A hot water storage tank (also hot water tank, thermal storage tank, hot water thermal storage unit) is a water tank that is used for storing hot water for space heating or domestic use.

Hot water tanks may use an external heat exchanger to heat water from another energy source system. Existin on the market in either stainless steel or in composite material, hot water tanks have similar operational perfomances. However, whilst composite tanks can store water having a high concentration of dissolved minerals without altering the tank structure, corrosion will develop only after a few years on stainless steel tanks.

This also applies for dissolved oxygen in the water, which accelerates the corrosion of the stainless steel tanks but composite tanks are made with a non-corrosive raw material (LDPE) able to avoid this effect.

Water heater for washing, bathing, laundry or solar heating system must be coupled with a Structural Hot Water Tanks.