This standard applies to gas-fired duct furnaces and unit heaters of any input for use with natural gas, liquefied petroleum gases and convertible for use with natural gas and liquefied petroleum gases. This standard covers self-contained, automatically controlled, vented unit heaters and duct furnaces intended for installation in locations where flammable gases or vapors are not generally present. Unit heaters shall have an integral means for circulation of air, either propeller fans or blowers. Unit heaters, unless for outdoor installation, are intended for installation in the space being heated. The unit heater may be either suspended or floor-mounted and of either the low- or high-static pressure types.