An immersion heater that has stopped working is one of those domestic problems that makes itself known immediately. No hot water means no shower, no washing up, and a rapidly cooling cylinder. The good news is that immersion heater faults follow a fairly predictable pattern, and understanding them helps you decide whether you need an electrician urgently or whether there is a simple check you can carry out yourself before making that call.
This guide works through the most common causes in order of how easy they are to identify, starting with the quick checks and moving through to the faults that require a qualified electrician.
How an Immersion Heater Works
Before looking at faults, a brief explanation of how the system works helps make sense of what can go wrong.
An immersion heater is an electric heating element fitted into a hot water cylinder. When energised, the element heats the surrounding water directly, much like a kettle element heats water in a kettle. The temperature is controlled by a thermostat, typically set between 60 and 65 degrees Celsius, which cuts off the supply to the element once the set temperature is reached and allows it to restart when the water cools below a lower threshold.
The immersion heater is supplied by a dedicated circuit from the consumer unit, protected by its own MCB or RCBO. A double-pole isolator switch, mounted on the wall near the cylinder, provides a local means of isolation. Between the isolator and the element itself, a length of heat-resistant flex carries the supply. On the element head, the thermostat and in many cases a thermal cut-out device are accessible behind a small cover.
Many properties, particularly those on Economy 7 or other off-peak tariffs, have two-element cylinders. A lower element, typically 3kW, heats the full cylinder volume overnight using cheap off-peak electricity. An upper element, typically 1kW or 3kW, heats only the upper portion of the cylinder quickly for a daytime boost when the overnight charge has been depleted. Each element has its own thermostat and is supplied by its own circuit or switching arrangement.
The Quick Checks: Start Here Before Calling Anyone
Check the Consumer Unit
The first thing to do when the hot water fails is to look at the consumer unit. The immersion heater circuit will be labelled, typically as “immersion” or “water heater”, and will have its own MCB or RCBO. If that device has tripped to the middle or off position, the circuit has lost power.
Reset the device by switching it fully off and then back on. If it stays on, power has been restored to the circuit and the problem may have been a transient fault. Go and check whether the cylinder is heating up as expected.
If the MCB or RCBO trips again immediately on reset, or within a short time of resetting, do not continue to reset it. A repeatedly tripping circuit is telling you that a fault exists on the circuit, most likely in the element itself or in the wiring, and continuing to reset protective devices against a live fault is not safe. This needs investigation by a qualified electrician.
Check the Thermal Cut-Out
Many immersion heaters are fitted with a thermal cut-out, sometimes called an overheat protector or high limit thermostat, in addition to the normal working thermostat. The thermal cut-out is a safety device designed to disconnect the element if the water temperature significantly exceeds the thermostat setting, which would happen if the thermostat failed to cut off at the correct temperature.
The thermal cut-out is a small button, usually red or white, located on the immersion heater head behind the cover plate. To access it, switch off and isolate the immersion heater at the isolator switch, remove the cover plate on the heater head (usually held by one or two screws), and look for the small reset button. Press it firmly until you feel or hear a click.
Replace the cover, restore the isolator, and switch on. If the heater now works, the thermal cut-out had operated because the water overheated. This is a symptom rather than a cure: the reason the water overheated was most likely that the working thermostat has partially or fully failed and allowed the temperature to rise beyond its set point before the cut-out intervened. The thermostat should be assessed and replaced by a qualified electrician to prevent the situation recurring.
Check the Timer
If the circuit is live and the thermal cut-out has not tripped, the next check is the timer. An immersion heater controlled by a mechanical or electronic timer will only heat the water during the programmed periods. A timer that has lost its programme after a power cut, a mechanical timer whose wheel or pins have seized, or a timer that has simply been set incorrectly will result in a heater that is electrically sound but never switched on.
Check the timer display or dial. Is the current time set correctly? Are the on and off periods programmed as expected? For a mechanical timer, does the override lever work when pressed manually? If operating the override or switching the timer to manual on produces heat, the timer programme or the timer unit itself is the fault rather than the heater.
Electronic timers that have lost their programme after a power cut will need reprogramming. If the timer display is blank or showing an error, check whether it has its own battery backup and whether that battery needs replacing.
Fault 1: Thermostat Failure
If the quick checks above have not identified the problem, the most common fault in a working-age immersion heater is thermostat failure. The thermostat is a relatively simple bimetallic or electronic device that has a finite service life. It can fail in one of two ways.
