What an Immersion Heater Circuit Actually Needs
An immersion heater is a high-load appliance — a typical 3kW element draws around 13A continuously. BS 7671 requires it to be on a dedicated 20A radial circuit, protected by a 20A RCBO or MCB. It must not share a circuit with sockets, lighting, or any other load.
The circuit runs from the consumer unit to a 20A double-pole fused connection unit (FCU) with a flex outlet and neon indicator, typically mounted near the airing cupboard. From the FCU, 2.5mm² heat-resistant cable (usually butyl rubber or XLPE-insulated) runs to the immersion heater boss. The FCU is the safe isolation point for maintenance: switching it off and removing the fuse gives full double-pole isolation before anyone touches the element or thermostat.
Older installations sometimes have the immersion heater wired from a standard 13A switched socket outlet. This is technically incorrect: a socket outlet is for portable appliances only, and a fixed water heater should be wired via an FCU. It may also be undersized if the socket circuit is shared with other loads. We flag this as a C3 improvement on any EICR that shows it.
Common Faults and What Causes Them
| Symptom | Most Likely Cause | Fix |
|---|---|---|
| RCD trips when immersion heater switches on | Element insulation failure — current leak to earth | Element replacement |
| Water warm but not hot enough | Thermostat set low, or thermostat failing early | Thermostat adjustment or replacement |
| No hot water at all, no trip | Element open-circuit failure, or failed thermostat | Element or thermostat replacement |
| MCB trips but RCD holds | Element drawing excessive current (short circuit) | Element replacement; check cable for damage |
| FCU neon on but water cold | Thermostat stat cut-out tripped manually (overtemp) | Reset stat; if recurring, replace thermostat |
| Slow heating — takes much longer than before | Limescale build-up on element sheath | Element replacement; consider scale inhibitor |
Diagnosing the Fault Properly
A proper diagnosis takes around 20–30 minutes with the right test equipment. An insulation resistance (IR) test with a calibrated instrument (a Megger or equivalent) tells us immediately whether the fault is in the element itself or in the cable and wiring. Testing at 500V DC between the element live conductor and earth: a reading below 1MΩ indicates element insulation failure. A reading above 1MΩ points the fault elsewhere.
If the element tests sound, we check the thermostat for continuity and correct temperature response. A thermostat that has had its thermal cutout trip is easily reset — there is usually a small reset button accessed through a hole in the thermostat cover with a thin screwdriver. If the cutout trips repeatedly, the thermostat is faulty or the element is overheating due to scale build-up.
The right thermostat temperature setting
Immersion heater thermostats are often shipped set at 55–60°C. The correct setting for Legionella control is 60°C — at this temperature, Legionella bacteria are killed within two minutes. Setting it lower saves energy but creates a health risk in stored hot water systems. We always check and confirm the thermostat setting as part of any immersion heater visit.
Element Replacement: What’s Involved
Element replacement is the most common immersion heater job. The process: isolate the circuit at the FCU, drain the top section of the cylinder, unscrew the element boss using a specialist immersion heater spanner, fit the new element with a new sealing washer, refill, and test. The job typically takes 45–90 minutes depending on the cylinder type and how corroded the element boss is.
Elements come in two main configurations:
- Flanged elements (11-inch boss, most common in older copper cylinders) — unscrew the large hexagonal boss and withdraw the element and thermostat together
- Rod elements with separate thermostat pocket — element and thermostat are independent; only the failed component needs replacing
We carry standard 11-inch 3kW elements on the van. Non-standard sizes (economy 7 dual elements, stainless steel cylinders, unvented cylinders) may need a specific part ordered in advance — we confirm this at survey or first call.
Unvented Cylinders: Different Rules
If your hot water cylinder is unvented (pressurised — fed directly from the mains cold water supply rather than a header tank), there are additional considerations. Unvented systems operate at mains pressure and have safety devices (temperature and pressure relief valves, thermostatic blending valves) that must be in working order. Electrical work on an unvented cylinder must be carried out alongside a check of these safety devices.
In practice, this means we always check the T&P relief valve and expansion vessel condition when working on an unvented cylinder. If these are showing signs of failure, we advise before proceeding — a failed T&P valve on an unvented cylinder is a serious safety hazard regardless of the electrical status of the element.
Part P: What Needs Certification
Like-for-like element or thermostat replacement in an existing, correctly-wired immersion heater is maintenance work and is not notifiable under Part P of the Building Regulations. No certificate is required.
The following immersion heater jobs are notifiable and require certification by a registered electrician:
- Installing an immersion heater where none existed before (new circuit from consumer unit)
- Replacing an incorrectly-wired circuit (e.g., upgrading from a shared socket to a dedicated FCU circuit)
- Adding a second element to an existing cylinder
- Relocating the consumer unit supply for the immersion heater circuit
If you are selling the property and the immersion heater circuit was added or significantly modified, a buyer’s solicitor may ask for a Part P certificate. If none exists, an EICR will flag the non-certified work. The cost of retrospective certification (via a formal inspection and test) is typically higher than getting it right at the time.
What It Costs in Mid Somerset
| Job | Typical Cost (ex VAT) |
|---|---|
| Fault diagnosis (IR test, thermostat check, report) | From £80 |
| Element replacement (standard 11-inch, part supplied) | From £120 |
| Thermostat replacement | From £90 |
| Element + thermostat replacement together | From £160 |
| FCU replacement (double-pole 20A with neon) | From £80 |
| New dedicated immersion heater circuit (new radial from consumer unit) | From £280 |
| Consumer unit upgrade required to add circuit | From £380 additional |
Parts costs vary by element type and cylinder manufacturer. We quote before starting any work where the part cost is above the standard item. All prices exclude VAT at 20%.
What We Cover in Mid Somerset
DS Electrical covers immersion heater fault diagnosis, element replacement, thermostat replacement, and circuit upgrades across Mid Somerset and BANES — Wells, Shepton Mallet, Bath, Frome, Radstock, Midsomer Norton, Street, Bruton, Castle Cary, Cheddar, Wincanton, and surrounding villages.
Call 07889 334849 or use the button below. Most immersion heater faults can be diagnosed and resolved in a single visit.