BoilerFaultCodesDaikin 8H-65 Fault Code
Also written as 8H65 — same fault.
The Daikin 8H-65 fault code means: the water inside your boiler is heating up too quickly because it cannot flow through your radiators properly.. It appears on 1 Daikin model in our database.
Technical description: Flow temperature steep rise – the radiator valves may not be open enough to circulate water
What causes the Daikin 8H-65 fault?
This fault occurs when the heat generated by the burner cannot be carried away because the water is not moving through the system fast enough. The most common reason is that too many radiator valves are turned off or blocked, causing the internal temperature to spike dangerously high in a matter of seconds. In some cases, it may also indicate that the pump is beginning to fail or the system has become restricted by debris.
This code often appears during the first few cold days of autumn when homeowners turn their heating back on but find their radiator valves have seized shut over the summer.
DIY fix possible
Ensure all your radiator valves (TRVs) are turned to the highest settingCheck that any bypass valves or zone valves in your system are openRestart the boiler once you have ensured water can flow through the radiators
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Gas Safe Register →Daikin models that show the 8H-65 fault code
Click a model for the full fault code list and model-specific repair notes.
Source
Daikin 8H-65 fault code information on this page is sourced and cross-checked against the official Daikin service / installation manuals for each affected model.
Reviewed by the BoilerFaultCodes Editorial Team · Last reviewed 27 April 2026