Immersion heater not working: checks and likely faults

How an immersion heater works

An immersion heater is an electric element that sits inside your hot water cylinder, a bit like the element in a kettle. It has a thermostat that switches it off when the water reaches the set temperature, and a separate safety cut-out that switches it off if the water gets far too hot. Some cylinders have one immersion at the top; many have two, a lower one used overnight on a cheaper tariff and an upper one for a quick boost.

In some homes the immersion is the only way the water gets heated. In others it is a backup to a gas boiler that normally heats the cylinder through a coil. Knowing which you have matters: if the boiler usually heats your water and the taps have gone cold, the fault is more likely to be the boiler, the programmer or a zone valve than the immersion.

Checks you can safely make

These checks use only the normal switches and controls. None of them involves taking off the immersion cover, which you should leave alone.

• Switch: find the switch near the cylinder, often labelled water heater. Make sure it is on and, if it has a light, that the light is lit.
• Timer: if the immersion runs on a timer or programmer, check the clock and the on times. A power cut or the clocks changing can leave it heating at the wrong time or not at all.
• Off-peak tariff: on Economy 7 and similar tariffs the lower element only heats overnight. If you have run out of hot water during the day, try the boost switch rather than assuming a fault.
• Consumer unit: look for a tripped breaker or RCD. You can reset it once. If it trips again, leave it off and call us, because repeated tripping usually means an electrical fault in the element.
• Boost test: if you have two elements, switch to boost for an hour. If the water heats on boost but not overnight, the lower element or the off-peak supply is the likely cause.
• Temperature: run a hot tap for a minute. Lukewarm water that never gets hot points to a different fault from water that stays completely cold.

Do not remove the immersion cover to press the reset on the thermostat. There are live terminals under the cover and the cut-out trips for a reason. Resetting it without finding the cause can leave the water overheating.

What the symptoms usually mean

The pattern of the problem narrows the cause down a long way before anyone arrives. Tell the engineer which of these you are seeing.

Element, thermostat or cut-out?

The element is the part that most often fails. It is a metal sheath that sits in water all day, and London's hard water coats it in scale, which makes it run hotter and shortens its life. When it fails it either stops heating or starts tripping the electrics. An engineer confirms this with electrical tests rather than guesswork.

The thermostat controls the temperature and can fail so that it never switches the element on, or never switches it off. On most modern immersions it is combined with the safety cut-out, which trips if the water gets dangerously hot. A tripped cut-out is a symptom, not the fault: something made the water overheat, and that needs finding before the heater goes back into use.

The switch and wiring are worth checking too. A switch that is scorched, crackles or feels warm should be left off, and so should any wiring that smells of burning. Cylinder thermostat faults covers the controls on boiler-heated cylinders, which behave in a similar way.

What replacing an immersion involves

A thermostat can usually be changed without draining anything. Changing the element is a bigger job: the cylinder has to be isolated and drained below the element, the old element unscrewed with a special spanner, and the new one fitted with a new seal before the cylinder is refilled and tested. Old elements can be seized in place, and on an older copper cylinder too much force can damage the boss the element screws into. That is one reason this is a job for someone who does it regularly.

The replacement has to suit the cylinder. Unvented cylinders in particular need the element and controls specified by the manufacturer, with the correct safety cut-out, because the immersion is part of the cylinder's safety arrangement. Our immersion heater repair service covers diagnosis, element and thermostat replacement and the checks afterwards.

When the cylinder is the real problem

Sometimes a failed immersion is the first sign of an old cylinder reaching the end of its life. Signs include water around the base, corrosion around the immersion boss, discoloured hot water or a cylinder that has needed several repairs. If you think you have a hot water cylinder leaking, turn off the immersion at the switch and read that guide first.

If the cylinder is sound, a new element or thermostat is normally a sensible repair. If it is not, replacing a hot water cylinder may be better value than repairing it, and it is a good moment to think about whether the immersion should remain your main source of hot water.

Getting it fixed

Switch the immersion off and call if the electrics trip, if the water is scalding or steaming, if water is leaking from the cylinder or the immersion, or if you can smell burning. Otherwise, a cold cylinder is inconvenient rather than dangerous, and an engineer can usually diagnose it in one visit.

Loyal Plumbers has 10 vans covering London and everywhere inside the M25, 24 hours a day. When you get in touch, tell us whether the immersion is your main hot water or a backup, whether you have one element or two, what the switch light is doing and whether anything has tripped. Our call-out charge is £60, and the engineer explains what has failed and what the repair involves before any work goes ahead.