In a home's central heating system, the furnace heats air, and the boiler heats water. Boilers typically use an electric switch to ignite an oil or gas supply that then heats pipes carrying water throughout the home.
Radiator and baseboard heating systems use boilers. An electric switch, usually controlled by a thermostat, is responsible for opening a natural gas, oil or propane valve that allows the fuel to enter the boiler's combustion chamber via jet spray system. An electric ignition system ignites the fuel, which begins to heat a coiled copper pipe known as a heat exchanger. The heat exchanger is further connected to a pipe carrying cold water. The heat exchanger may heat the cold water up to 140 degrees Fahrenheit, and an electric pump circulates the hot water out to the home's pipework and radiators.
The combustion process at work in conventional boilers releases excess steam through a flue vent. In a more efficient condensing boiler, the steam is condensed back into liquid water while helping to pre-warm cold water entering the boiler. The waste water is then drained out. Different types of boiler designs include open-vented boilers with hot water tanks and combination boilers that heat water on demand for both central home heating and hot faucet water.