Abstract: A furnace having a nitrogen-heat separator comprised of a heat absorbing mass, such as cast iron plates, containing passages through which products of combustion can pass and deposit heat into the metal mass to thereafter be extracted and utilized, a baffle for directing products of combustion through the nitrogen-heat separator, a fiberglass cloth heat-retaining curtain for blocking the furnace exit and trapping heat inside the furnace when fuel combustion stops, a fan for extracting the stored heat from the nitrogen-heat separator when combustion stops and circulating it throughout the furnace interior.