No hot water: how to fault-find an immersion heater

Helpful video reference. This UK tutorial "How to test, install, fault find hot water immersion heater - No hot water - unvented cylinder" covers the full process of testing and fault-finding an immersion heater, including element resistance testing and thermostat checks. It is aimed at the UK market and covers both vented and unvented cylinders.

1. Check the obvious first

Before reaching for a multimeter, work through the simple stuff:

• Is the immersion heater isolating switch (usually a double-pole switched spur near the cylinder) switched on?
• If there is a timer or programmer, is it set correctly? Try switching to a manual boost if available.
• Has the MCB on the immersion circuit tripped at the consumer unit? Reset it and see if it holds.
• Is there actually hot water in the cylinder? Check the temperature by running the hot tap for 30 seconds - the water may just be lukewarm rather than cold.

2. Reset the overheat thermostat

Remove the plastic dome cover from the element head (it usually unclips or has one central screw). Inside you will see the thermostat body - a cylindrical component clamped against the element. On the thermostat there is a small reset button, often red or white, sometimes recessed so you need a thin screwdriver to reach it.

Press the button firmly. A distinct click means the cutout has reset. Replace the cover, switch the supply back on, and wait 30 minutes. If the water starts to warm, the overheat cutout had tripped - usually because the main thermostat is set too high or was sticking. If the cutout trips again, do not keep resetting it: there is an underlying fault.

3. Test the main thermostat with a multimeter

Isolate the circuit. Remove the cover. The thermostat has two spade or screw terminals (some have three - consult your cylinder's datasheet). Set the multimeter to continuity mode and place a probe on each terminal.

When the cylinder is cold (below the thermostat set point), you should get continuity - the thermostat is closed, allowing current through. If the multimeter shows open circuit when the water is definitely cold, the thermostat is stuck open and needs replacing.

Turn the thermostat adjustment wheel to the maximum setting and test again. If continuity appears when the dial is moved but not at a normal setting, the thermostat calibration has drifted.

4. Test the element

Isolate the circuit and disconnect the wires from the element terminals (take a photo first). Set the multimeter to resistance (ohms) mode.

Measure resistance across the two element terminals:

• A healthy 3 kW element reads roughly 19 ohms.
• A healthy 2 kW element reads roughly 28 ohms.
• An open circuit (OL, 1, or "overload" on a digital meter) means the element wire has broken internally - the element is dead and needs replacing.
• A very low resistance (under 5 ohms) suggests a partial short, which would trip the MCB quickly.

Also measure from each element terminal to earth. Any reading below several megaohms (MΩ) means water or scale is providing a leakage path - this is what trips the RCD. The element needs replacing.

5. Inspect the wiring and terminal block

While the cover is off, look at the terminal block where the supply cable connects. Any signs of overheating - brown or black discolouration on the plastic, melted insulation, or a burnt smell - mean there has been a loose connection arcing over time. Tighten all terminals and, if the terminal block is damaged, it needs replacing.

Also check the supply cable for damage: flex that has been pinched under the cylinder lagging jacket, or sat against the hot cylinder for years, can degrade the insulation.

6. Test the timer (if fitted)

If the immersion has a mechanical time switch (the type with push-in pins around a dial), listen for the motor - it should hum faintly when the supply is on. If the dial is not rotating the motor is dead. Plug-in or wired timers can be checked by bypassing them temporarily with a direct connection to confirm the heater itself works.

7. Restore supply and retest

Once you have replaced the faulty component - thermostat, element, or timer - reconnect everything, replace the cover, and restore the supply. Give the heater 30 minutes and check the hot tap again. A 3 kW element on a 120-litre cylinder should produce noticeably warmer water within that time.

When to call us

The thermostat reset and multimeter tests are well within what a careful homeowner can do. Replacing an element on a vented copper cylinder is also manageable - our sister guide covers that step by step. But anything involving a sealed, pressurised cylinder, a tripping RCD, or a wiring fault at the consumer unit is the point to pick up the phone. Richard covers fault-finding in Sandwich at £10 per 10 minutes.