Low Pressure Heating Boiler

A unit consisting of a series of metal shells and/or tubes assembled in integral form, utilizing a space enclosed within itself as a combustion chamber for burning fuel. The heat emanating from the burning fuel is absorbed by water circulating through the shells and/or tubes, raising the water temperature to a predetermined degree or changing its physical properties to steam. Excludes high pressure steam boilers over 15 pounds (6.804 kg) gauge working steam pressure.