BoilerFaultCodesYour boiler has detected that water isn't moving through the system fast enough, causing a large temperature gap between the outgoing and incoming pipes.
Book a Gas Safe engineer within 24–48 hours. Your boiler may be unsafe or could break down completely if left.
Technical description: Water shortage (Temperature difference between flow and return NTC is too large)
What causes this fault?
This fault occurs when the boiler's internal sensors detect that water is heating up too quickly or isn't circulating fast enough to carry heat away. The most common reasons are a seized or failing pump, a blockage in the pipework, or air trapped within the heat exchanger. Essentially, the boiler shuts down to prevent itself from overheating because the hot water hasn't moved out of the unit.
This fault is more common in autumn when heating systems are switched on after a long summer break, as circulation pumps can often seize up from lack of use.
DIY fix possible
Check that all radiator valves are fully openEnsure the boiler pressure is between 1.0 and 1.5 bar on the gaugeBleed all radiators to remove trapped airTry resetting the boiler once the system has cooled down
Find a Gas Safe engineer
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A boiler cover plan would cover faults like F.23 — no call-out fees, no surprises.
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