A mechanical stoker suitable for small boilers, such as the vertical, water-tube, and locomotive types. Coal is conveyed direct from a bunker or hopper by a feed worm, which forces the fuel up through the bottom of the retort in which it is burned. Volatiles driven off must pass through the ignited fuel, thus eliminating smoke. An underfeed stoker operates most successfully on graded coals with an upper size limit of 1 to 2 in (2.5 to 5.1 cm).