If your cylinder has stopped producing hot water and the boiler is working, the immersion heater is the first suspect. Here is how to tell, and what replacing one involves.
Gas Safe registered 25 years in the trade 12 years running the business
What the immersion heater does
An immersion heater is an electric element screwed into the top or side of your hot water cylinder, with its own thermostat. It heats the water directly, independent of the boiler. Many homes use it as a backup for when the boiler is off, and some rely on it entirely.
Because it is electric, it heats slowly compared with a boiler — a full cylinder can take a couple of hours. If your hot water has suddenly gone lukewarm or cold and the boiler is fine, the element or its thermostat has likely failed.
Signs the immersion heater has failed
Hot water runs out quickly or never gets properly hot on the immersion setting
The switch or fuse spur trips when the immersion is turned on
The water is scalding at the tap and the thermostat will not control it — a safety issue that needs attention
Nothing happens at all when the immersion is switched on
What replacement involves
The electrical supply is isolated and tested dead before anything is touched. For a top-entry element the cylinder usually has to be partially drained; for a side-entry one, drained below the level of the element. The old element is unscrewed — on older cylinders this can be the hardest part, as years of scale can seize it in place — and the new element and thermostat are fitted with a fresh washer.
The cylinder is then refilled, the air vented, and the heater tested. The electrical connection itself must be done safely and correctly, and we will tell you at the survey whether any part of the job needs a qualified electrician.
Repair the immersion or look at the cylinder?
If the cylinder is old, scaled up or showing other signs of age, a new element can be money spent on a tank that is near the end. Our hot water cylinder installation page explains the signs a cylinder is failing, and the hot water cylinder replacement cost guide explains what drives the price of a bigger job. We will tell you honestly which situation yours is in.
Using the immersion sensibly
Immersion heaters are expensive to run as a main source of hot water compared with a gas boiler, so most households use them as a backup. A timer or a well-set thermostat stops the element holding a full cylinder at temperature all day. If your boiler has stopped heating the water, read no hot water from the boiler before assuming the immersion is the problem.
Common questions
Immersion Heater Replacement FAQs
Clear answers before you arrange a visit.
How do I know if my immersion heater is broken?+
If the boiler is heating water normally but the immersion setting gives you cold or lukewarm water, the element or thermostat has likely failed. A switch that trips the fuse is another clear sign.
Can I replace an immersion heater myself?+
It involves electrical isolation, draining the cylinder and freeing a possibly seized element. It is doable for a confident person but easy to get wrong — a leaking element seal can flood the airing cupboard. Most people prefer to have it done.
How long does an immersion heater last?+
Years, but hard water shortens their life because scale builds on the element and makes it overheat. If yours has failed and the water in your area is hard, it is worth mentioning when you call.
Is it cheaper to heat water with the immersion or the boiler?+
For most homes the gas boiler heats water more cheaply. The immersion is best treated as a backup or used on a timer for a top-up.
