Last edited 29 Mar 2019

Hot water


Hot water is an essential building service used for washing, cleaning, drinking, cooking, heating and so on. It requires energy to heat water, and the type of hot water system installed in a building has a direct impact on its energy consumption. The main design considerations are the fuel type that will be used, whether the hot water will be generated locally or centrally and whether it will be stored, or generated on demand.

The two components of a typical hot water system are:

  • The central heating system which supplies hot water to radiators located around a building. Radiators are heat exchanging devices that use the heat from hot water (or sometimes steam) to warm the surrounding space.
  • Domestic hot water supplied to taps and sometimes to appliances. Section G3 of Approved Document G defines ‘domestic hot water’ as water that has been heated for cooking, food preparation, personal washing or cleaning purposes. This can be generated instantaneously when there is a demand, or stored in a hot water cylinder/tank. The term 'domestic' is used irrespective of the type of building in which the hot water system is installed (ie it does not have to be a domestic building).

The water and pipework used for these two systems are not mixed, although both share some of the same components.

Boilers can be used to heat water to supply a central heating system, or domestic hot water, or both. Boilers are generally fuelled by mains gas, but they can also be fuelled by liquid petroleum gas (LPG), wood, coal, oil, or electricity.

Electric heating systems commonly heat up a hot water tank overnight using off-peak electricity and store it for use during the day. Immersion heaters are electric water heaters that heat water in hot-water cylinders in a similar way to a kettle. They can provide a building’s hot water and can also be used as a back-up for boilers.

For more information see: Boilers.

Heat pumps can also be used domestically or commercially to provide hot water. Heat pumps transfer heat from a lower temperature source, such as the air, ground or water, to one of a higher temperature.

For more information see: Heat pumps.

Micro-CHP, or micro combined heat and power, is the small-scale generation of heat and electricity from a single energy source. Micro-CHP is becoming more common in domestic buildings, where it can be installed as direct replacement for gas-fired boilers, as micro-CHP units (sometimes referred to as CHP boilers) are similar in size and shape to conventional domestic boilers and can be floor standing or wallhung.

For more information see: Micro-CHP.

District energy (DE) is the process of heating and / or cooling a group of buildings from a central thermal energy generation plant(s) via a network of fluid distribution pipes. It is widely used for urban environments including residential, commercial, local authority, government, and industrial buildings. It is also used extensively for universities and hospitals where there are a variety of discrete buildings located around a campus. District energy is an alternative to the more traditional installation of individual heating or cooling plants in each building.

For more information see: District energy.

Other forms of heating hot water include solar thermal panels, geothermal energy and so on.

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I’m planning to buy a thermal store. I really want to change the way I manage my heat source and maximise the use of my solar panels, so I’m researching a lot before placing my purchase. Thanks for this, really a huge help.

Good luck and thanks for your kind feedback! Ed.