F.81 Medium

Vaillant 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 1 Vaillant model in our database.

Book a service

This fault should be looked at by a Gas Safe engineer within the next week or two. It may get worse if left.

Technical description: Fault:cylinderchargingpump OnlyinconjunctionwithF.91 Cylinderisnotfullychargedafterspecifiedtime. – Checkcylinderchargingsensorandcylindersensor – AirintheactoSTORpump – Inspectcableharnessforpump – Checktheimpellersensorand/orlimiterintheproduct – Prioritisingdivertervalvedefective – Secondaryheatexchangerblocked – Pumpdefective
Typical repair cost £150-350 (including labour)

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 27 April 2026