Hot water storage

A hot water storage tank (also called a hot water tank, thermal storage tank, hot water thermal storage unit, heat storage tank and hot water cylinder) 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, compared to other substances, it can store more heat per unit of weight (and volume). Water is non-toxic and in many countries available free of charge or at low cost.

An efficiently insulated tank can retain stored heat for days, reducing fuel costs. Hot water tanks may have a built-in gas or oil burner system, electric immersion heaters, an external heat exchanger such as a central heating system, or heated water from another energy source such as a wood-burning stove, district heating system, solar collectors, a biomass heat source, or an air, water or earth source heat pump or any combination of the above. The most typical, in the domestic context, is a fossil-fuel burner supplemented by electric immersion elements, whether grid powered or using renewable generation such as wind power as an independent installation or a district heating scheme.

Largely due to the move towards smaller households in developed countries, along with high density housing and high levels of private home ownership, the trend has been increasingly away from maintaining any hot water storage tanks, opting instead for integrated central heating systems supplied by combination boilers that can supply mains pressure hot water on demand.

Water heaters for washing, bathing, or laundry have thermostat controls to regulate the temperature, in the range of 40 to 60 °C (104 to 140 °F), and are connected to the domestic cold water supply.

Where the local water supply has a high content of dissolved minerals such as limestone, heating the water causes the minerals to precipitate in the tank (scaling) and a tank may develop leaks due to corrosion after only a few years, a problem exacerbated by dissolved oxygen in the water which accelerates corrosion of both tank and fittings.

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