This is a question that genuinely confuses homeowners, and with good reason. Replacing an immersion heater element sits at the boundary between electrical and plumbing work, and the correct answer depends on what type of cylinder you have, what the specific fault is, and who is doing the work. This guide explains what the job involves, what each trade contributes, when both are needed, and what you should expect in terms of testing and documentation when the work is done.
What Does Replacing an Immersion Heater Element Actually Involve?
An immersion heater element is a resistance heating element that screws into a threaded boss on the side of the hot water cylinder. The element is in direct contact with the water in the cylinder and is connected to the electrical supply via a flex running to an isolator switch on the airing cupboard wall.
Replacing it involves two distinct sets of work. The electrical side: isolating the circuit, disconnecting the flex from the element terminals, making the electrical connections to the new element, testing the new installation, and re-energising. The plumbing side: draining the cylinder down below the level of the element boss, removing the old element using a purpose-made immersion heater spanner, inspecting the boss and thread, fitting the new element with the correct seal, refilling the cylinder, and checking for leaks.
Both sides must be done correctly and in the right order. The cylinder must be drained before the element is removed, and the electrical connections must not be made until the element is physically in place, correctly sealed, and the water side has been confirmed as sound. Getting the sequence wrong, or cutting corners on either side, creates a risk of water damage, electrical fault, or both.
What the Electrician Does
A qualified electrician carries out all of the electrical aspects of the job. Before any work starts, the circuit is isolated at the consumer unit and at the double-pole isolator switch on the airing cupboard wall. The electrician will confirm the circuit is dead using a voltage indicator before touching any connections.
The flex is then disconnected from the element terminals. At this point, the failed element can be tested to confirm the nature of the fault. An insulation resistance test between the element terminals and the element sheath identifies an earth fault. A continuity test across the element terminals identifies an open-circuit failure. Confirming the fault before the element is removed matters: if the element tests electrically sound, the problem may be elsewhere in the circuit, and changing the element will not resolve it.
Once the new element has been physically fitted by the plumber or by the electrician themselves in appropriate circumstances (see below), the electrician makes the electrical connections to the new element, observing the correct terminal connections and ensuring the heat-resistant flex is correctly dressed and secured. The new element is then tested: insulation resistance of the element windings, continuity of the earth conductor, and polarity before re-energising. Only when the test results confirm the new element is electrically sound is the circuit re-energised and the heater switched on.
On completion, the electrician issues a Minor Electrical Installation Works Certificate. This document records what work was done, confirms it was tested to BS 7671, and provides the homeowner or landlord with evidence that the electrical work was carried out by a competent person. For landlords in particular, this documentation is worth keeping alongside the property’s EICR records.
What the Plumber Does
The plumber carries out all of the water side of the job. For a vented gravity-fed cylinder, this typically involves shutting off the cold feed from the loft tank and either draining the cylinder down from a drain cock or connecting a hose from a low point to run the water to a drain. The cylinder does not need to be completely empty: it only needs to be drained below the level of the element boss, which is usually at roughly two-thirds of the way up the cylinder.
With the water level below the boss, the old element can be unscrewed using an immersion heater spanner. This is a large, purpose-made tool sized to fit the hexagonal element boss. It is not a job that can be done with a standard spanner or a wrench: attempting to remove an element without the correct tool risks damaging the boss thread or the cylinder body. The old element is withdrawn and inspected. Scale deposits on the element sheath give a good indication of the water hardness and how long the element has been in service.
The boss thread and seating surface are inspected for corrosion, damage, or signs of previous leaks. If the thread is in good condition, the new element is fitted with fresh sealing compound, or with a new fibre or rubber washer as appropriate for the boss type, and torqued to the correct specification. The cylinder is then refilled and the area around the boss inspected for any sign of weeping or leaking. Only when the plumber is satisfied the water side is sound is the electrician asked to make the connections and re-energise.
Can One Trade Do Both?
In some circumstances, yes. But this depends on the cylinder type and the specific situation.
For a standard vented hot water cylinder, gravity-fed from a cold water tank in the loft, the drain-down and element change is a relatively straightforward plumbing task. Some electricians who regularly work on immersion heater systems carry an immersion heater spanner and are competent to drain down a vented cylinder, change the element, and then complete the electrical connections and testing themselves. If you are using an electrician who is confident and equipped to handle the full job, this is a practical and cost-effective option that avoids coordinating two separate trades.
However, the electrical reconnection and testing side must always be carried out by someone who is qualified to test and certify the work. A plumber who changes the element and reconnects the flex themselves, without being competent to test the installation and issue a certificate, is carrying out unregistered electrical work. The connections may be perfectly adequate in practice, but the work has not been tested, the installation has not been verified as safe, and no documentation has been issued. For homeowners, that is a gap in the safety record. For landlords, it is a compliance issue that could become significant if the installation is later subject to scrutiny.
Vented vs Unvented Cylinders: Why the Difference Matters
The type of cylinder installed has a significant bearing on how the element replacement job is scoped and who can carry it out.
