BoilerFaultCodesYour boiler has sensed that the water getting sent out is significantly hotter than the water coming back, suggesting that water is not circulating through your system properly.
Book a Gas Safe engineer within 24–48 hours. Your boiler may be unsafe or could break down completely if left.
Technical description: Flow/Return Differential > 50°C
What causes this fault?
This fault occurs when water is heating up inside the boiler but cannot move around your radiators quickly enough to shed that heat. The most common reasons are a pump that has seized or failed, or a blockage in the system caused by a buildup of sludge and debris. Essentially, the boiler shuts down to prevent itself from overheating because the hot water has nowhere to go.
This issue often appears during the first cold spell of autumn when the heating is turned back on after being idle all summer, which can cause internal pump parts to seize.
DIY fix possible
Check that all radiator valves are fully openEnsure the central heating system pressure is between 1.0 and 1.5 barListen for air in the system and bleed radiators if necessary
Find a Gas Safe engineer
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A boiler cover plan would cover faults like F0U — 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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