Abstract: A boiler has a cylindrical body separated into two chambers by a transverse inner water chamber. A fuel magazine leads to one of the chambers, while a cyclone with a flue leads from the second chamber, the two chambers being connected by a sinuous combustion gas passage. A horizontal wall extends across the top of the cyclone and the combustion gas enters the second chamber adjacent the wall to descend into the cyclone. On ascending, a portion of the gases impinge on the wall and is caused to descend to create a circulatory motion in the cyclone.