High Lockout
25 on Viessmann Vitodens 333-F

Your boiler is getting too hot too quickly and is shutting itself down to prevent damage.

Call an engineer soon

Book a Gas Safe engineer within 24 to 48 hours. Your boiler may be unsafe or could break down completely if left.

Technical description

Boiler operates with high boiler water temperature

What causes this fault?

This fault occurs when water cannot travel through the boiler fast enough to carry heat away, causing the internal temperature to spike dangerously high. The most common reasons are a failed circulation pump that isn't pushing the water or a significant blockage within the pipework or heat exchanger. In some cases, it may simply be that the internal sensors have become faulty and are providing inaccurate heat readings to the control board.

This fault often appears when the heating is first turned on after summer, as the internal pump can seize up during long periods of inactivity.

Engineer required

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

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