Abstract: A fuel-fired heating appliance, representatively a gas-fired water heater, has a combustion chamber with a flame arrestor plate forming a bottom wall thereof. The arrestor plate has a spaced series of flame quenching perforations therein through which combustion air upwardly flows into the combustion chamber during firing of the water heater. A shield structure within the combustion chamber overlies the arrestor plate perforations in an upwardly spaced relationship therewith and blocks operating scale on the bottom side of the top combustion chamber wall, and/or a flue extending upwardly from the top combustion chamber wall, from falling on and blocking the combustion air transfer perforations in the flame arrestor plate.