Boilers

What is a Boiler?

If you want hot water you need to have a boiler. In basic terms, a boiler is a container that heats water therein. This is usually accomplished by electricity or the burning of natural gas, wood or coal. For heating water on a large scale nuclear fission can be used to create steam.

Methods of heating water

Water for space heating may be heated by fossil fuels in a boiler. Potable water may be heated in a separate appliance: this is common practice in the USA where warm-air space heating is usually employed. The most useful heating transmitter is known as the radiator.

Gravity system
Where a space-heating water boiler is employed the traditional arrangement in the UK is to use boiler-heated ("primary") water to heat ("secondary") water in a cylindrical vessel (usually made of copper) containing potable water supplied from a cold water storage vessel/container, usually in the roof space of the building. This produces a fairly steady supply of DHW at low static pressure head but usually with a good flow. Water heating appliances in most other parts of the world do not use cold water storage vessel/container but heat water at pressures close to that of the incoming mains water supply.

On-demand water heaters (Instant hot water dispenser)
Stand-alone appliances for quickly heating water for DHW (Domestic Hot Water) are known in North America as tankless heaters, elsewhere as multipoint heaters, geysers or Ascots. In Australia and New Zealand there was a similar wood fired appliance known as the chip heater.
A common arrangement where hot-water space heating is employed is for the boiler to also heat potable water giving a continuous supply of DHW without any extra equipment required. Appliances capable of supplying both space-heating and DHW are known as combination (or "combi") boilers. Although on-demand heaters can give a continuous supply of DHW, the rate at which they can produce it is limited by the thermodynamics of heating water from the available fuel supplies.

Storage systems
Another popular arrangement where higher flow rates are required (although for limited periods) is to heat water in a pressure vessel capable of withstanding a hydrostatic pressure close to that of the incoming mains supply. (A pressure reducing valve is usually employed to limit the pressure to a safe level for the vessel.)