustion chamber. A heat exchanger includes finned tubes disposed in the combustion chamber for continuous circulation therethrough of water to be heated during heater operation. The heater housing includes an access opening through which the burner box together with the combustion surface may be disconnected from the combustion chamber and withdrawn as a unit for service or repair without removal of the heater from its installed position below the counter.