A furnace has a solid fuel hopper 35 above a combustion chamber 10, an ash outlet 11 below said combustion chamber, a combustion gas outlet 11 at or below the level of the combustion chamber, and an auger 22 in an ash chamber 20 below the ash opening 11 to regulate the size of an ash pile 21 below the combustion chamber 10 and hence control the escape of ash or fuel or non-combustibles through the ash opening 11. This enables the combustion gas outlet 11 to be covered by hot coals 53 within the combustion chamber 10 so that any combustion gas from the combustion chamber or any pre-combustion zone must pass through a filter of hot coals 53 before reaching the combustion gas outlet 11.
