High
B 30 on Keston C40

Your boiler is struggling to move water around your heating system properly, causing it to overheat because the hot water isn't getting away from the heat exchanger fast enough.

Reviewed by the BoilerFaultCodes Editorial Team · Last reviewed

Call an engineer soon

Book a Gas Safe engineer within 24 to 48 hours. Your boiler may be unsafe or could break down completely if left.

What the manual says

Manufacturer manual

Difference between flow and return temperatures too high. Check water circulation

From this model's manual
Verified against the Keston C40 installation & service manual, page 31.
Plain-English guidance below. Severity, DIY steps, costs and parts are our interpretation to help you act, not text from the manual.

Engineer required

This is a high severity fault. Stop using the boiler and call a Gas Safe registered engineer. Do not attempt gas-related repairs yourself.

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