F.684 Burner in a fault state Back draught safety de- vice faulty Check the back draught safety de- vice and replace if necessary
The F684 error code on Viessmann boilers indicates f.684 burner in a fault state back draught safety de- vice faulty check the back draught safety de- vice and replace if necessary.
Viessmann F684 means: f.684 burner in a fault state back draught safety de- vice faulty check the back draught safety de- vice and replace if necessary. The usual causes are blocked or partially blocked flue system, incorrect flue installation (too many bends, excessive length) or terminal guard blocked with debris or ice. You can safely reset the boiler once — if F684 returns, do not keep resetting. A qualified heating engineer should diagnose the root cause.
What Does F684 Mean
The F684 code is shown by the boiler’s control board when it detects f.684 burner in a fault state back draught safety de- vice faulty check the back draught safety de- vice and replace if necessary. The boiler then stops instead of continuing to run in that condition: the code is the appliance reporting a problem, not proof that the boiler is broken.
| What you see on the boiler |
|---|
| The display shows F684 and the boiler stops until the fault is cleared |
Common Causes
Causes documented in 2 technical sources.
| Cause |
|---|
| Blocked or partially blocked flue system |
| incorrect flue installation (too many bends, excessive length) |
| terminal guard blocked with debris or ice |
| condensate drain blocked causing back-pressure in the flue |
| combustion issue — incorrect gas/air ratio |
Safety Warning
F684 involves electrical or pressure-related components. Do not attempt repairs without proper qualifications. Ensure the boiler is electrically isolated before any inspection. If you notice water leaks, unusual sounds, or pressure anomalies, contact a qualified heating engineer immediately.
Parts & Prices
Prices shown are indicative. Actual prices and availability vary by region.
Step-by-Step Diagnosis
684 Burner in a fault state Back draught safety de- vice faulty Check the back draught safety de- vice and replace if necessary
How to Fix It
Clear any flue blockage
correct the flue installation if it exceeds maximum length or bends
clear ice from the flue terminal (insulate external condensate pipe)
clear the condensate drain
if combustion analysis shows incorrect values, adjust the gas valve or service the burner
Refer to the Viessmann service documentation for the detailed procedure. Only a qualified engineer should carry out these repairs.
Diagnostic Tools
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Affected Models
Viessmann models that can display fault code F684:
| Model Family | Output Range | Frequency |
|---|---|---|
| All Viessmann models | — | Possible |
Frequently Asked Questions
Clear any flue blockage. According to the technical documentation, this is the first check on the list for F684.
F684 is not classified as a high-severity fault, so the boiler can be reset once. If F684 comes back, stop resetting: the underlying cause is still there and needs a qualified heating engineer.