A residential boiler system is used to heat a household. The boiler heats water inside a tank and distributes this hot water (or, in some cases, steam) via pipes, radiators and/or a radiant in-floor system in order to heat the inside of a home. Once the water has cooled and can no longer provide heat, it is sent back to the boiler to be reheated.
Although the name might suggest otherwise, modern-day boilers do not boil water. The term boiler comes from the fact that, in the past, steam boilers were common. Steam boilers boil water in order to create steam for heating purposes. Today, however, most boilers are water heating systems that distribute hot water, rather than steam, in order to heat a house.