an outdoor furnace including a conditioned air chamber surrounding a fire box and having an air inlet and outlet connected to the space to be heated within the building . the furnace is characterized by a pre - heat chamber for the combustion inlet air as well as deflectors and baffles for maximizing contact of conditioned air flow over the fire box for maximizing the efficiency of the furnace . the furnace may also include an after - burner chamber and a damper for varying the degree of combustion within the after - burner chamber . the furnace also preferably includes an exhaust damper having a thermal control for exhausting excessively heated air from the conditioned chamber .