Immersion heater not working: what to check

Usually a switch or a timer. Check those first.

The answer in 2 lines. When an immersion heater stops heating, the cause is often simple: its wall switch is off, the timer or Economy 7 settings have changed (after a power cut or clock change), or its circuit has tripped at the consumer unit. Check these with dry hands. If the water is still cold after a full heating period, the element or its thermostat has probably failed: do not take the cover off the cylinder, it hides live wiring. Call a registered electrician or a heating engineer.

The first 3 steps

Check the immersion's wall switch (often a red switch, sometimes marked "water heater") is on, and its light is lit if it has one.
Check the timer: right time of day, on periods set, not on "off". After a power cut or clock change, it may need resetting. Try the boost button if it has one.
At the consumer unit, with dry hands and nothing wet, see if the immersion's breaker has tripped. Reset it once. If it trips again, leave it off.

Stop: danger

Water near electrics, a burning smell, scorch marks or a hot switch: do not touch anything. If the consumer unit is dry, switch off the main switch; call a registered electrician. Smoke or fire: get out and call 999.
Never take the cover off the immersion heater to press a reset or adjust the thermostat yourself: there are live parts and very hot water behind it.
Unvented cylinder (no tank in the loft) with hot water pouring into the tundish (the small open funnel on the pipework): switch off the immersion and the boiler, and call a qualified unvented cylinder installer (G3-qualified).
Cylinder leaking: switch off the immersion at its switch, turn off the cold water feed to the cylinder or the stop tap, and call a plumber.

What you see, and what it usually means

No hot water at all: switch, timer, tripped breaker, or a failed element.
Hot water only when you press boost: the off-peak (Economy 7) side or its timer is not working. Check the timer and, on a dual-rate meter, ask your supplier or an electrician to check the off-peak supply.
Lukewarm water that runs out quickly: one of two elements has failed (many cylinders have a top and bottom one), a thermostat set too low, or heavy limescale.
Breaker trips when the immersion comes on: often a failing element. Leave it off and call an electrician.
Hot water from the boiler but not the immersion (or the other way round): they are separate; the immersion is only the electric backup on many systems.

The timer and Economy 7

On an Economy 7 tariff, the immersion usually heats the cylinder at night, on cheaper electricity, and boost gives an extra hour or so in the day.
Check the timer shows the correct time: after the clocks change, it may still be on the old hour. Your meter's off-peak hours are on your bill or from your supplier.
A full cylinder can take several hours to heat. Wait one full night before deciding it is broken.
Tips for getting the most from Economy 7: National Energy Action.

What not to do

Resetting a breaker that keeps tripping.
Opening the immersion cover to "reset the thermostat" yourself.
Leaving the immersion on permanently or on boost all day: it is expensive.

Who to call and who pays

Registered electrician for breakers, wiring, switches, timers and elements. Find one on the government-approved Registered Competent Person Electrical register (search by postcode and choose one registered for domestic work; their checklist), or Electrical Safety First for an emergency.
Plumber or heating engineer for a leaking cylinder; a G3-qualified installer for an unvented cylinder.
If you rent: your landlord must keep the hot water installation in repair (Landlord and Tenant Act 1985, section 11). Report it in writing; Shelter England explains what to do if repairs are not done. Council and housing association tenants can also rely on Awaab's Law timescales for emergency hazards (GOV.UK).

Good to know

Stored hot water should be kept at about 60°C to limit the growth of Legionella bacteria (HSE). Do not turn the cylinder thermostat right down to save money.
Limescale shortens the life of immersion elements in hard-water areas; your water company's website shows how hard your water is.

In Brighton, Chloe has no hot water on the Monday after the clocks go back. The immersion's switch is on and nothing has tripped, but the timer is an hour out and its on period has slipped past the off-peak window. She resets the clock, presses boost for an hour, and next morning the cylinder is hot again.

A made-up story, for illustration.

Bobby

Sources, verification, confidence

Verified on 1 November 2026, for England. Confidence: high for the safety steps and the landlord's duty to repair hot water in England; medium for typical causes, which depend on your cylinder and wiring, and for the 60°C figure, which comes from HSE guidance on Legionella written mainly for workplaces and landlords. We found no official statistics on immersion heater faults (checked 1 November 2026). This guide describes common cases, not a diagnosis.