BoilerFaultCodesVaillant F26 Fault Code
The Vaillant F26 fault code means: your boiler has detected that the water is heating up far too quickly or there is a blockage preventing hot water from flowing out of the unit.. It appears on 2 Vaillant models in our database.
Technical description: Maximum delta t
What causes the Vaillant F26 fault?
This fault occurs when the boiler's internal sensors detect that the temperature is rising too rapidly or is too high compared to what the system can handle. The most common reason is a lack of water circulation, often caused by a failing pump or a blockage in the pipework that prevents heat from being carried away from the boiler.
This issue is most frequent when the heating is first turned on after summer, as pump components can seize up during long periods of inactivity.
DIY fix possible
Ensure all radiator valves and lockdowns are fully openCheck that the external filling loop valves are closed properlyReset the boiler to see if the error clears once the system has cooled down
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Gas Safe Register →Vaillant models that show the F26 fault code
Click a model for the full fault code list and model-specific repair notes.
Source
Vaillant F26 fault code information on this page is sourced and cross-checked against the official Vaillant service / installation manuals for each affected model.
Reviewed by the BoilerFaultCodes Editorial Team · Last reviewed 27 April 2026