A water heater including a water tank, a combustion chamber beneath the tank, a gas burner in the combustion chamber, an air plenum upstream of the combustion chamber, a flame arrestor located such that air in the air plenum passes through the flame arrestor to reach the combustion chamber, and a flue extending upwardly from the combustion chamber and through the water tank. An inner plate defines the lower boundary of the combustion chamber, and supports the flame arrestor. The inner plate is rolled around the bottom edge of the skirt. A raised ring is formed in the inner plate, and the flame arrestor is seated against the raised ring. An o-ring seal is positioned in a peripheral groove of the flame arrestor to substantially gas-tightly seal the periphery of the flame arrestor with respect to the inner plate. An access door is stamped and fit over a access opening in the skirt. The access door includes a boot wall for receiving a gas manifold tube boot in a slip fit fashion, and includes a flat portion for a sight glass.