BoilerFaultCodesYour boiler has shut down because the water leaving the unit is getting much hotter than the water returning, suggesting a circulation problem through your radiators.
Book a Gas Safe engineer within 24–48 hours. Your boiler may be unsafe or could break down completely if left.
Technical description: Safety switch-off: Temperature difference too great (flow/return)
What causes this fault?
This fault occurs when the water leaving the boiler is significantly hotter than the water coming back, indicating that heat isn't being moved away from the unit quickly enough. The most common reasons are a failing internal pump, a build-up of sludge blocking the pipes, or even just having too many radiator valves turned off at once.
This issue often appears when the heating is first turned on after summer, as the pump can seize or get stuck during long periods of inactivity.
DIY fix possible
Check that all radiator valves are fully openEnsure there is enough pressure in the system (typically 1.0 to 1.5 bar)Bleed any air out of your radiators and then top up the boiler pressure
Find a Gas Safe engineer
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Gas Safe Register →Protect yourself from future repair bills
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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