BoilerFaultCodesThe water inside your boiler is heating up too quickly because it cannot flow through your radiators 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 temperature steep rise – the radiator valves may not be open enough to circulate water
What causes this fault?
This fault occurs when the heat generated by the burner cannot be carried away because the water is not moving through the system fast enough. The most common reason is that too many radiator valves are turned off or blocked, causing the internal temperature to spike dangerously high in a matter of seconds. In some cases, it may also indicate that the pump is beginning to fail or the system has become restricted by debris.
This code often appears during the first few cold days of autumn when homeowners turn their heating back on but find their radiator valves have seized shut over the summer.
DIY fix possible
Ensure all your radiator valves (TRVs) are turned to the highest settingCheck that any bypass valves or zone valves in your system are openRestart the boiler once you have ensured water can flow through the radiators
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 8H-65 — 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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