A boiler of the kind in which a bed of solid fuel is carried on a moving conveyor (l3) in one direction through a combustion chamber (ll). A fan (2l) supplies air via pipes (40, 4l) to injection nozzles (42, 43) which direct streams of air respectively upstream and downstream within the boiler casing to enhance combustion of the bed of fuel on the conveyor and of the combustion gases issuing therefrom, and to distribute cooling air evenly across the boiler chamber at its downstream end. Advantages accrue from efficient combustion within the boiler chamber and subsequent efficient cooling of exhaust gases which helps to solidify molten in suspension solids thus preventing deposition on the boiler walls and gas exit tubes.