- Heat pumps
- Air-to-water heat pump
CH20
LG CH20 on Therma V heat pump: backup heater thermal fuse open
Risk level
High
Professional inspection is recommended.
Urgency
High
Quick answer
Error CH20 or 20 on an LG Therma V heat pump means the internal electric heater thermal fuse is damaged or its sensing circuit is open, so stop the unit and check only external water pressure and shut-off valves. Never bypass internal thermal protections; call an authorised LG technician.
Safety first
Stop and check this first
- Severe shock and fire hazard: never alter, jumper, or bypass the thermal fuse or safety devices on the electric heater, and never power the appliance through an external switching device such as a timer that could inadvertently reset a thermal cut-out.
- If there is any burning smell or loud abnormal noise from the unit, stop operation and switch off the circuit breaker immediately; continuing to run the unit risks fire or electric shock.
Safe checks you can perform
- Stop the heat pump from the controller and note code CH20 or 20
- Check water circuit pressure on the front gauge or remote controller
- Confirm external water inlet and outlet shut-off valves are fully open
Leave these tasks to a qualified professional
- Do not attempt this professional task: Test thermal fuse continuity, harness integrity, and electric heater coil condition
- Do not attempt this professional task: Purge trapped air from the hydraulic circuit and inspect the water strainer
What a technician must be able to justify
Technician visit
These are the steps they must carry out and show you. Tick them as they work: if steps are skipped, do not accept a part swap.
These are not instructions for opening or handling the equipment yourself. They are the checks that should support a technical conclusion.
Test thermal fuse continuity, harness integrity, and electric heater coil condition
What they doAfter isolating mains supplies and waiting at least 10 minutes, an authorised technician must test whether the thermal fuse opened due to electric heater tank overheating past 80 °C, a mechanical fault in the fuse, or a damaged wire, and inspect the electric heater element.
They must show youthe continuity test results on the thermal fuse and wiring harness plus verification that the electric heater tank stays below 80 °C.
In writingthe root cause identified (overheating, mechanical fuse fault, or wire damage), resistance readings, and genuine LG replacement part numbers fitted.
Purge trapped air from the hydraulic circuit and inspect the water strainer
What they doBefore re-energising the backup heater, the technician must bleed all trapped air via the air purge vent, clean the water strainer, and stabilise water pressure at 2.0 to 2.5 bar so the heater coil cannot dry-fire again.
They must show youair-free water circulation, a clean strainer mesh, the air vent cap set two to three turns open as specified in the manual, and water pressure between 2.0 and 2.5 bar.
In writingthe final water pressure reading, confirmation of complete air purging, and the official LG service report or case reference.
Before accepting a part or repair
Nothing has been shown yet. Do not accept a part swap based on the code alone.
Follow these steps
Note: Follow the documented order. An error code identifies the affected system, but it does not prove by itself which part has failed.
Stop the heat pump from the controller and note code CH20 or 20
- Who:
- you.
- How:
- switch the system off at the wired controller, write down code 20, and immediately switch off the dedicated circuit breaker if you notice any burning smell or abnormal noise from the unit.
- Result that matters:
- having the appliance safely stopped and the fault code recorded for the authorised LG service provider.
- If there is no burning smell or unusual noise:
- proceed to check the external water pressure reading.
- If you smell burning or hear electrical arcing:
- trip the main breaker immediately and stay clear of the enclosure.
- Evidence to keep:
- photo of error 20 or CH20 on the wired remote controller screen before turning the system off.
Evidence: LG air to water heat pump owner's manual MFL68026705
Check water circuit pressure on the front gauge or remote controller
- Who:
- you.
- How:
- without using tools or opening panels, read the water pressure on the front pressure gauge (3 Series) or wired remote display (4 Series): LG specifies a normal range of 2.0 to 2.5 bar and a minimum of 0.3 bar.
- Result that matters:
- recording the current hydraulic pressure reading safely from outside the casing.
- If pressure is below 2.0 bar or near 0.3 bar:
- note the low reading so the technician checks for air pockets and water loss.
- If pressure sits between 2.0 and 2.5 bar:
- move on to inspect the external water isolation valves.
- Evidence to keep:
- photo of the front pressure gauge needle or remote controller pressure screen.
