Abstract: A charging shaft in a furnace housing, communicating at its bottom with an adjoining combustion chamber, is topped by a sloping lid having an air-intake opening near its lower end, that opening registering with an underlying baffle defining with that lid a narrow rising channel for incoming air. Distillation gases evolving in the shaft, admixed with some of the entering air but prevented by that air from escaping through the channel, are recirculated through a port at the top of the shaft to an entrance near its bottom -- just above a horizontal grate -- by way of an adjacent duct provided with a lateral air inlet, the duct narrowing at that inlet into a constricted throat from which the recirculated gases are aspirated by the entering additional air. The connection between the charging shaft and the combustion chamber may be formed by an upright grill, or by a diverging passage accommodating an ancillary combustion device such as an oil burner.