- Boilers
- Gas boiler
F20
Glow-worm F20 on a gas boiler: safety thermal cut out operated
Risk level
High
Professional inspection is recommended.
Urgency
High
Quick answer
On Glow-worm boilers, F.20 means the safety thermal cut out has operated and shut the boiler off. Check the pressure is 1 to 1.5 bar and top up if needed, make sure radiator valves are open, then reset. If it returns, call your heating installer.
Safety first
Stop and check this first
- Do not keep resetting F.20: it is a thermal safety cut. If it returns after pressure and valves are right, leave the boiler off.
- If you smell gas, Glow-worm says to leave the property immediately and call the National Gas Emergency Service on 0800 111 999. Do not operate switches on the way out and make the call from outside.
Safe checks you can perform
- Pressure and refill
- Radiator valves and leaks
- One reset
- Circulation, sensor and ignition check
Leave these tasks to a qualified professional
- Do not attempt this professional task: System circulation check
- Do not attempt this professional task: Sensor tests
- Do not attempt this professional task: Combustion check after an overheat
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.
System circulation check
What they doThe installer checks external pumps, zone valves and radiator valves and removes airlocks.
They must show youcirculation restored.
In writingwhat was found in the system.
Sensor tests
What they doThe engineer tests the flow and return sensors, the water pressure sensor and the volume flow sensor against the manufacturer's data.
They must show youthe readings.
In writingvalues and the reference of any part replaced.
Combustion check after an overheat
What they doAfter a thermal cut out, the engineer checks the flue and measures combustion with an analyser.
They must show youthe analyser printout.
In writingreadings and flue condition.
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.
Pressure and refill
- Who:
- you.
- How:
- read the gauge; if it is below 1 bar, refill the system to 1 to 1.5 bar with the filling loop as your manual describes.
- Result that matters:
- whether pressure was low.
- If it was low:
- refill and reset; low water fits F.20.
- If it was correct:
- move on to radiators and valves.
- Evidence to keep:
- a photo of the gauge before and after.
Evidence: Glow-worm boiler fault codes: what they mean and how to fix them
Radiator valves and leaks
- Who:
- you.
- How:
- check radiator valves are open and look for leaks around radiators and pipes, as Glow-worm advises.
- Result that matters:
- whether water can circulate.
- If valves were closed:
- open them and reset.
- If a leak shows:
- tell the installer where.
- Evidence to keep:
- a note of valves and any leak.
Evidence: Glow-worm boiler fault codes: what they mean and how to fix them
One reset
- Who:
- you.
- How:
- reset the boiler as your user manual describes.
- Result that matters:
- whether F.20 stays away.
- If it returns:
- stop; contact your heating installer.
- If it runs:
- watch whether radiators heat evenly.
- Evidence to keep:
- a note of each reset.
Evidence: Glow-worm boiler fault codes: what they mean and how to fix them
Circulation, sensor and ignition check
- Who:
- your heating installer or a Gas Safe registered engineer.
- How:
- checks external pumps, zone valves and airlocks, then the boiler sensors and ignition parts.
- Result that matters:
- what stopped heat being carried away.
- If the system is at fault:
- valves, pump or air are corrected.
- If the boiler is at fault:
- the sensor or ignition part is replaced.
- Evidence to keep:
- a job sheet with findings.
Glow-worm advises against accessing the inside of the boiler; ignition parts and sensors are engineer work.
Evidence: HSE gas safety for homeowners · Glow-worm boiler fault codes: what they mean and how to fix them
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
- Boilers
- Brand
- Glow-worm
- Product type
- Gas boiler
Known code variants
- 20
- A21
- calentador Cointra F20
- cocina LG error F20
- code erreur F20 chaudière Vaillant
- code erreur F20 lave-linge Siemens
- codice E20 Siemens
- codice F20 Vaillant
- código E20 Siemens
- Cointra F20
- Cointra water heater F20
- CPE T F20
- E00-20
- erreur E20 Siemens
- Error F20 caldera
- error F20 lavadora Siemens
- Error F20 Saunier Duval
- errore F20 caldaia Vaillant
- errore F20 lavatrice Siemens
- F 20
- F20 Siemens
- Fehlercode F20 Vaillant
- Glow worm boiler F20
- Glow worm F20
- Glow-worm F.20
- Glow-worm F20
- Glow-worm F20 overheat
- lavadora Miele anomalía de calentamiento
- LG F20 bloqueo puerta horno inferior
- LG F20 lower oven door lock
- LG horno F20
- LG oven error F20
- LG range F20
- Miele F20 no calienta
- Miele F20 not heating
- Miele lavadora F20
- Miele Thermostop
- Miele washer F20
- Miele washing machine heating fault
- panne F20 Vaillant
- Saunier Duval F20
- Siemens E20
- Siemens F20
- Siemens Fehlercode E20
- Siemens Waschmaschine Fehler F20
- Siemens washer F20
- Siemens washing machine E20
- Supreme E TS F20
- Thema NOx F20
- Vaillant boiler F20
- Vaillant F.20
- Vaillant F20
- Vaillant Fehler F20
What it means
Glow-worm's fault code page gives F.20 as the safety thermal cut out having operated and shut the boiler off. Possible causes it lists are system pressure below the normal 1 to 1.5 bar, faulty wiring or flow and return sensors, a circulation fault (pump, valve, blockage), a defective water pressure sensor or volume flow sensor, and the ignition cable, plug or electrode. Glow-worm sends F.20 to the installer first because it often relates to how the boiler works with the heating system.
Warnings and stop conditions
- Higheditorial warning
Do not keep resetting F.20: it is a thermal safety cut. If it returns after pressure and valves are right, leave the boiler off.
Evidence: Glow-worm boiler fault codes: what they mean and how to fix them
- Criticaleditorial warning
If you smell gas, Glow-worm says to leave the property immediately and call the National Gas Emergency Service on 0800 111 999. Do not operate switches on the way out and make the call from outside.
Evidence: HSE gas safety for homeowners · Glow-worm boiler fault codes: what they mean and how to fix them
Symptoms and causes
Probable causes · in order
- Low pressure or airCommon
Fits when the gauge is under 1 bar or radiators gurgle; Glow-worm asks you to check pressure and airlocks first.
Evidence: Glow-worm boiler fault codes: what they mean and how to fix them
- Circulation fault in the systemPossible
Fits if radiator valves, external pumps or zone valves were closed or not working; water stops moving and overheats.
Evidence: Glow-worm boiler fault codes: what they mean and how to fix them
- Sensors or ignition partsPossible
Fits when pressure and circulation are right; Glow-worm lists flow and return sensors, pressure and flow sensors and ignition parts.
Evidence: Glow-worm boiler fault codes: what they mean and how to fix them
Virtual technician
Does F20 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
Health and Safety Executive · official_doc
- 2
Glow-worm boiler fault codes: what they mean and how to fix them
Glow-worm · official_doc
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