Abstract: An assembly for purifying the gases and combustion fumes from a heating apparatus, particularly from a closed chimney furnace of the type using a catalyst device (7) in the gas and combustion fume exhaust duct (5). The assembly has an electrically powered rapid acting heater for raising the temperature, in and/or near the catalyst device (7), to initiate a catalytic reaction. A temperature-sensitive sensor or detector (12) is provided in the control circuit (11) of the heater. The assembly is of interest to builders of heating apparatuses using solid fuel, particularly wood.