BoilerFaultCodesDaikin HJ-08 Fault Code
Also written as HJ08 — same fault.
The Daikin HJ-08 fault code means: your boiler has too much water pressure and needs some drained out to reach a safe operating level.. It appears on 1 Daikin model in our database.
Technical description: High system pressure – the boiler needs to discharge water down to 0.8 bar
What causes the Daikin HJ-08 fault?
This fault occurs when the water pressure inside your heating system rises above the safe operating limit, causing the boiler to shut down for safety. The most common reasons are that the filling loop was left slightly open after topping up the system, or an internal component like the expansion vessel has failed and can no longer manage the pressure changes.
This fault is more frequent when you first turn the heating on after summer, as the water expands when heated and can reveal underlying pressure issues or a faulty expansion vessel.
DIY fix possible
Check that the filling loop valves are fully closedBleed water from a radiator using a bleed key to reduce pressureMonitor the pressure gauge until it reaches approximately 1.5 barRestart the boiler
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Gas Safe Register →Daikin models that show the HJ-08 fault code
Click a model for the full fault code list and model-specific repair notes.
Source
Daikin HJ-08 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