BoilerFaultCodesVaillant F.5 Fault Code
Also written as F5 — same fault.
The Vaillant F.5 fault code means: your boiler has got too hot and has automatically shut down for safety to prevent damage.. It appears on 4 Vaillant models in our database.
Technical description: The system has suffered a fault due to overheating.
What causes the Vaillant F.5 fault?
This fault occurs when the internal water temperature rises too quickly or exceeds a safe limit, causing the boiler to shut down to prevent internal damage. The most common reasons are a failed pump that isn't moving water away from the heat exchanger fast enough, or a build-up of debris causing a blockage in the pipework. It can also be triggered if air is trapped in the system or if the radiators are turned off, leaving the heat with nowhere to go.
This fault is more common in autumn when heating systems are turned back on after a long summer break, as pumps can seize during periods of inactivity.
DIY fix possible
Turn off the boiler and wait 20 minutes for it to cool down.Ensure all radiator valves and the boiler's isolation valves are fully open.Locate the reset button (often marked with a flame symbol or 'R') and press it once.
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Gas Safe Register →Vaillant models that show the F.5 fault code
Click a model for the full fault code list and model-specific repair notes.
Source
Vaillant F.5 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