How-to · Fault-finding

How to test an immersion heater

No hot water from the cylinder and unsure whether it is the element, the thermostat, or something else? This guide walks through how immersion heaters work and the multimeter checks that tell you what has failed before you spend money on parts. Some of these tests are fine for a careful homeowner — replacing a failed element is where most people call an electrician.

Helpful video reference. We use John Ward's detailed tutorial "Immersion Heaters — How they work, fault finding & testing" as the video reference for this guide. John is a UK electrician and one of the most thorough electrical educators on YouTube. He covers the anatomy of the heater, the role of the thermostat and cut-out, and demonstrates the multimeter tests that identify a failed element. Worth watching in full before you open the heater head.

Before you start. The immersion heater circuit is a dedicated radial, usually protected by its own 15 A or 20 A fuse or MCB. Switch it off at the consumer unit, not just at the wall switch, and confirm dead at the heater head with a voltage tester before removing the cover. Hot water cylinders hold water under pressure — do not disconnect any pipework.

1. Understand what is inside the heater head

Lift or unscrew the plastic cover on the top of the cylinder. Inside you will find: the element terminals (two or three screws, depending on whether it is single or dual element), the thermostat (a long probe that sits in the water), and a small thermal cut-out that breaks the circuit if the temperature exceeds a safe limit.

Take a photograph of the wiring before you touch anything. Modern immersion heaters use a brown live, blue neutral, and green/yellow earth. Older installations may have different colours — photograph first, then check.

2. Confirm the circuit is dead

With the cover off, use a voltage tester on the live terminal of the element. Dead in all positions? Good. If the tester still shows voltage even with the circuit switched off at the board, stop — there may be a second supply feeding this, or the consumer unit labelling is wrong.

3. Test the element for continuity

Disconnect the two wires from the element terminals. Set your multimeter to the resistance setting. Place one probe on each terminal of the element. A healthy element will read somewhere between 10 ohms and 40 ohms, depending on its wattage rating (a 3 kW element at 230 V works out at around 17 ohms from P = V2/R). A reading of zero indicates a short; a reading of OL or infinity means the element has burned through and needs replacing.

While you have the meter on resistance, also check between each element terminal and the element body (earth). You should read a very high resistance or open circuit. A low reading here means the element insulation has failed and the element is fault to earth — it will trip an RCD every time.

4. Test the thermostat

The thermostat is wired in series with the element. If the element tests good but there is still no heat, the stat is the next suspect. With the stat probe at room temperature and the dial turned to maximum, you should read continuity (close to zero ohms) through the stat. No continuity at room temperature means the stat contacts are stuck open — the stat needs replacing.

Also check for continuity through the thermal cut-out, which is in series with the stat. On most heaters there is a small reset button accessible through a hole in the cover or by pressing the cut-out itself. If it has tripped, press it firmly and retest continuity. A cut-out that keeps tripping is a sign the thermostat has failed and is allowing the water to overheat.

5. Inspect the wiring and fused spur

While you have access, check the cable entering the heater for any heat damage or scorch marks on the insulation. Look at the fused spur or double-pole switch on the wall: the fuse inside (often 13 A) should be intact and the contacts should not show signs of burning. A blown fuse on the spur is usually a symptom, not the root cause — find out what blew it before fitting a new one.

6. Replacing a failed part

A failed element on a vented (open-vent) cylinder can be swapped by a competent person, though the job involves draining the cylinder down and using a large immersion heater spanner — usually a 55 mm or 85 mm A/F. A failed thermostat is simpler to change and does not require draining. On unvented (pressurised) cylinders, any work on the hot water side should be done by a qualified plumber or electrician who is qualified to work on unvented systems.

Whichever part you replace, work through the same continuity checks before restoring power.

7. Restore the circuit and test

Reconnect all wires exactly as photographed. Replace the cover. Switch the circuit back on at the consumer unit and allow 30 to 45 minutes before checking whether the water is heating. If the MCB or RCD trips immediately, switch off and recheck your connections.

Stop and call an electrician if: the circuit trips immediately on restoration, there are signs of arcing or burning inside the heater head, the cable entering the heater is heat-damaged or discoloured, the cylinder is unvented, or you cannot confidently identify what is live and what is not.

When to call us

The diagnostic checks in this guide are fine for a careful homeowner with a multimeter. Actually replacing an element — draining the cylinder, handling a 55 mm spanner, and reconnecting the circuit correctly — is where most people find it worth getting a qualified electrician to finish the job. Richard covers immersion heater callouts in Sandwich and across east Kent, usually on the same day for urgent no-hot-water situations.

No hot water in Sandwich?

Richard can diagnose and repair immersion heater faults the same day for most locations in east Kent.

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