BoilerFaultCodesWhat the manual says
Manufacturer manualSystem flow fault. Check pump operation and speed setting. Check by-pass isolating valve is open
What causes this fault?
This fault happens when the boiler detects that water isn't moving through the internal pipework quickly enough to safely carry heat away from the burner. The most common reason is a failing circulation pump that can no longer push the water, or a blockage such as a closed valve that is choking the flow. To prevent the unit from overheating, the system shuts down as a safety precaution.
This issue often surfaces in autumn when the heating is turned back on after a long summer break, as internal pump components can seize up during periods of inactivity.
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 A41: no call-out fees, no surprise bills.
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