- Boilers
- Gas boiler
F33
Saunier Duval F33 on a Thema NOx: air flow fault
Risk level
High
Professional inspection is recommended.
Urgency
High
Quick answer
Saunier Duval F33 means an air flow fault on the Thema NOx after 5 start attempts (St33 flashing). Switch the boiler out of service, wait 5 seconds and switch it on. If it persists, call the official service centre.
Safety first
Stop and check this first
- An air flow fault can mean flue gases are not clearing properly. If you smell smoke or fumes, switch the boiler off, ventilate and do not use it until the service centre checks it. A carbon monoxide alarm in the home adds an early warning.
- 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
- Flue terminal from outside
- Out of service, 5 seconds, back on
- Air and flue circuit check
Leave these tasks to a qualified professional
- Do not attempt this professional task: Duct and terminal inspection
- Do not attempt this professional task: Fan and air monitoring test
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.
Duct and terminal inspection
What they doThe technician checks the flue duct and terminal are sealed and clear.
They must show youphotos of the duct and terminal.
In writingthe condition found and what was cleaned or repaired.
Fan and air monitoring test
What they doThe technician checks the fan turns and the air flow monitor changes state at start-up.
They must show youthe test with the boiler starting.
In writingthe part replaced and the final combustion analysis.
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.
Flue terminal from outside
- Who:
- you.
- How:
- look at the boiler's flue terminal from the ground or a window, without climbing or touching it.
- Result that matters:
- whether it is blocked, damaged or has ice or objects in it.
- If you see a blockage:
- tell the service centre when you call; do not handle it at height.
- If it looks clear:
- the cause may be inside; go on to the restart.
- Evidence to keep:
- a photo of the terminal taken safely.
Evidence: Saunier Duval Thema NOx operating instructions
Out of service, 5 seconds, back on
- Who:
- you.
- How:
- with F33 on the display, switch the boiler out of service, wait 5 seconds and put it back into operation, as Saunier Duval says.
- Result that matters:
- whether it starts without showing St33 or F33 again.
- If it starts and runs:
- note the date.
- If St33 or F33 returns:
- Saunier Duval says to contact the official service centre.
- Evidence to keep:
- a photo of the display with St33 or F33 and the time.
Evidence: Saunier Duval Thema NOx operating instructions
Air and flue circuit check
- Who:
- gas technician from the official service centre.
- How:
- checks the duct and terminal, the fan and the device that monitors air flow, and runs a combustion analysis.
- Result that matters:
- whether air and flue gases move and the control detects it.
- If they find the duct blocked:
- that confirms the outside cause.
- If the duct is clear:
- it points to the fan or the flow monitoring.
- Evidence to keep:
- a report with what was checked, the readings and any part replaced.
The flue, fan and air flow monitoring make up the combustion circuit: only the gas technician touches them.
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
- anomalía F33 Miele
- anomalie F33 Miele
- Bosch E33 espuma
- Bosch E33 foam
- Bosch F33 lavadora
- Bosch F33 washer
- Bosch washer F33 error
- code erreur F33 lave-vaisselle Miele
- E33
- error F33 lavadora Bosch
- error F33 lavavajillas Miele
- Error F33 Saunier Duval
- errore F33 lavastoviglie Miele
- F.33
- Fehlercode F33 Miele
- guasto F33 Miele
- Miele dishwasher F33
- Miele F33
- Miele fault F33
- Miele Geschirrspüler Fehler F33
- Saunier Duval F33
- St33
- Thema NOx F33
- turboMAG F.33
- turboMAG plus F33
- Vaillant F.33
- Vaillant F33 calentador
What it means
On the Thema NOx, Saunier Duval calls St33, F33 and F37 "air flow fault". The boiler makes 5 start attempts at 5 minute intervals, showing St33, before the safety system cuts out and shows F33. The control cannot confirm combustion air and flue gases are moving as they should. It can come from a blocked duct or terminal, the fan or the device that monitors air flow. The code does not say which.
Warnings and stop conditions
- Criticaleditorial warning
An air flow fault can mean flue gases are not clearing properly. If you smell smoke or fumes, switch the boiler off, ventilate and do not use it until the service centre checks it. A carbon monoxide alarm in the home adds an early warning.
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
- Blocked flue terminal or ductCommon
Nests, ice or an object at the terminal cut the flow; it is often visible from outside or coincides with storms.
Evidence: Saunier Duval Thema NOx operating instructions
- Fan not delivering flowPossible
If the terminal is clear and the fault returns, the fan is a suspect; the technician checks it.
Evidence: Saunier Duval Thema NOx operating instructions
- Air flow monitoringPossible
With duct and fan fine, the device that monitors flow or its connection can raise the alarm without a real lack of air.
Evidence: Saunier Duval Thema NOx operating instructions
Virtual technician
Does F33 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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