How much does it cost to fix Keston error code B 25?

Typical repair cost for the Keston B 25 fault is £120-250 including labour.

B 25 High

Keston B 25 Fault Code

Q: What does the Keston B 25 fault code mean?

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.

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

Typical repair cost £120-250 (including labour)

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 27 April 2026