- Boilers
- Gas boiler
F25
Vaillant F.25 on a gas boiler: flue gas temperature too high
Risk level
High
Professional inspection is recommended.
Urgency
High
Quick answer
Vaillant F.25 is a safety shutdown because the flue gas temperature is too high. Vaillant treats it as a job for an expert: if you smell fumes ventilate and leave, and call the official service centre.
Safety first
Stop and check this first
- F.25 is about removing combustion gases. If you notice a fumes smell, headache, dizziness or nausea at home, switch the boiler off, ventilate, leave and get help. Do not cover or tamper with the flue terminal.
- If you smell gas: do not operate switches, plugs, doorbells or phones, do not light flames or smoke, open doors and windows, close the meter valve or the gas shut-off valve if you can, leave the building and call the gas company's emergency service from outside. This is what Vaillant asks.
Safe checks you can perform
- Fumes smell at home
- Flue terminal from outside
- Water pressure on the gauge
- Flue, pump and limiter check by the technician
Leave these tasks to a qualified professional
- Do not attempt this professional task: Pump, bleeding and pressure check
- Do not attempt this professional task: Flue check with an analyser
- Do not attempt this professional task: Limiter and wiring check
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.
Pump, bleeding and pressure check
What they doThe technician checks the heating pump, internal bleeding and system pressure.
They must show youthe pump running and the pressure.
In writingwhat was found and corrected.
Flue check with an analyser
What they doThe technician checks the flue gas route and measures flue temperature, CO and CO2 or O2 with an analyser.
They must show youthe analyser printout and the flue's condition.
In writingthe flue length and condition and the values measured.
Limiter and wiring check
What they doThe technician checks the safety temperature limiter connector and the cable harness.
They must show youthe connector checked.
In writingthe repair and the part number of anything replaced.
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.
Fumes smell at home
- Who:
- you.
- How:
- notice whether there is a smell of burnt gases near the boiler or in other rooms.
- Result that matters:
- whether there is a smell or symptoms such as headache or dizziness.
- If there is a smell or symptoms:
- switch the boiler off, open windows, leave and call the official service centre; do not switch it on again.
- If there is no smell:
- carry on.
- Evidence to keep:
- a note of the time and what you noticed.
Evidence: Vaillant ecoTEC plus VMW ..6/5-5, instrucciones de instalación y mantenimiento 0020244991_01
Flue terminal from outside
- Who:
- you.
- How:
- look at the flue terminal from the ground or a window, without touching it or climbing.
- Result that matters:
- whether it is covered, damaged or blocked by something in front (scaffolding, sheeting, plants).
- If it is blocked or damaged:
- tell the technician; Vaillant lists a blocked flue route.
- If it is clear:
- it points to water, pump or wiring.
- Evidence to keep:
- photos of the terminal.
Evidence: Vaillant ecoTEC plus VMW ..6/5-5, instrucciones de instalación y mantenimiento 0020244991_01
Water pressure on the gauge
- Who:
- you.
- How:
- read the gauge with the boiler cold. If it is low, Vaillant says to top up with the boiler's filling key to about 1.5 bar.
- Result that matters:
- whether the pressure is low.
- If it was low and you top up:
- note how much and watch whether F.25 returns.
- If it is fine:
- low water becomes less likely.
- Evidence to keep:
- a photo of the gauge before and after.
Evidence: Lista oficial de códigos de error de calderas Vaillant · Vaillant ecoTEC plus VMW ..6/5-5, instrucciones de instalación y mantenimiento 0020244991_01
Flue, pump and limiter check by the technician
- Who:
- authorised gas technician.
- How:
- checks the flue gas route, pump, bleeding and the safety temperature limiter connector, and runs the test programme Vaillant specifies.
- Result that matters:
- which cause is present.
- If the flue is blocked or wrongly sized:
- that confirms the cause.
- If the flue is fine:
- it points to circulation or wiring.
- Evidence to keep:
- a report of what was checked and the measurements.
The flue, the limiter and internal parts are for an authorised gas technician; Vaillant treats this as a job for an expert.
Evidence: Lista oficial de códigos de error de calderas Vaillant · Vaillant ecoTEC plus VMW ..6/5-5, instrucciones de instalación y mantenimiento 0020244991_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
- Vaillant
- Product type
- Gas boiler
Known code variants
- anomalía F25 Miele
- anomalie F25 Miele
- code erreur F25 lave-linge Siemens
- code erreur F25 lave-vaisselle Miele
- codice E25 Siemens
- código E25 Siemens
- E00-25
- ecoTEC F.25
- erreur E25 Siemens
- error F25 lavadora Siemens
- error F25 lavavajillas Miele
- Error F25 Vaillant
- errore F25 lavastoviglie Miele
- errore F25 lavatrice Siemens
- F.25
- F25 Siemens
- Fehlercode F25 Miele
- guasto F25 Miele
- Miele dishwasher F25
- Miele F25
- Miele fault F25
- Miele Geschirrspüler Fehler F25
- Siemens E25
- Siemens F25
- Siemens Fehlercode E25
- Siemens Waschmaschine Fehler F25
- Siemens washer F25
- Siemens washing machine E25
- Vaillant F.25
What it means
In the ecoTEC plus table, F.25 is a safety shutdown for flue gas temperature too high. Vaillant says to check the safety temperature limiter connector and the cable harness, the pump cable, bleeding, the flue gas route (blockage, unfavourable wind, flue pipe too long), low water in the circuit and the heating pump. Vaillant Spain's official list adds a wiring problem. The code does not say which of those causes is at work.
Warnings and stop conditions
- Criticaleditorial warning
F.25 is about removing combustion gases. If you notice a fumes smell, headache, dizziness or nausea at home, switch the boiler off, ventilate, leave and get help. Do not cover or tamper with the flue terminal.
Evidence: Vaillant ecoTEC plus VMW ..6/5-5, instrucciones de instalación y mantenimiento 0020244991_01
- Criticalsource verified
If you smell gas: do not operate switches, plugs, doorbells or phones, do not light flames or smoke, open doors and windows, close the meter valve or the gas shut-off valve if you can, leave the building and call the gas company's emergency service from outside. This is what Vaillant asks.
Evidence: Vaillant ecoTEC plus VMW ..6/5-5, instrucciones de instalación y mantenimiento 0020244991_01
Symptoms and causes
Probable causes · in order
- Flue gas routePossible
Vaillant lists blockage, unfavourable wind or a flue pipe that is too long. It fits if F.25 shows up in strong wind or after work near the terminal.
Evidence: Vaillant ecoTEC plus VMW ..6/5-5, instrucciones de instalación y mantenimiento 0020244991_01
- Low water or poor circulationPossible
Vaillant includes low water in the circuit, bleeding and the heating pump. It usually goes with low gauge pressure or cold radiators.
Evidence: Vaillant ecoTEC plus VMW ..6/5-5, instrucciones de instalación y mantenimiento 0020244991_01
- Limiter wiring or connectorPossible
Vaillant Spain lists a wiring problem, and the manual says to check the limiter connector and harness. It is seen with the boiler open.
Evidence: Lista oficial de códigos de error de calderas Vaillant · Vaillant ecoTEC plus VMW ..6/5-5, instrucciones de instalación y mantenimiento 0020244991_01
Virtual technician
Does F25 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
Lista oficial de códigos de error de calderas Vaillant
Vaillant Group · 2026 · official_doc
- 2
Vaillant ecoTEC plus VMW ..6/5-5, instrucciones de instalación y mantenimiento 0020244991_01
Vaillant · 2017 · manual
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