There are two main types of water storage cylinder: direct and indirect.
Direct cylinders are a standard hot water tank found in most homes, with water sourced from a cold water tank usually in the loft or roof space. This type of cylinder is used when water for washing from the cylinder circulates directly through the boiler. There is no heating coil, so the water is heated directly by the immersion heaters located inside the cylinder – often meaning the recovery time is longer than with an indirect cylinder.
Indirect cylinders are used where water that’s used to heat the boiler and radiators is used to heat the hot water in the cylinder, but is kept totally separate from the water used for washing in the cylinder. Water from the boiler and radiators continuously re-circulates through the coil in the cylinder to heat the water, but never comes into direct contact with the water used for washing. These cylinders normally operate at around 80°C.