Immersion heaters are electric heating elements that are found in water cylinders to heat water. They act like a kettle to heat the surrounding water in the cylinder. The benefit of an immersion heater element within your heating system is that it means even the boiler fails, you should still be able to get hot water. It should also have its own power supply which means that it can be turned on and off when needed so that water is not constantly being heated unnecessarily. Look for thermostatic immersion heaters that switch themselves off when the water reaches a desired temperature for even more convenience.