Abstract: Method to operate a combustion device in a house or similar building, with fossil fuel furnaces, such as wood or oil furnaces. Outdoor air is supplied to a heat exchanger that is connected to the smoke tube of the furnace, in such an amount that an interior excess pressure is created, as excess air is led out of the interior of the building through one or more exhaust openings, especially exhaust devices, which are located at low places in the building at its outer side. This offers better combustion efficiency, better energy-utilization and a more uniform temperature in the building. A device for operating the method, comprises a fan (23) which takes in air from an inlet (19) near the outer walls of the building, through a tube (24) leading to the furnace, with a heat exchanger (33, 34) enclosing some of the smoke tube (18) of the furnace (17). The fan (23) is dimensioned and operated to create an overpressure in the building.