F9Viessmann
Viessmann F9: fan speed too low when the burner starts
Checked against Viessmann's own documentation: Viessmann Vitodens 100-W (B1HC/B1KC) installation and service instructions, 5697 787 GB. Last reviewed October 2026. How we check
Engineer only
The fan isn't running fast enough at start-up.
What you can safely do
There is nothing you can safely do yourself with this one. Leave the boiler off and book an engineer.
Before you book an engineer
- Write down the code exactly as the display shows it: F9.
- Find your boiler's model name. It is usually on the front panel or in the user manual. The table below shows why it matters: Viessmann codes can mean different things on different models.
- Note when it started and what the boiler was doing (heating, hot water, or starting up).
- If the boiler is still under warranty, check your warranty paperwork before you book anyone.
Call a Gas Safe engineer if
- Book a Gas Safe registered engineer.
Find a Gas Safe engineer near you
Opens the official Gas Safe Register search.
Applies to Vitodens 100-W (B1HC/B1KC). Codes can differ between models, so check your manual.