Immersion / tank heater not working
An immersion or tank heater isn't heating the water/fluid — it stays cold despite the thermostat calling, or trips its protection.
Safety first
Immersion heaters combine water and electricity — earth-leakage and shock risk if the element fails. Isolate, prove dead, and never bypass earth-leakage protection. Hot water scald risk.
Isolate, lock out / tag out, and prove dead before working unless a live test is specifically required, authorised, and carried out under proper supervision. Always follow local regulations, your site procedures, and the equipment manufacturer's documentation.
Full detail — causes, the why, and common mistakes.
Likely causes
Ranked from most to least likely.
- 1
Thermostat tripped or set low / faulty
Most likelyThe control thermostat (or a tripped high-limit cut-out) is stopping the element, or the setpoint is too low.
- 2
Open (burnt-out) element
#2The immersion element has failed open and draws no current.
- 3
No supply / blown fuse / tripped protection
#3Lost supply, a blown fuse, or tripped breaker/RCD removes power.
- 4
Element leaking to earth (tripping protection)
Least likelyA failing element leaks to earth and trips the RCD/breaker.
Reports are saved on this device to reflect what you actually find.
Testing sequence
Work through one test at a time. Expected reading and what each result means.
Check the thermostat/high-limit: is it calling for heat and not tripped, with a sensible setpoint?
Thermostat calling for heat; high-limit not tripped.
Heat is called — check supply and the element.
Setpoint low or cut-out tripped — adjust/reset (find why a cut-out tripped).
View all expected readings at once
Fault-finding flowchart
The same logic as a decision tree.
- 1start
Immersion heater not heating
→ step 2 - 2decision
Is the thermostat calling and the high-limit not tripped?
Yes→ step 3No→ step 4 - 3decision
Is supply present at the heater?
Yes→ step 5No→ step 6 - 4result
Adjust setpoint / reset cut-out (find why it tripped).
- 5decision
Is the element resistance sensible and insulation good?
Yes→ step 7No→ step 8 - 6result
Lost supply / tripped protection — find why before restoring.
- 7result
Element healthy — re-check thermostat/control.
- 8result
Open element or low insulation — replace the element.
Common mistakes apprentices make
- Resetting a high-limit cut-out without finding why it tripped.
- Bypassing the RCD when a leaking element trips it.
- Not testing the element's insulation to earth.
- Assuming the element when the thermostat is the issue.
When to stop & escalate
A leaking element is a replacement — never run it on defeated protection. A repeatedly tripping high-limit indicates a control fault to find before resetting.
If you're past your competence, authorisation, or the safe limits of the job — stop and hand it on. There's no fault worth getting hurt over.
Related faults
Heater not heating at all
A heater (element, band, or bank) produces no heat — temperature won't rise, the process stays cold, despite the control calling for heat.
Heater tripping the RCD / earth leakage
A heater circuit trips its RCD/earth-leakage protection — often when cold and first switched on, or once it's been damp — pointing to leakage to earth from the element.
Thermocouple open or reversed (wrong temperature reading)
A thermocouple gives a wrong temperature — reads way off, shows an open/sensor-break, or moves the wrong way — so the heat control can't work properly.