Backup immersion heater

A backup immersion heater is an electric heating element fitted to a domestic hot water cylinder so stored hot water can be heated directly when the main heat source cannot cover demand, is unavailable or needs support for a specific hot-water programme. In heat pump documents it may be described as an immersion heater, booster heater, backup heater, auxiliary heater or second heat source, depending on the manufacturer. Running it can use direct electricity rather than heat-pump output, so its cost and role depend on the control settings, tariff, cylinder size, water temperature targets, commissioning records and manufacturer instructions.