Emergency
F.684 on Viessmann Vitodens 100-W WB1A

Your boiler has detected that combustion gases are not correctly exiting through the flue, preventing it from operating safely to protect you from fumes.

Reviewed by the BoilerFaultCodes Editorial Team · Last reviewed

Act immediately

Turn off your boiler and stop using gas appliances. Call a Gas Safe engineer now. This fault can be dangerous if ignored.

What the manual says

Manufacturer manual

Burner in a fault state, Back draught safety device faulty

From this model's manual
Verified against the Viessmann Vitodens 100 W Wb1a installation & service manual, page 99.
Plain-English guidance below. Severity, DIY steps, costs and parts are our interpretation to help you act, not text from the manual.

Engineer required

This is a emergency severity fault. Stop using the boiler and call a Gas Safe registered engineer. Do not attempt gas-related repairs yourself.

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