Oil-fired boilers have been in use to heat buildings for more than 100 years but are rarely installed with new residential heating systems. These systems, which operate on steam and hot water, tend to be costly to maintain and aren't energy-efficient compared to newer heating systems.
Oil-fired, hot water (hydronic) boiler systems use steam or hot water to deliver heat through a heat distribution system, which includes fans, pumps, ducts and piping, to supply heat to each room through baseboards or radiators. These systems typically operate from a room thermostat.
The boiler is a large container made of steel, copper or cast iron, which holds the water. When the boiler burns oil, it creates hot gases used to heat the water inside and deliver heat through a series of pumps.
With a steam or hot water boiler, the boiler heats water to a boiling point causing steam to rise through a series of steam riser pipes and radiators to the rooms needing heat. At the radiator, the steam condensate is collected, and flows by gravity back to the boiler to be reheated. The condensate can either return to the boiler through the same pipes (one-pipe system) or through a system of return piping (two pipe system).