Abstract: A water heater is shown which has a water tank having a water inlet, a water outlet, and an opening in a sidewall of the tank. A combustion chamber assembly has a submergible portion which is adapted to be received within the opening in the tank sidewall. The combustion chamber assembly has a mounting flange for detachably engaging a tank mounting flange for mounting the assembly within the tank. The combustion chamber assembly has a plurality of curved fire tubes. Each of the tubes has at least one end which communicates through the mounting flange with the tank exterior. The combustion chamber assembly is removable from the tank by detaching the mounting flanges and sliding the assembly out of the tank opening.