BoilerFaultCodesYour boiler has detected that water is not moving through its internal heating unit fast enough, which can cause the system to overheat and shut down.
Book a Gas Safe engineer within 24–48 hours. Your boiler may be unsafe or could break down completely if left.
Technical description: Flow rate in heat exchanger is too low
What causes this fault?
This fault occurs when water is unable to circulate through the boiler's main heat exchanger at the required speed to safely carry heat away. The most common reasons are a failing internal pump, a build-up of sludge or debris blocking the pipework, or a secondary heat exchanger that has become restricted over time.
This issue often comes to light during the first cold spell of autumn when the heating is turned back on and the pump or system is pushed to work harder after sitting idle all summer.
DIY fix possible
Check that your system pressure is between 1 and 1.5 bar on the front displayEnsure all radiator valves are fully open to allow water to circulateInspect the magnetic system filter, if fitted, to see if it needs cleaning
Find a Gas Safe engineer
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A boiler cover plan would cover faults like 2964 B — 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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