BoilerFaultCodesWhat the manual says
Manufacturer manualHigh Flow/Return Differential
What causes this fault?
This fault occurs when the water temperature leaving the boiler is significantly higher than the water returning, suggesting that heat isn't being moved away fast enough. The most common cause is a restriction in the system, such as a build-up of sludge in the heat exchanger or a failing pump that can no longer push water through the radiators effectively.
This issue is most frequently seen during the first cold spell of autumn when the heating is turned back on after sitting idle for several months, often causing the pump to seize.
Engineer required
This is a high severity fault. Stop using the boiler and call a Gas Safe registered engineer. Do not attempt gas-related repairs yourself.
Avoid the next call-out bill
A cover plan would handle faults like P5: no call-out fees, no surprise bills.
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