Abstract: A furnace shell has a fire pot centrally located therein, the walls of the shell and fire pot being spaced to form a fluid heat transfer chamber therebetween. Heat transfer is effected between the pot and the chamber to heat the fluid medium in the chamber. The shell is located in an interior to be heated, such as a building or home, including mobile and motor homes. An inlet conduit has a first end exteriorly positioned of the building interior and a second end substantially pneumatically sealed from the interior and in fluid communication with the fire pot enclosure to provide combustion supporting fluid exclusively from the exterior. A flue has a first end positioned exteriorly of said interior and a second end positioned in the upper portion of the fire pot enclosure to conduct combustion gases to the exterior. The flue is pneumatically sealed from the interior to prevent combustion gases from entering the interior.