- Boilers
- Tankless gas water heater
F20
Cointra F20 on a gas water heater: combustion control fault
Risk level
High
Professional inspection is recommended.
Urgency
High
Quick answer
F20 on Cointra CPE T and Supreme E TS water heaters indicates a combustion control fault: the fan, a wrong flue restrictor, or a flue that is wrongly sized or blocked. Six occurrences in ten minutes turn it into A21. It needs a gas technician.
Safety first
Stop and check this first
- If you detect a gas smell with F20 on screen, get out straight away leaving doors and windows open, without switching anything on or off or using your phone indoors. From outside, contact the gas emergency service.
- F20 and A21 mean combustion is poor, which can produce carbon monoxide. If a CO alarm sounds or you feel drowsy or nauseous, stop using hot water, get into fresh air and call the emergency services.
Safe checks you can perform
- Watch the burner indicator and A21
- Recall recent changes to the flue
- Request a combustion measurement
Leave these tasks to a qualified professional
- Do not attempt this professional task: Check the fan and wiring
- Do not attempt this professional task: Inspect the complete flue
- Do not attempt this professional task: Verify the fitted restrictor
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.
Check the fan and wiring
What they doThe authorised gas technician checks the modulating fan and its wiring, the first remedy the table gives for F20.
They must show youthe condition of the fan and its connections.
In writingthe check carried out, any part replaced and the combustion test result.
Inspect the complete flue
What they doThe technician checks the length, bends, diameter and terminal of the air and flue duct and clears any obstruction, making sure the maximum permitted length is not exceeded.
They must show youthe duct route and the terminal inspected.
In writingthe flue condition, corrections made and the final combustion reading.
Verify the fitted restrictor
What they doThe technician confirms that the flue restrictor matches the model and duct length according to the manual's tables, and that it is correctly seated with its seal.
They must show youthe diameter of the fitted restrictor against the specified one.
In writingthe restrictor found, the final restrictor and the duct length calculation.
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.
Watch the burner indicator and A21
- Who:
- you, only looking at the panel.
- How:
- see whether the burner on indicator is flashing and count how many times F20 appears over a short period.
- Result that matters:
- telling a one-off F20 apart from escalation to A21.
- If A21 appears:
- do not keep resetting and call the service engineer.
- If F20 keeps returning without reaching A21:
- stop using hot water and report it anyway.
- Evidence to keep:
- photos of F20 or A21 with the time.
Evidence: Cointra CPE T, instrucción de uso, instalación y mantenimiento (cod. A73022110 Rev.03) · Cointra SUPREME E TS, instrucciones de uso, instalación y mantenimiento (cod. 3541A273 Rev.01)
Recall recent changes to the flue
- Who:
- you, with the installation paperwork.
- How:
- note whether the flue duct was changed or extended, whether the heater was installed recently, or whether there has been facade work near the terminal.
- Result that matters:
- giving the technician leads on the restrictor and flue.
- If there were changes:
- hand over the invoice or job sheet for that work.
- If there were none:
- say so, without touching or dismantling the duct.
- Evidence to keep:
- the flue installation paperwork.
Evidence: Cointra CPE T, instrucción de uso, instalación y mantenimiento (cod. A73022110 Rev.03) · Cointra SUPREME E TS, instrucciones de uso, instalación y mantenimiento (cod. 3541A273 Rev.01)
Request a combustion measurement
- Who:
- the gas technician diagnosing F20.
- How:
- they check the fan, restrictor and flue and measure combustion with an analyser.
- Result that matters:
- which of the three F20 causes applies to your installation.
- If the restrictor is wrong:
- it must be swapped for the one in the table.
- If the flue exceeds the permitted length:
- the route must be corrected.
- Evidence to keep:
- the analyser printout and the F20 job sheet.
Removing ducts or restrictors affects flue extraction; it is work for the technician only.
Evidence: Cointra CPE T, instrucción de uso, instalación y mantenimiento (cod. A73022110 Rev.03) · Cointra SUPREME E TS, instrucciones de uso, instalación y mantenimiento (cod. 3541A273 Rev.01)
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
- Cointra
- Product type
- Tankless gas water heater
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
The water heater monitors combustion quality and shows F20 when it detects that combustion is not right. Cointra's table lists three origins: a fault in the fan or its wiring, an incorrect flue restrictor (diaphragm), and a flue that is wrongly sized or obstructed. During the fault the burner indicator flashes. F20 is temporary, but if it occurs six times in the last ten minutes the appliance moves to the permanent lockout A21, poor combustion, which refers back to the same causes.
Warnings and stop conditions
- Criticaleditorial warning
If you detect a gas smell with F20 on screen, get out straight away leaving doors and windows open, without switching anything on or off or using your phone indoors. From outside, contact the gas emergency service.
Evidence: Cointra CPE T, instrucción de uso, instalación y mantenimiento (cod. A73022110 Rev.03) · Cointra SUPREME E TS, instrucciones de uso, instalación y mantenimiento (cod. 3541A273 Rev.01)
- Criticaleditorial warning
F20 and A21 mean combustion is poor, which can produce carbon monoxide. If a CO alarm sounds or you feel drowsy or nauseous, stop using hot water, get into fresh air and call the emergency services.
Evidence: Cointra CPE T, instrucción de uso, instalación y mantenimiento (cod. A73022110 Rev.03) · Cointra SUPREME E TS, instrucciones de uso, instalación y mantenimiento (cod. 3541A273 Rev.01)
Symptoms and causes
Probable causes · in order
- Fan faultPossible
Cointra asks for the fan and its wiring to be checked. The modulating fan plays a direct part in combustion on this room-sealed appliance, so a fan fault or a cut wire can trigger this control.
Evidence: Cointra CPE T, instrucción de uso, instalación y mantenimiento (cod. A73022110 Rev.03) · Cointra SUPREME E TS, instrucciones de uso, instalación y mantenimiento (cod. 3541A273 Rev.01)
- Incorrect flue restrictorPossible
The manual requires the restrictor matching the duct length to be fitted; on the CPE T 11 it is 43 mm and on the 14 and 17 it is 50 mm. A wrong or badly fitted restrictor upsets the draught and must be replaced.
Evidence: Cointra CPE T, instrucción de uso, instalación y mantenimiento (cod. A73022110 Rev.03) · Cointra SUPREME E TS, instrucciones de uso, instalación y mantenimiento (cod. 3541A273 Rev.01)
- Flue wrongly sized or obstructedPossible
The third F20 cause is a flue whose length or diameter is outside the permitted range, or which is blocked. The stated remedy is a check of the complete flue, including bends and terminal.
Evidence: Cointra CPE T, instrucción de uso, instalación y mantenimiento (cod. A73022110 Rev.03) · Cointra SUPREME E TS, instrucciones de uso, instalación y mantenimiento (cod. 3541A273 Rev.01)
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
Cointra CPE T, instrucción de uso, instalación y mantenimiento (cod. A73022110 Rev.03)
Cointra · 2018 · manual
- 2
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