A vented cylinder is an open-vented system. The cold feed is gravity-fed from a cold water storage tank in the loft, and there is an open vent pipe that rises above the cold water tank to allow air to escape and prevent vacuum formation. The system operates at low pressure, determined by the height of the loft tank above the cylinder. Draining a vented cylinder is straightforward: shut the cold feed valve at the tank, open a hot tap to admit air, and the water runs down to drain. The element boss is accessed at low pressure and the job carries relatively low risk.
An unvented cylinder is connected directly to the mains cold water supply without a loft tank. It operates at mains water pressure, typically 1.5 to 3 bar or more. It is fitted with a pressure and temperature relief valve, commonly called a P&T valve, which opens automatically if the pressure or temperature in the cylinder exceeds safe limits. An expansion vessel absorbs the pressure increase as the water heats.
Working on an unvented cylinder is fundamentally different from working on a vented one. The mains water supply must be isolated, the expansion vessel must be isolated if fitted with a service valve, and the P&T valve must be manually opened to release the stored pressure before any work is carried out on the element boss. Getting this wrong exposes the person working on the cylinder to a sudden release of scalding pressurised water.
For this reason, the specification and commissioning of unvented hot water cylinders, and any significant work on them including element replacement, must be carried out by a qualified person holding the G3 Unvented Hot Water Storage Systems qualification. This is a specific qualification that demonstrates competence in working with pressurised hot water systems. Neither an electrician nor a general plumber without the G3 qualification should be attempting to change an element in an unvented cylinder. If you are not certain which type of cylinder you have, the presence of a cold water tank in the loft feeding the cylinder suggests a vented system. If the cylinder is fed directly from the mains with no loft tank and has a pressure relief valve with a discharge pipe running to a tundish, it is almost certainly unvented.
Confirming the Element Has Failed: Test Before You Replace
An element should always be tested before replacement is confirmed as the course of action. Not every immersion heater fault is caused by the element, and replacing a sound element while the real fault remains unresolved wastes money and delays the proper repair.
An electrician testing an immersion heater circuit will measure the insulation resistance between the element conductors and the element sheath. A healthy element will show a very high insulation resistance, typically many megaohms. An element with an earth fault will show a low insulation resistance, often below 1 megaohm and sometimes effectively zero. This is the failure mode that trips the circuit protective device when the heater is switched on.
The continuity of the element resistance wire is measured between the element terminals. A healthy element has a measurable resistance determined by its wattage rating. A 3kW element at 230V has a resistance of approximately 17.6 ohms. An open-circuit element will show infinite resistance. This is the failure mode that leaves the heater apparently dead without tripping the circuit.
Where the element tests as sound and the fault is not in the element, the investigation moves to the thermostat, the thermal cut-out, the flex, the timer, and the wiring. Replacing the element in this scenario would leave the hot water problem unresolved.
Limescale and Element Life in Hampshire and Surrey
Hampshire, Surrey, and Berkshire are hard water areas. The chalk geology of the region means that groundwater picks up calcium and magnesium minerals before it reaches the tap, and those minerals deposit as limescale on any surface where water is heated. The immersion heater element, which heats water directly, accumulates scale faster than almost any other surface in the plumbing system.
Scale builds up as a white or cream crust on the element sheath. As it thickens, it acts as an insulator, reducing the efficiency of heat transfer from the element to the water. The element has to run longer and hotter to achieve the same water temperature. The elevated operating temperature accelerates the degradation of the element windings. In a hard water area, an element that might last fifteen to twenty years in a soft water area may fail in eight to twelve years.
When a failed element is withdrawn from the cylinder, the scale deposit gives a clear picture of how hard the water is and how long the element has been in service. A thick, hard scale coat on a relatively new element suggests the water is very hard and scale prevention measures would be worthwhile. Inline scale reducers fitted on the cold feed to the cylinder can slow the rate of scale accumulation. A whole-house water softener eliminates scale entirely but requires its own installation and ongoing salt replenishment.
Part P and Certification
The electrical reconnection and testing of an immersion heater element is electrical installation work and must be carried out by a competent person. In most cases, element replacement as a like-for-like repair is not notifiable under Part P of the Building Regulations. However, a Minor Electrical Installation Works Certificate must still be issued on completion, confirming that the work has been tested to BS 7671.
For homeowners this provides documented evidence that the work was done properly. For landlords it is part of the compliance record for the property and should be retained alongside the EICR documentation. An immersion heater element replaced without any documentation or testing is a gap in the electrical record that may become relevant during a future EICR, a property sale, or an insurance claim.
Talk to Flodman Electrical
Flodman Electrical are NAPIT-approved electricians based in Farnborough, serving homeowners and landlords across north Hampshire and Surrey including Aldershot, Fleet, Camberley, Farnham, Guildford, Basingstoke, Hook, and Yateley.
We carry out immersion heater fault diagnosis, element testing, and full electrical replacement including testing and certification. Where a plumber is also needed, particularly for unvented cylinders requiring G3-qualified work, we can advise on how to coordinate the two trades for an efficient and properly documented repair.
Call us on 01252 751322 or get in touch below.
Contact Flodman Electrical to discuss an immersion heater element replacement
Flodman Electrical Ltd, NAPIT Approved Contractor. Serving Farnborough, Aldershot, Fleet, Camberley, Farnham, Guildford, Basingstoke, Hook, Yateley and the surrounding area.
