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Viessmann Error Code F5
Air pressure switch or fan fault — the boiler cannot confirm safe air flow.
Before a boiler lights, it proves it can move air safely — drawing combustion air in and pushing flue gas out. F5 means that proof failed: the air pressure switch did not change state the way the sequence expects. The boiler refuses to fire, which is the correct response, because burning gas without confirmed flue flow is how carbon monoxide problems start.
What causes F5
1. A blocked flue or air intake terminal
Snow drifted across the terminal, leaves, or a bird nest. The most common cause and often visible from outside without any tools.
2. A failing fan
A fan running slow, noisily, or not at all cannot generate the pressure difference the switch is looking for. Bearings wear and this is a normal end-of-life failure.
3. The pressure switch or its sensing tubes
The small tubes connecting the switch can perish, crack, or fill with condensate, so a healthy fan still reads as a fault. Cheap to fix once identified.
4. Condensate backing up into the flue path
A blocked condensate route can put water where air should be, which disrupts the pressure reading and often produces other codes alongside F5.
What you can safely check first
- Look at the flue terminal outside for snow, leaves or nesting, and clear what you can reach safely from the ground.
- Listen to the boiler on a start attempt — a fan that is silent, or noticeably louder and rougher than usual, is a useful clue.
- One reset is reasonable after clearing an obvious blockage.
When to stop and call us
If the terminal is clear and F5 returns, call. Anything inside the sealed combustion circuit — fan, switch, sensing tubes — is licensed work, and this is the fault family where getting it wrong has combustion-safety consequences.
Seeing F5 on your Viessmann?
Book online and we will diagnose the actual cause, explain what we find, and price the repair before starting.
Viessmann F5 FAQs
What does Viessmann fault code F5 mean?
An air pressure switch or fan fault. The boiler could not confirm safe air flow through the flue before lighting, so it refused to fire. That refusal is the safety system working correctly.
Is F5 dangerous?
The lockout itself is protective — the boiler is declining to burn gas without confirmed flue flow. What you should not do is keep resetting it, because the checks it is failing are the combustion-safety ones.
Can snow really cause F5?
Yes, and it is one of the most common causes here. A drift or ice across the external flue terminal is enough to disrupt the airflow the boiler is trying to prove, and clearing it often resolves the code.
How much does an F5 repair cost?
Our $99 service fee applies toward the repair with an upfront price before work starts. A blocked terminal or a perished sensing tube is inexpensive; a replacement fan is a larger part, and we tell you which before proceeding.
Other codes on this unit? See all Viessmann error codes.
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Viessmann F5 diagnosed properly
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All Viessmann error codes
The other codes this unit can show, and what each one means.
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Viessmann F5 diagnosed properly
We diagnose the cause rather than clearing the code and hoping. You get an upfront price before any work starts.
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