Evidence: LG Therma V R32 Monobloc air to water heat pump installation manual MFL68026607
Confirm external water inlet and outlet shut-off valves are fully open
- Who:
- you.
- How:
- look at the external isolation valves on the water inlet and outlet pipes connected to the unit to verify neither handle has been left closed or partially throttled.
- Result that matters:
- visual confirmation that both external water shut-off valves allow unobstructed water flow.
- If either valve was closed or partially throttled:
- inform the technician, as restricted flow can overheat the heater tank.
- If both valves are fully open:
- wait for an authorised engineer to test the thermal fuse and wiring.
- Evidence to keep:
- photo of the external water inlet and outlet shut-off valve handles.
Evidence: LG Therma V R32 Monobloc air to water heat pump installation manual MFL68026607
Still not solved
If the error persists, stop here. See what the technician must do and show you.
Where this code applies
A code is meaningful only inside the right product and version context.
Primary scope
- System
- Heat pumps
- Brand
- LG
- Product type
- Air-to-water heat pump
Known code variants
- 20
- CH 20
- CH20 HN0916M
- CH20 LG Therma V
- error CH20 aerotermia LG
- fallo 20 bomba de calor LG
- fusible termico resistencia LG Therma V
- LG air to water code 20
- LG backup heater thermal fuse damaged
- LG heat pump error CH20
- LG Therma V CH20
What it means
In the LG Therma V R32 Monobloc installation manual MFL68026607 and owner's manual MFL68026705 for HM, HN, and HU air-to-water heat pumps, code 20 (CH20) is listed as Thermal fuse is damaged. Other LG heat pump families may use different diagnostic tables. LG notes that this fault does not occur while the electric heater tank temperature stays below 80 °C; however, the error code alone never proves the thermal fuse or heater element is defective, because damaged wiring or trapped air starving the heater coil of water flow can trigger the fault.
Warnings and stop conditions
- Higheditorial warning
Severe shock and fire hazard: never alter, jumper, or bypass the thermal fuse or safety devices on the electric heater, and never power the appliance through an external switching device such as a timer that could inadvertently reset a thermal cut-out.
Evidence: LG Therma V R32 Monobloc air to water heat pump installation manual MFL68026607 · LG air to water heat pump owner's manual MFL68026705
- Higheditorial warning
If there is any burning smell or loud abnormal noise from the unit, stop operation and switch off the circuit breaker immediately; continuing to run the unit risks fire or electric shock.
Evidence: LG air to water heat pump owner's manual MFL68026705
Symptoms and causes
Probable causes · in order
- Thermal fuse cut off by abnormal overheating above 80 °C in the electric heaterCommon
The internal electric backup heater tank exceeded its 80 °C normal operating limit due to restricted water circulation or air pockets around the heating coil, melting the safety thermal fuse.
Evidence: LG Therma V R32 Monobloc air to water heat pump installation manual MFL68026607
- Trapped air in the water circuit or partially closed shut-off valvesPossible
Manual MFL68026607 warns that air trapped inside the water piping causes noise and mechanical damage on the electric heater coil surface because heat cannot dissipate into the water stream.
Evidence: LG Therma V R32 Monobloc air to water heat pump installation manual MFL68026607
- Mechanical fault inside the thermal fuse assembly itselfPossible
The thermal protection link opens prematurely due to internal mechanical fatigue even though the electric heater tank temperature remained below 80 °C.
Evidence: LG Therma V R32 Monobloc air to water heat pump installation manual MFL68026607
- Damaged wire or loose connector in the thermal fuse circuitLess common
A chafed conductor or loose terminal between the internal electric heater thermal fuse and the control PCB breaks continuity and raises code 20.
Evidence: LG Therma V R32 Monobloc air to water heat pump installation manual MFL68026607
Virtual technician
Does CH20 still appear after these checks?
Answer a few questions to narrow down the likely cause and the safest next action. Guidance only - it never replaces a professional.
Sources and technical references
- 1
LG Therma V R32 Monobloc air to water heat pump installation manual MFL68026607
LG Electronics · 2026 · manual
- 2
LG air to water heat pump owner's manual MFL68026705
LG Electronics · 2026 · manual
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