9F:8 Medium

Viessmann 9F:8 Fault Code

Also written as 9F8 — same fault.

The Viessmann 9F:8 fault code means: your secondary heating circuit is not reaching the required temperature difference to operate correctly, likely due to a flow or sensor issue in that specific zone.. It appears on 5 Viessmann models in our database.

Book a service

This fault should be looked at by a Gas Safe engineer within the next week or two. It may get worse if left.

Technical description: Differential temperature 8K; only in conjunction with a mixer circuit

What causes the Viessmann 9F:8 fault?

This fault occurs when the second heating zone, typically controlled by a mixing valve, fails to reach the required temperature spread to heat your home effectively. It is usually caused by a sticking mixer motor, a faulty temperature sensor, or a pump that isn't circulating the water through that specific circuit correctly.

This issue often surfaces at the start of autumn when the heating is turned back on after summer, as the moving parts in the mixing valve can seize up during long periods of inactivity.

Typical repair cost £120-280 (including labour)

Engineer required

This fault requires a Gas Safe registered engineer. Do not attempt gas-related repairs yourself.

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Viessmann models that show the 9F:8 fault code

Click a model for the full fault code list and model-specific repair notes.

Source

Viessmann 9F:8 fault code information on this page is sourced and cross-checked against the official Viessmann service / installation manuals for each affected model.

Reviewed by the BoilerFaultCodes Editorial Team · Last reviewed 27 April 2026