BoilerFaultCodesVaillant F25 Fault Code
The Vaillant F25 fault code means: your boiler is heating up too quickly, causing it to shut down to prevent damage from overheating.. It appears on 2 Vaillant models in our database.
Technical description: Maximum temperature rise slope
What causes the Vaillant F25 fault?
This fault occurs when the boiler's internal sensors detect that the water temperature is rising much faster than it should. The most common reason is that water isn't moving through the system quickly enough, often due to a failing pump or a buildup of debris and sludge that creates a blockage. Because the heat cannot be carried away to your radiators, the boiler shuts down to prevent itself from overheating and becoming damaged.
This issue often surfaces in autumn when the central heating is switched back on after a summer of inactivity, which can cause internal pump components to seize.
DIY fix possible
Check that all radiator valves are fully openEnsure the central heating system pressure is between 1.0 and 1.5 barBleed all radiators to remove trapped air that might be blocking water flow
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Gas Safe Register →Vaillant models that show the F25 fault code
Click a model for the full fault code list and model-specific repair notes.
Source
Vaillant F25 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