BoilerFaultCodesVaillant F.81 Fault Code
Also written as F81 — same fault.
The Vaillant F.81 fault code means: your boiler is struggling to heat your hot water tank effectively, causing it to time out before the water reaches the requested temperature.. It appears on 3 Vaillant models in our database.
Technical description: Fault:cylinderchargingpump OnlyinconjunctionwithF.91 Cylinderisnotfullychargedafterspecifiedtime. – Checkcylinderchargingsensorandcylindersensor – AirintheactoSTORpump – Inspectcableharnessforpump – Checktheimpellersensorand/orlimiterintheproduct – Prioritisingdivertervalvedefective – Secondaryheatexchangerblocked – Pumpdefective
Engineer required
This fault requires a Gas Safe registered engineer. Do not attempt gas-related repairs yourself.
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Vaillant models that show the F.81 fault code
Click a model for the full fault code list and model-specific repair notes.
Source
Vaillant F.81 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 7 July 2026