Immersion heater not working: causes and repairs

An immersion heater is an electric element that sits inside a hot water cylinder, a bit like the element in a kettle. In some homes it is the main way of heating water. In others it is the backup for when the boiler is off. When it stops working, the fix is usually simple, but it is worth knowing what to check first.

Quick checks

1. Is it switched on? Immersion switches are usually near the cylinder or in the kitchen. Some have a red light. Some have two switches, labelled “bath” and “sink”, for two elements.
2. Is there a timer? Many immersions are on a timer to use cheaper overnight electricity. Check the time and settings, especially after a power cut or the clocks changing.
3. Has the fuse or breaker tripped? Check the consumer unit. An immersion is usually on its own circuit. If the breaker trips again when you switch it back on, stop and get an electrician or plumber, as there may be a fault in the element.
4. Is the thermostat set right? Under the cover on top of the element there is a thermostat dial. It should usually be around 60°C. Turn the power off at the switch before taking the cover off.
5. Try the reset button. Many immersion thermostats have a small red reset button. With the power off, press it. If it clicks, it had tripped from overheating. If it keeps tripping, the thermostat or element is failing.

Why elements fail

Scale in harder water

This is the biggest cause in parts of Sheffield and almost all of Rotherham. When water is heated, calcium in it forms scale on the hot surface. On an immersion element, that scale builds up as a crust. The element gets hotter and hotter trying to heat the water through it, and eventually burns out.

Yorkshire Water’s 2025 water quality report shows how much this varies:

Area	Supply zone	Type	Average calcium (mg/l)
Crookes, Walkley, Fulwood	Fulwood	Soft	9.5
Dore, Totley, Ecclesall	Ringinglow	Soft	9.3
Handsworth, Woodhouse, Aston	Aughton and Woodhouse	Moderately hard	77.0
Rotherham centre, Broom, Moorgate	Rotherham and Tinsley	Hard	89.9

So an element in Handsworth faces about eight times as much calcium as one in Crookes. Expect to replace it more often, and expect it to have a thick crust when it comes out. Our Handsworth area page covers the other effects of harder water there.

Age and wear

In soft water areas such as west Sheffield, scale is rarely the reason. Elements still wear out with age, and thermostats fail. The element can also corrode if the cylinder’s own protection, a small rod called an anode in some cylinders, has worn away.

Thermostat faults

The thermostat switches the element on and off. If it sticks off, the water stays cold. If it sticks on, the water gets too hot and the overheat cut out trips. Either way the thermostat needs replacing.

Signs it is the element or thermostat

• The switch is on, the breaker is fine, and the water stays cold.
• The breaker trips when you switch the immersion on. That often means a failed element.
• The water is scalding. That often means a stuck thermostat. Turn the immersion off and get it checked.
• The reset button keeps tripping.
• You can hear crackling or fizzing from the cylinder, a sign of heavy scale.

What the repair involves

Replacing an immersion element usually means:

1. Turning off the power and isolating the circuit.
2. Turning off the cold feed and draining the cylinder below the element.
3. Unscrewing the old element with a special spanner. In old copper cylinders this needs care, as the cylinder can be thin.
4. Fitting a new element and thermostat with a new seal.
5. Refilling, checking for leaks and testing.

In a hard water area, it is worth asking for an element designed for scale, sometimes sold as a long life or titanium element. If the cylinder is old and the copper is thin, the plumber may advise replacing the cylinder rather than risking a leak when the old element comes out.

When to think about a new cylinder

Replacing the immersion is usually the answer. A new cylinder makes more sense if:

• The cylinder is leaking or weeping at the base or seams.
• The copper is badly corroded around the element boss.
• You want better showers and your mains flow is good enough for an unvented cylinder.
• You are changing your boiler and the whole system is being redesigned.

Our hot water cylinders page explains the choices, and our post on Sheffield water hardness covers what your local water means for your appliances.

If your immersion has stopped, tell us your postcode, the cylinder type and what you have checked. We will tell you what we expect it needs.