CH20 on a LG heat pump: Electric (backup) heater overheated – thermal fuse / thermo switch cut out
Short answer
On a LG heat pump, CH20 means: Electric (backup) heater overheated – thermal fuse / thermo switch cut out. The heat pump stops until the fault clears or is reset. An engineer is needed to fix the cause. The wording differs by model range — each is shown below.
Reviewed 23 September 2026 · 2 sources
What it means
On the LG, all models
The manual says: Electric (backup) heater overheated – thermal fuse / thermo switch cut out.
Part involved: backup heater thermal fuse / thermo switch (CN_SOLAR/CN_THMO).
When it shows: Heater tank/outlet temperature >= 80 °C (thermal fuse).
Likely causes:
- Thermal fuse cut off by abnormal overheating of internal electric heater (e.g. air in heater vessel, no flow); mechanical fault at thermal fuse; wiring wrong or damaged.
On the THERMA V Water Heater (heat pump DHW cylinder)
The manual says: Lower element is not working (THERMA V water heater).
Part involved: electric heating element.
When it shows: Lower element is not working.
Likely causes:
- Lower element is not working.
Is it safe?
On the LG, all models
Switch the heat pump off at its isolator and call an engineer.
| Does the heat pump stop? | Yes |
|---|---|
| Severity | Safety lockout |
| Engineer needed? | Yes |
The heat pump stops until the fault clears or is reset.
Resetting itself: no.
Manual reset: Service-engineer repair and restart required (LG: resumed only after certificated professional's check).
On the THERMA V Water Heater (heat pump DHW cylinder)
| Does the heat pump stop? | Partly |
|---|---|
| Severity | Warning |
| Engineer needed? | Yes |
The heat pump keeps running with some functions stopped.
Resetting itself: unknown.
Manual reset: Per LG corrective action; service engineer for most codes.
What you can check
On the LG, all models
- Read the code on the controller (LG shows 'CH' + number; older wired controllers alternate 'CH' and the number) and note it together with the model number. Report it to your installer or an LG-authorised service centre – LG owner's manuals say an error code generated by self-diagnosis is a reason to call the service. This is an 'Error'-class fault: LG says the system can only be resumed after a qualified professional has checked it, so do not keep trying to restart it. If you notice a burning smell, switch the heat pump off at its isolator/breaker and call the service.
On the THERMA V Water Heater (heat pump DHW cylinder)
- Note the code shown on the water heater display. LG's corrective action: call LG technical support. While the fault is present the unit operates as: Use heat pump only – hot water will be heated more slowly/at higher cost.
What the engineer will check
On the LG, all models
- Check harness on PCB, test thermo switch/thermal fuse continuity, find cause of overheat (flow, air); 1ph models: manual reset button on thermo switch; 3ph models: auto reset once below 70 °C then re-power. Replace thermo switch or heater.
Who to ask for: Heat pump engineer / electrician (inverter and PCB diagnostics).
Certification: Competent person for electrical work (BS 7671); isolate and wait 3 min for DC-link discharge.
On the THERMA V Water Heater (heat pump DHW cylinder)
- Isolate; test element resistance and insulation, relay output and wiring; replace element.
Who to ask for: Heat pump engineer / electrician (inverter and PCB diagnostics).
Certification: Competent person for electrical work (BS 7671); isolate and wait 3 min for DC-link discharge.
What you might notice
Sources
- AIR-TO-WATER HEAT PUMP SERVICE MANUAL (General) – R32 Hydrosplit, MFL68682007 ↗ · MFL68682007 · page 71, 94 · Reviewed 23 September 2026
- LG Heat Pump Water Heater Owner's Manual WH20S F5 / WH27S F5 (EU/UK multilingual), MFL71409209 Rev.00 (2023) – ManualsLib 3195149 ↗ · MFL71409209 Rev.00_090823 · page 32 · Reviewed 23 September 2026
A code explains what the controller detected, not a confirmed root cause.
Reviewed: 23 September 2026