- Boilers
- Gas boiler
F20
Saunier Duval F20 on a Thema NOx: overheating fault
Risk level
High
Professional inspection is recommended.
Urgency
High
Quick answer
Saunier Duval F20 means an overheating fault on the Thema NOx. Saunier Duval gives no user reset for this code: contact the official service centre. Meanwhile, read the pressure on the display and note it.
Safety first
Stop and check this first
- Saunier Duval says to run the appliance only after the official service centre has fixed the fault and not to tamper with safety devices. Do not force resets with F20.
- If you smell gas: do not use a flame to look for the leak, do not touch switches, a phone or anything that can spark, open doors and windows, close the gas valve, warn your neighbours and call your gas supplier's emergency number.
Safe checks you can perform
- Circuit pressure on the display
- Call the service centre with the details
- Overheat diagnosis
Leave these tasks to a qualified professional
- Do not attempt this professional task: Circulation test and bleeding
- Do not attempt this professional task: Sensor and protection measurement
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.
Circulation test and bleeding
What they doThe technician checks the pump moves water and bleeds air from the boiler and the system.
They must show youthe pump running and a stable temperature.
In writingthe cause found.
Sensor and protection measurement
What they doIf there is circulation, the technician measures the sensor and temperature protection with a multimeter.
They must show youthe readings against the specification.
In writingthe part replaced and why.
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.
Circuit pressure on the display
- Who:
- you.
- How:
- check the pressure on the display with the MODE button, as Saunier Duval describes. If the indicator flashes at 0.5 bar or less, open the blue tap under the boiler to about 1 bar.
- Result that matters:
- whether the pressure is low.
- If it was low:
- it is a key fact for the service centre; if you top up often, Saunier Duval warns of a possible leak.
- If pressure is normal:
- lack of water does not explain it.
- Evidence to keep:
- a photo of the pressure indicator before and after.
Evidence: Saunier Duval Thema NOx operating instructions
Call the service centre with the details
- Who:
- you.
- How:
- call the official service centre, which is Saunier Duval's remedy for F20, with the code, the model and the pressure you noted.
- Result that matters:
- that the technician arrives with the fault information.
- If F20 appeared after topping up or bleeding:
- say so; it points to air in the circuit.
- If it appeared with no change to the system:
- say so; it points to the boiler.
- Evidence to keep:
- a photo of the display with F20 and of the rating plate.
Evidence: Saunier Duval Thema NOx operating instructions
Overheat diagnosis
- Who:
- official service centre technician.
- How:
- checks circulation, bleeds air and measures the sensor and the temperature protection.
- Result that matters:
- whether water circulates and whether the protection trips at the right temperature.
- If it does not circulate:
- that confirms the pump, air or a blockage.
- If it circulates:
- it points to the sensor or the protection.
- Evidence to keep:
- a report with the readings and any part replaced.
The temperature protection, pump and sensors are inside the boiler, next to gas and mains voltage: service centre only.
Evidence: Saunier Duval Thema NOx operating instructions
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
- Saunier Duval
- 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
On the Thema NOx, Saunier Duval groups F20 and F25 as "overheating fault" and the only remedy it gives is to contact the official service centre. A temperature protection has detected excessive heat. It can come from water not circulating (low pressure, air or the pump), a sensor or the protection itself. The code does not say which, so forcing resets is not advisable.
Warnings and stop conditions
- Highsource verified
Saunier Duval says to run the appliance only after the official service centre has fixed the fault and not to tamper with safety devices. Do not force resets with F20.
Evidence: Saunier Duval Thema NOx operating instructions
- Criticalsource verified
If you smell gas: do not use a flame to look for the leak, do not touch switches, a phone or anything that can spark, open doors and windows, close the gas valve, warn your neighbours and call your gas supplier's emergency number.
Evidence: Saunier Duval Thema NOx operating instructions
Symptoms and causes
Probable causes · in order
- Lack of water or low pressurePossible
If the pressure indicator flashes at 0.5 bar or less, water is missing and heat is not carried away; Saunier Duval gives its own remedy for that.
Evidence: Saunier Duval Thema NOx operating instructions
- Insufficient circulationPossible
With correct pressure, air or a pump that does not push leave the water still in the heat exchanger; the service centre checks it.
Evidence: Saunier Duval Thema NOx operating instructions
- Temperature sensor or protectionPossible
If circulation is good, the protection that trips or the sensor feeding it may be faulty; measuring confirms it.
Evidence: Saunier Duval Thema NOx operating instructions
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
Saunier Duval Thema NOx operating instructions
Saunier Duval · 2012 · manual
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