Hot water storage

A hot water storage tank (also called a hot water cylinder or hot water tank) is a water tank used for storing hot water for space heating or domestic use.

Water is a convenient heat storage medium because it has a high specific heat capacity. This means that, compared to other substances, water can store more heat per unit of weight.

How it works

Water storage tanks or cylinders can be heated directly or indirectly.

Directly heated storage tanks are a combination of a vessel and a heater, for example a burner, an electric resistance heater or an air-source heat pump.

Indirectly heated storage tanks, on the other hand, incorporate one or more internal heat-exchangers, which are connected with external heaters, like heat pumps, gas, oil or biomass boilers, solar panels, district heating or combinations of those.

A specific storage technology is the stratified storage tank, which is often used in solar systems. This technology is specifically designed to divide the water it contains into layers at different temperatures (hence ‘stratified’). The upper part of the tank will contain warmer water, which can be used as domestic hot water; the lower and colder part of the tank receives the heat coming from the solar collectors. Between the cold and the hot parts of the tank lies a second heat exchanger which can – if necessary – heat lower layer of water.

Demand response

Hot water cylinders can also help provide demand response services to the grid. How? An efficient hot water storage tank allows consumers to heat water with electricity, consuming the electricity when prices are lower. Energy is then stored in the tank in the form of hot water, ready to be used for washing or to heat the house when it is needed.


  • Great comfort by supplying hot water for simultaneous use, for example when several showers are running at the same time;
  • Flexibility of use: a hot water storage tank provides hot water almost immediately;
  • The water in the cylinder will be released exactly at the desired temperature;
  • Well-insulated hot water tanks are a great way to store energy: they can be coupled with electric resistance or a heat pump, and run when electricity is most abundant and inexpensive;
  • A hot water storage tank can help reduce energy consumption. This is because it takes less energy to keep water warm, once it has already been heated, than it takes to heat cold water.