Hot water cylinders are found in many households and are used in mains-pressure and gravity-fed heating systems. The primary role of a hot water cylinder is to store water and heat it via immersions or a coil. Generally, an indirect hot water cylinder will be heated by a coil with an immersion heater(s) as a back-up heat source. A direct hot water cylinder would be heated exclusively by immersion heaters. Cylinders are mostly made from stainless steel, although copper hot water cylinders are also popular.