BoilerFaultCodesYour boiler has detected that the water is heating up far too quickly or there is a blockage preventing hot water from flowing out of the unit.
Book a Gas Safe engineer within 24–48 hours. Your boiler may be unsafe or could break down completely if left.
Technical description: Maximum delta t
What causes this fault?
This fault occurs when the boiler's internal sensors detect that the temperature is rising too rapidly or is too high compared to what the system can handle. The most common reason is a lack of water circulation, often caused by a failing pump or a blockage in the pipework that prevents heat from being carried away from the boiler.
This issue is most frequent when the heating is first turned on after summer, as pump components can seize up during long periods of inactivity.
DIY fix possible
Ensure all radiator valves and lockdowns are fully openCheck that the external filling loop valves are closed properlyReset the boiler to see if the error clears once the system has cooled down
Find a Gas Safe engineer
Search the official register for a qualified engineer in your area.
Gas Safe Register →Protect yourself from future repair bills
A boiler cover plan would cover faults like F26 — no call-out fees, no surprises.
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Parts for discontinued models can be hard to find and expensive. A new A-rated boiler could save you money on both repairs and energy bills.
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