A warm air furnace is a device in which gaseous or liquid fuel is directly fired or electric resistance heaters are used to heat the warm supply air. Natural gas, liquefied petroleum gas, oil, electric energy, or occasionally wood may be used as the fuel or energy input. There are two types of combustion systems in a natural gas-fired warm air furnace: natural vent or power vents combustion systems. In a natural vent or atmospheric vent combustion system discharging from the chimney or vent pipe. A hot water boiler is an enclosed pressure vessel used as a heat source for space heating in which water is heated to a required temperature and pressure without evaporation. The sizing of low-temperature hot water pipes is usually based on a pressure drop of 1 to 3 feet per 100 feet of pipe length. For a small low-temperature hot water heating system, an open-type expansion tank is often used.