immersion heater 
noun
An electric heating element that is positioned in the liquid to be heated, typically in a domestic hot-water tank.It is believed the cause of the fire was an electrical fault in the wiring of an electric immersion heater....
- Heating is by means of an oil-fired central heating system which is augmented by a separate immersion heater for the hot water supply.
- She has hot water because that is heated through an immersion heater.