The boiler consists of a lined combustion chamber located in the bottom section and a system of heat exchangers in the upper section. A steel grate with primary air inlet from beneath is placed in the combustion chamber and secondary air inlet holes are located in the side walls of the chamber. The primary/secondary air ratio is adjustable by a flap; however, there is only one air fan. The pellets are fed from a storage container by a screw feeder to the grate