BoilerFaultCodesKeston B 25 Fault Code
Also written as B25 — same fault.
The Keston B 25 fault code means: your boiler is overheating because water isn't moving through the system fast enough, likely due to trapped air or a blockage in your pipes.. It appears on 3 Keston models in our database.
Technical description: Flow temperature climbing too quickly. Possible air lock or circulation blockage
Engineer required
This fault requires a Gas Safe registered engineer. Do not attempt gas-related repairs yourself.
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Keston models that show the B 25 fault code
Click a model for the full fault code list and model-specific repair notes.
Source
Keston B 25 fault code information on this page is sourced and cross-checked against the official Keston service / installation manuals for each affected model.
Reviewed by the BoilerFaultCodes Editorial Team · Last reviewed 7 July 2026