BoilerFaultCodesVaillant F5 Fault Code
The Vaillant F5 fault code means: your boiler has got too hot and stopped working to protect itself, likely because the water isn't moving through the system properly.. It appears on 2 Vaillant models in our database.
Technical description: Overheat fault
What causes the Vaillant F5 fault?
This fault occurs when the internal temperature sensors detect that the water inside the boiler has exceeded a safe limit, triggering an emergency shutdown to prevent damage. The most common reason is a failure in the circulation pump or a blockage in the pipework, which stops heat from being carried away from the burner quickly enough. It can also be caused by 'kettling' due to a build-up of limescale or sludge within the main heat exchanger.
This fault is more frequent in autumn when heating systems are first turned on after being dormant all summer, as pump components are more likely to seize during periods of inactivity.
DIY fix possible
Check that all radiator valves are fully openEnsure the boiler pressure is between 1 and 1.5 bar on the gaugePress the 'Reset' button (flame symbol with a cross) to restart the appliance
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Gas Safe Register →Vaillant models that show the F5 fault code
Click a model for the full fault code list and model-specific repair notes.
Source
Vaillant F5 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