BoilerFaultCodesYour boiler has detected that water is heating up too quickly or not circulating properly, which usually means something is blocking the flow of water through the system.
Book a Gas Safe engineer within 24–48 hours. Your boiler may be unsafe or could break down completely if left.
Technical description: Diff greater than 50c check isolation valves are open
What causes this fault?
This fault occurs when the boiler detects a rapid rise in temperature because heated water isn't moving away from the burner fast enough. The most common reasons are a pump failure, a blockage in the pipework, or an internal part that has become restricted with sludge and debris.
This fault is more common in autumn when heating systems are first turned back on after the summer, as pumps can seize up during long periods of inactivity.
DIY fix possible
Check that all blue-handled isolation valves underneath the boiler are fully turned to the open positionEnsure all radiator valves (TRVs) are turned up to a high settingCheck your central heating filter for any obvious blockages if you have one installed
Find a Gas Safe engineer
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A boiler cover plan would cover faults like FU — 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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