A thermostat that fails open circuit stops the element from heating at all, even when the water is cold. The heater appears completely dead despite the circuit being live, the cut-out being in, and the timer being on. Replacing the thermostat resolves the fault.
A thermostat that fails closed circuit allows the element to run continuously, heating the water beyond its set temperature until the thermal cut-out operates. This produces the reset cut-out scenario described above.
Thermostat replacement is a job for a qualified electrician. The thermostat sits inside the immersion heater boss in contact with the water, and accessing it requires the heater to be isolated and the boss to be partially withdrawn. In a pressurised unvented cylinder, this also requires the cylinder to be partially drained and the pressure to be released before the thermostat can be safely accessed. A plumber may be involved in this work depending on the type of cylinder installed.
Fault 2: Element Failure
The heating element is the component that does the actual work, and it has a finite lifespan. In areas with hard water, which includes much of Hampshire, Surrey, and Berkshire, limescale builds up on the element surface over time, progressively reducing its efficiency and eventually causing failure. An element that has been in service for ten to fifteen years in a hard water area is approaching or at the end of its expected life.
Element failure takes two forms. An open-circuit element has a break in the resistance wire, producing no heat and no fault current. The circuit will not trip but the water will simply not heat. An earth-fault element has a breakdown in the insulation between the resistance wire and the element sheath, allowing fault current to flow to earth. This will typically trip the RCBO or RCD protecting the circuit.
Both types of element failure are confirmed by testing with a multimeter and insulation resistance tester. An open-circuit element will show infinite resistance between its terminals. An element with an earth fault will show a low insulation resistance reading between the element terminals and earth.
Element replacement requires the cylinder to be drained down sufficiently to allow the element to be withdrawn from the boss without flooding the airing cupboard. In a vented cylinder this is relatively straightforward. In a pressurised unvented cylinder it requires the pressure and temperature relief valve to be operated and the cylinder pressure to be released safely. This is typically a combined job between the electrician, who handles the electrical connections and tests the new element before energising, and a plumber or heating engineer who handles the drain-down and refill.
Limescale on a failed element will often be visible as a thick white or cream crust. The scale acts as an insulator, causing the element wire inside to run hotter and eventually fail. In hard water areas, fitting an in-line scale reducer or water softener upstream of the cylinder can extend element life significantly.
Fault 3: Timer Failure
If the timer override check described above showed that the heater works when manually switched on but does not come on automatically at the programmed times, the timer unit itself may have failed rather than simply lost its programme.
Mechanical timers can suffer from seized or broken mechanisms, particularly in damp airing cupboard environments. Electronic timers can fail due to component degradation, condensation damage, or power surge damage. A timer that responds to manual override but fails to operate automatically, or one that shows signs of physical damage or failed displays, needs replacing.
Timer replacement is within the capability of a qualified electrician and does not usually require any work on the cylinder itself. The replacement timer must be of the correct type for the circuit arrangement and the cylinder specification.
Fault 4: Wiring and Connection Faults
The heat-resistant flex that connects the isolator switch to the immersion heater head is in a challenging environment. It is exposed to the heat rising from the cylinder, to steam from the airing cupboard, and to the vibration of the cylinder expanding and contracting with thermal cycling over many years. Over time the flex can harden, crack, or develop loose connections at either end.
Signs of flex or connection problems include visible cracking or discolouration of the flex, scorch marks or discolouration at the heater head cover or at the isolator, a burning smell from the airing cupboard, or a heater that works intermittently.
Heat-resistant flex used on immersion heater connections must be of the correct specification for the temperature and current involved. Standard PVC flex is not suitable. Where heat-resistant flex has deteriorated, replacement is required. This is electrician work.
Loose connections at the immersion heater head are also a common finding, particularly in older installations. A loose connection generates heat, which accelerates the deterioration of surrounding insulation and can in serious cases cause a fire at the connection point. An electrician inspecting the heater head will tighten all connections, check for signs of overheating, and replace any damaged components.
Fault 5: Isolator Switch Failure
The double-pole isolator switch on the wall of the airing cupboard is an often-overlooked component. It is operated relatively infrequently but is in a warm, potentially damp environment, and it can fail over time.
A failed isolator may feel warm or hot to the touch, may make a clicking or buzzing sound, may show signs of discolouration or scorch marks on the faceplate, or may operate the switch mechanism without actually making or breaking the circuit. An isolator that fails closed presents a safety risk because the element cannot be safely isolated for maintenance.
