Theoretically, it is

Theoretically, it is confirmed by laboratory tests that the temperature of the flue gases in the combustion chambers is in the range from 300 to 330 C °. At this temperature in the heat exchanger, about 4-5 kW can be obtained. If a room with a fireplace is located on the same floor, but the house has other floors, as well as an attic, the air ducts should pass through these rooms.