A boiler comprising a water jacket surrounding upper and lower combustion chambers for receiving heat for heating water or other fluid therein. The upper and lower combustion chambers are defined by a refractory structure which extends entirely across the inner casing. A vertical passage through the refractory structure provides for flow of combustion gases from the upper to lower combustion chamber. Oxygen is provided through a first forced air inlet to the upper combustion chamber for burning of wood or biomass in the upper combustion chamber. Oxygen is provided through a forced air passage in the refractory structure and opening into the vertical passage thereby providing at cast one second forced air inlet to the vertical passage for burning of the combustion gases and particulates passing therethrough from the upper combustion chamber. The refractory structure sealingly engages the inner casing in a manner to seal the upper combustion chamber from the lower combustion chamber so that the upper combustion chamber can be made substantially air tight whereby the upper combustion chamber can be pressurized by forced air thereto to effect expulsion of combustion gases and particulates from the upper combustion chamber through the vertical passage for burning thereof. Refractory material in the lower combustion chamber provides refractory surface on which the burning combustion gases and particulates expelled through the vertical passage impinge.