Circulating hot water can be used for central heating.
Sometimes these systems are called hydronic heating systems.15 Common components of a central heating system using water-circulation include: A supply of fuel, electric power or district heating supply lines A Boiler (or a heat exchanger for district heating) which heats water in the system Pump to circulate the water Radiators which are wall-mounted panels through which the heated water passes in order to release heat into rooms. The circulating water systems use a closed loop; the same water is heated and then reheated.A sealed system provides a form of central heating in which the water used for heating circulates independently of the building's normal water supply. An expansion tank contains compressed gas, separated from the sealed-system water by a diaphragm.
This allows for normal variations of pressure in the system.
Sealed systems offer an alternative to open-vent systems, in which steam can escape from the system, and gets replaced from the building's water supply via a feed and central storage system. Heating systems in the United Kingdom and in other parts of Europe commonly combine the needs of space heating with domestic hot-water heating.These systems occur less commonly in the USA.
In this case, the heated water in a sealed system flows through a heat exchanger in a hot-water tank or hot-water cylinder where it heats water from the regular potable water supply for use at hot-water taps or appliances such as washing machines or dishwashers. Expansion tank in a sealed system Hydronic radiant floor heating systems use a boiler or district heating to heat water and a pump to circulate the hot water in plastic pipes installed in a concrete slab.
Hydronic heating systems are also used with antifreeze solutions in ice and snow melt systems for walkways, parking lots and streets.
They are more commonly used in commercial and whole house radiant floor heat projects, whereas electric radiant heat systems are more commonly used in smaller "spot warming" applications.Źródło: https://en.wikipedia.org/wiki/Central_heating#Water_heating
A boiler that has a loss of feed water and is permitted to boil dry can be extremely dangerous. If feed water is then sent into the empty boiler, the small cascade of incoming water instantly boils on contact with the superheated metal shell and leads to a violent explosion that cannot be controlled even by safety steam valves. Draining of the boiler can also happen if a leak occurs in the steam supply lines that is larger than the make-up water supply could replace.
The Hartford Loop was invented in 1919 by the Hartford Steam Boiler and Insurance Company as a method to help prevent this condition from occurring, and thereby reduce their insurance claimsŹródło: https://en.wikipedia.org/wiki/Boiler#Safety
This connection is more often found in bathrooms and sometimes requires an additional connection to the existing water supply and sewerage.