Hot water storage tank
（重定向自Hot water cylinder）
![2 red parallelled hot water storage tanks connected to a wood-fueled furnace.](/Images/godic/202501/18/Heatingsystem_for_wood5553.jpg")
![A hot water storage tank where one of the heat sources is solar heating A, that is sent into the hot water storage tank via a smaller pump B (circle with triangle) and the heat exchanger spiral in the hot water storage tank. The other spiral C can be used for a e.g. oil-fired boiler or a wood burner. At D the hot water gets out and domestic cold water is sent back at the bottom at E.](/Images/godic/202501/18/InstalacjaSlonecznaCWU.svg5553.png")
![A hot water storage tank where one of the heat sources is solar heating. Almost the same example as above, but in a domestic habitat.](/Images/godic/202501/18/Принципиальная_схема_установки_солнечного_теплоснабжения_RKraft.svg5553.png")
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.