BoilerFaultCodesViessmann 81:1 Fault Code
Also written as 811 — same fault.
The Viessmann 81:1 fault code means: your boiler is notifying you that it has automatically switched between heating and hot water mode based on the outside temperature.. It appears on 5 Viessmann models in our database.
Technical description: Automatic summer/winter changeover
What causes the Viessmann 81:1 fault?
This notification occurs when the outdoor temperature sensor detects a change in the weather that crosses a pre-set threshold. The boiler is simply informing you that it has automatically turned the central heating off for the summer or back on for the winter to save energy. It is a sign that the system's intelligent weather compensation is working correctly to manage your home's efficiency.
This code is most common during the transition periods of spring and autumn when outdoor temperatures fluctuate across the changeover trigger point.
DIY fix possible
Check the outside temperature sensor is not covered or obstructedAdjust the summer/winter threshold temperature in the user settings if the house feels too coldMonitor the boiler to ensure it switches back when the temperature drops
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Gas Safe Register →Viessmann models that show the 81:1 fault code
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Source
Viessmann 81:1 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