BoilerFaultCodesYour boiler is struggling to move water around the system properly, causing it to overheat and shut down for safety.
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 too large
What causes this fault?
This fault occurs when the boiler detects that water isn't circulating through the pipes and radiators fast enough to carry heat away from the burner. The most common reasons are a seized internal pump or a blockage in the system, such as a buildup of sludge or a clogged heating filter, which prevents the water from moving freely. Because the heat cannot escape, the boiler reaches a dangerous temperature and shuts down to prevent internal damage.
This fault is frequently seen at the start of autumn when the heating is turned on for the first time after summer, as the pump can seize during long periods of inactivity.
DIY fix possible
Check that the boiler pressure is between 1 and 1.5 bar on the displayEnsure all radiator valves and the boiler service valves are fully openGently bleed air from your radiators using a radiator key
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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Could it be time for a new boiler?
If repair costs are adding up, a new energy-efficient boiler could be the smarter long-term choice.
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