High efficiency wood or biomass boiler

A boiler comprising an inner casing, an outer casing defining with said inner casing a space in which fluid can be received for heating thereof, a fluid inlet to the space, a fluid outlet from the space, a refractory structure in said inner casing and defining an upper combustion chamber and a lower combustion chamber in said inner casing, at least one vertical passage through said refractory structure for flowing combustion gases from said upper combustion chamber to said lower combustion chamber, at least one first forced air inlet to said upper combustion chamber for providing oxygen for burning of material in said upper combustion chamber, at least one forced air passage in said refractory structure and opening into said vertical passage thereby providing at least one second forced air inlet to said vertical passage for providing oxygen in said vertical passage for burning of the combustion gases and particulates passing therethrough from said upper combustion chamber, wherein said refractory structure extends entirely across and sealingly engages said inner casing in a manner to seal said upper combustion chamber from said lower combustion chamber so that said upper combustion chamber can be made substantially air tight whereby said upper combustion chamber can be pressurized by forced air thereto to effect expulsion of combustion gases and particulates from said upper combustion chamber through said at least one vertical passage for burning thereof, and refractory material disposed in said lower combustion chamber and having an upper surface positioned so that the burning combustion gases and particulates expelled through said at least one vertical passage impinge on said upper surface.