Abstract: A boiler assembly including primary and secondary combustion chambers includes primary and secondary air controls for controlling combustion air to the chambers respectively. The air controls are operable between open positions to allow combustion in the chambers and a closed position in which the combustion air passages are fully sealed to fully interrupt combustion responsive to heating demands. The air controls are supported externally on the main housing of the boiler assembly so as to be modular and readily releasable from the main housing and the respective combustion air passages. The primary combustion chamber communicates with the secondary combustion chamber at a location partway in between introduction of the secondary combustion air and the exhaust passage.