An instantaneous water heater heats DHW. In contrast to a cylinder, which keeps a certain amount of DHW ready in a storage tank, an instantaneous water heater heats water as it flows through the appliance. This means it only heats the water when hot water is actually required. Due to its high heating output, an instantaneous water heater can produce hot water continuously. Instantaneous water heaters are usually used for decentralised DHW provision. Instantaneous water heaters for heating systems are used in combi boilers.