An immersion heater not working is a cold shower and, in a flat with no gas, no hot water at all. The good news is that the fault is nearly always in one of four places, and one of them can be reset in a minute. This guide sets out what to check before you call anyone, how to tell a thermostat fault from a dead element, whether the job belongs to an electrician or a plumber, and what an appliance repair visit costs in Croydon.
How an immersion heater works
An immersion heater is a 3 kW electric element that screws into the top or side of a hot water cylinder, with a thermostat in the same housing that switches it off when the water reaches the set temperature, usually 60 to 65 degrees. It is fed by its own circuit from the consumer unit through a double-pole switch, often with a neon, and in many homes through a timer or an Economy 7 controller. Some cylinders have two elements: a short one at the top for a quick boost and a long one at the bottom for an overnight heat on the cheap tariff.
Built into the thermostat is a thermal cut-out that trips if the water gets too hot, to prevent scalding and damage to the cylinder. It is resettable with a small button under the cap, and a tripped cut-out is the most common cause of an immersion heater not working.
Checks before you call: immersion heater not working
Work through these in order. The first three need nothing but your eyes; the fourth means switching the circuit off first.
- Confirm the switch is on. The wall switch is often in an airing cupboard and gets knocked off by a stack of towels. A dead neon with the switch on suggests a tripped breaker.
- Check the consumer unit for a tripped breaker or RCD. Reset it once. If it trips again straight away, the element has failed and is leaking to earth; leave it off and call an electrician.
- Check the timer. Economy 7 and programmable timers lose their settings after a power cut, and clocks drift. A boost button on the controller will tell you within an hour whether the heater works when told to.
- Reset the thermal cut-out. Switch the circuit off at the consumer unit, unscrew the cap on the immersion head and press the small reset button, then screw the cap back on and switch on. If the cut-out trips again within a day, the thermostat has failed and is letting the water overheat.
Do not remove the cap with the power on. If you are not confident isolating the circuit, stop at step three.
Thermostat, element or timer: telling them apart
A failed thermostat shows itself in one of two ways. Either the water never heats because the thermostat has gone open circuit, or it heats too far, trips the cut-out and you get scalding water followed by cold. Replacement thermostats are a cheap part and a short job for an electrician.
A failed element is different. Elements in hard water areas furr up with limescale, which insulates them, makes them run hotter and eventually splits the sheath. Water gets into the element, it leaks to earth and the RCD trips.
Replacing it means draining the cylinder below the element, unscrewing the old one with a box spanner, fitting a new one with a fresh washer, then refilling and reconnecting. Many plumbers do the mechanical part, but the electrical connection is electrician territory, and a single engineer who does both saves a second call-out. A timer fault is the cheapest to confirm and the most variable to fix.
What is different in Croydon
Croydon has a high proportion of homes that rely on immersion heaters full time. The tower blocks and low-rise flats in the town centre, New Addington, South Norwood and Thornton Heath were built with electric heating and no gas supply, so the cylinder is the only source of hot water and a failure is urgent rather than inconvenient. Economy 7 is common across these estates and timer faults make up a large share of calls. The 1930s semis of Shirley, Purley and Coulsdon usually have gas heating with the immersion as a backup that has not been switched on for years, so it fails the first time it is needed.
South London’s water is hard, drawn from the chalk, and limescale kills immersion elements faster here than in soft water areas. Prices across Croydon follow London rates, which run above the national average. Our booking day checklist for washing machine repair in Croydon covers the practicalities of a visit that apply to any appliance call.
What the repair costs
As an indicative range, a call-out with diagnosis is £60 to £100, and a completed repair including parts is £100 to £250 all in, with Croydon at the upper end. A thermostat swap sits at the bottom of that range because the part is inexpensive and the cylinder does not need draining. An element replacement sits at the top: the part is £20 to £60, but draining and refilling takes longer and a seized element can turn a one-hour job into two. A new timer is £40 to £120 as a part plus fitting.
Electrical work on the immersion circuit should be done by an electrician registered with NICEIC or NAPIT, and Electrical Safety First explains how to check a registration. Replacing the cylinder itself is a much larger job, from £800 upwards, and only worth considering if it is leaking or badly scaled. Tenants should report a failed immersion to the landlord, whose duty to keep installations for water heating in repair under section 11 of the Landlord and Tenant Act 1985 covers it; our Bromley guide to who pays for appliance repair sets out the rules.
When to call a professional
Call when the breaker will not stay reset, when the cut-out trips again after a reset, when the cylinder or pipework is leaking, or when the checks above have not found the cause. A tripping RCD means an element leaking to earth and the circuit should stay off until it is fixed. For a smell of burning from the airing cupboard switch the circuit off and call an electrician; for fire call 999.
Our help desk reviews enquiries about an immersion heater not working, confirms availability and checks credentials before anything is agreed. It is free, and nothing is booked until the team confirms. Request help here.
Questions people ask
Where is the reset button on an immersion heater?
Under the plastic or metal cap on the head of the immersion, where it screws into the cylinder. Switch the circuit off at the consumer unit, remove the cap and look for a small button, often red, next to the thermostat adjustment. Press it firmly until it clicks, replace the cap and switch back on. If it trips again within a day the thermostat has failed and needs replacing.
Do I need an electrician or a plumber for an immersion heater?
The electrical side, including the thermostat, the wiring, the switch and the fault that trips a breaker, is a job for an electrician registered with NICEIC or NAPIT. Replacing the element means draining the cylinder, which plumbers and many appliance engineers also do. The best option is a single tradesperson who does both, so you are not paying two call-outs for one fault.
How much does it cost to replace an immersion heater element?
As an indicative range, a call-out with diagnosis is £60 to £100 and a completed element replacement including the part is £100 to £250 all in, with Croydon and the rest of London at the upper end. The element itself costs £20 to £60; most of the bill is the time taken to drain the cylinder, remove a scaled element and refill. A thermostat alone is cheaper.
Why does my immersion heater keep tripping the electrics?
An element that trips the RCD or breaker has almost always split its outer sheath, usually because limescale made it overheat, and water is reaching the live conductor. Resetting it repeatedly is unsafe. Leave the circuit off and have the element replaced. If the trip happens only when the thermal cut-out operates, the thermostat is overheating the water and needs replacing instead.
Ready to take the next step?
Tell the household desk what has happened, where you are and when you need help. We will review it and explain your options.
Describe your job →Guidance is general and refers to the authority that sets each rule. Costs are indicative ranges; always get written quotes. Trade availability and credentials are confirmed when you enquire.