Isolator replacement is straightforward electrician work and is a relatively low-cost fix. If the isolator shows signs of significant overheating or arcing, the wiring at the isolator terminals should also be inspected and the insulation condition assessed before the new isolator is energised.
The Boost Element: A Get Out of Jail Free Card
If your cylinder has two elements and only one has failed, the other may still be functional and can provide a temporary supply of hot water while you arrange for the repair.
In a dual-element cylinder, the lower element heats the full volume of the cylinder on overnight Economy 7 or off-peak power. The upper element, often referred to as the boost element, heats only the top portion of the cylinder quickly and is typically used for a midday or evening top-up when the overnight charge has been exhausted.
If the lower element has failed and the overnight charging circuit is dead, the upper boost element may still be working perfectly. Switching on the boost element manually will heat the upper portion of the cylinder, typically 50 to 80 litres depending on the cylinder size and the position of the upper element, in 30 to 60 minutes. This is not a long-term solution, and it will cost more to run than using off-peak overnight charging, but it gives you functional hot water while the main element fault is diagnosed and repaired.
Similarly, if the boost element has failed and you only have overnight charging available, you can manage by ensuring the overnight charge is fully utilised and adjusting your hot water use accordingly while the repair is arranged.
If you are not sure which element or which circuit has failed, checking whether the boost switch or the overnight circuit produces any warmth in the cylinder after being on for 30 minutes is a useful diagnostic step. A cylinder that warms from the top has a working boost element. One that warms from the bottom has a working overnight element.
Economy 7 and Dual-Element Cylinders: Specific Considerations
Properties on Economy 7 or other off-peak tariffs typically have dual-element cylinders supplied by separate circuits. The overnight charging circuit may be on a separately switched supply from the meter, controlled by a ripple control signal from the supplier or by a timed relay at the consumer unit. The boost circuit is usually a standard mains circuit controlled by a timer or a manual switch.
A common fault pattern on Economy 7 systems is that the overnight charging circuit stops working while the boost circuit remains functional, or vice versa. If the overnight supply has failed, the cylinder may be cold in the morning despite the boost element working correctly. If the boost circuit has failed but the overnight circuit is fine, the cylinder will be hot in the morning but there will be no way to top it up during the day.
Diagnosing which circuit has failed, and why, requires an electrician to test both circuits and both elements. The Economy 7 switching arrangement, which may involve a timeswitch, a ripple control receiver, or a timed relay at the consumer unit, adds a layer of complexity that is worth explaining to the electrician before they attend so they can bring the correct test equipment and replacement components if needed.
If the overnight supply appears to have failed but was previously working, it is also worth contacting your electricity supplier. In some cases the off-peak switching signal from the network has been changed or discontinued in certain areas, and the supplier can advise on whether this is the cause and what your options are.
When to Call a Qualified Electrician
The checks a homeowner can safely carry out are limited to resetting the MCB or RCBO at the consumer unit once, pressing the thermal cut-out reset button on the heater head, and checking and reprogramming the timer. Everything beyond that is work for a qualified electrician.
Electrical Safety First is clear that electrical work on fixed installations, including immersion heater repairs, should be carried out by a qualified and competent electrician. An immersion heater element carries 230V mains voltage in direct contact with water. The consequences of unsafe work on this type of installation are serious.
When the electrician attends, they will isolate the circuit at the consumer unit and at the isolator switch, use test equipment to identify the specific fault, and advise on the repair or replacement needed. Where a plumber is also required, for example for a drain-down to replace an element in a pressurised cylinder, the electrician can advise on the sequence of work needed and whether the two trades need to be on site simultaneously or in sequence.
As NAPIT-registered electricians, Flodman Electrical carry out immersion heater fault diagnosis and repair to BS 7671 standards, with all work tested and certified on completion.
Talk to Flodman Electrical
Flodman Electrical are NAPIT-approved electricians based in Farnborough, covering Hampshire, Surrey, and Berkshire. If your immersion heater has stopped working and the basic checks have not resolved the problem, we can attend, diagnose the fault, and carry out the repair or advise on replacement.
Contact Flodman Electrical to book an immersion heater repair
Flodman Electrical Ltd, NAPIT Approved Contractor. Serving Farnborough, Hampshire, Surrey and Berkshire.
