BoilerFaultCodesRemeha b9 Fault Code
Also written as B9 — same fault.
The Remeha b9 fault code means: your boiler has shut down because it is heating up much faster than it can circulate water, indicating a blockage or a pump issue.. It appears on 3 Remeha models in our database.
Technical description: Lock-out: maximum temperature rise is exceeded, dT between supply and return >= 22°C, rate of rise > 2,7°C/sec, or start with no flow/insufficient water. Also activated if shut down input (terminals 5 and 6) is opened.
What causes the Remeha b9 fault?
This fault occurs when heat builds up inside the boiler faster than the system can carry it away to your radiators. The most common reasons are a failed internal pump that isn't pushing water through the pipes or a physical blockage, such as sludge or debris, restricting the flow. Essentially, the boiler is protecting itself from overheating because the water is essentially standing still while being heated.
This issue often comes to light in autumn when the heating is turned back on after a long summer break, as pump components can seize during periods of inactivity.
DIY fix possible
Check that all radiator valves are fully openEnsure the boiler pressure is between 1.0 and 1.5 barBleed all radiators to remove trapped air from the system
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Gas Safe Register →Remeha models that show the b9 fault code
Click a model for the full fault code list and model-specific repair notes.
Source
Remeha b9 fault code information on this page is sourced and cross-checked against the official Remeha service / installation manuals for each affected model.
Reviewed by the BoilerFaultCodes Editorial Team · Last reviewed 27 April 2026