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Viessmann fault code

Viessmann F9: fan speed too low when the burner starts

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.

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.

From Viessmann's own fault tables. Your model name is usually on the front of the boiler or in the user manual.

Model	Meaning	Causes listed for engineers
Vitodens 100-W (B1HC/B1KC)	Fan speed too low during burner start	Check the fan, fan cables, fan power supply and fan control

Can I fix F9 myself?

No. It is a fan fault that needs a Gas Safe registered engineer.