BoilerFaultCodesYour boiler has detected that water isn't moving through the internal pipes correctly, causing it to shut down to prevent overheating.
Book a Gas Safe engineer within 24–48 hours. Your boiler may be unsafe or could break down completely if left.
Technical description: A blockage in the appliance is preventing the system from flowing as it should.
What causes this fault?
This fault occurs when the boiler's internal sensors detect a massive temperature difference between the water entering and leaving the unit, usually because the water has stopped moving. The most common reason is a failed internal pump or a heavy buildup of sludge and debris blocking the main heat exchanger. Without proper flow, the water inside the unit reaches a boiling point almost instantly, forcing the system to shut down for safety.
This issue is most frequently seen during the first cold snap of autumn when a boiler is switched back on after being idle all summer, often causing a seized pump to finally give up.
DIY fix possible
Check that all radiator valves are fully openEnsure the internal boiler valves underneath the unit are in the open positionCheck if the pump is making a humming sound, which may indicate it is stuck
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A boiler cover plan would cover faults like F.11 — no call-out fees, no surprises.
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