Medium Warning
9F:8 on Viessmann Vitodens 200-W

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.

Reviewed by the BoilerFaultCodes Editorial Team · Last reviewed

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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 this 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.

Engineer required

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

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