Immersion Heater Not Working — UK Fault Guide
An immersion heater has just three failure points: thermostat, element, or timer/switch. All are DIY-diagnosable with a multimeter.
Step-by-step method
1. Check power at the switch
Switch on. Confirm neon indicator glows. No glow = no supply — check fused spur (16A) and RCD.
2. Test thermostat
Isolate power at consumer unit. Pop off immersion cover. Multimeter across thermostat terminals — should show continuity when cold. If not, replace (£12).
3. Test the element
Multimeter across element terminals — should read 15–20Ω. Open circuit (infinite) means blown element — replace (£25).
4. Replace element (drain first)
Isolate mains water and drain cylinder via drain-off cock. Unscrew old element with immersion spanner. Fit new with PTFE tape and refill.
5. Reset and time
Set thermostat to 60°C (Legionella-safe). If economy 7, set timer for cheap-rate hours.
Frequently asked questions
How long does an immersion take to heat a cylinder?+
A 3kW element heats 120L from cold in ~3.5 hours. 180L takes ~5 hours.
Immersion running cost UK 2026?+
3kW × 4 hours = 12kWh. At 27p/kWh = £3.24 per full heat. Economy 7 tariff halves this.