Daikin HJ-08 Fault Code: High System Water Pressure
The HJ-08 code appears on Daikin boilers when the heating system's water pressure has climbed above the safe operating threshold. The boiler detects this excessive pressure and triggers a safety lockout to protect internal components from damage. You'll typically need to bring the system pressure back down to around 0.8–1 bar before the boiler will restart. Until the underlying cause is identified and resolved, the fault is likely to keep returning.
General guidance only — not a substitute for professional advice. Any gas work must be carried out by a Gas Safe registered engineer. If you smell gas or suspect carbon monoxide, leave the property and call the National Gas Emergency line on 0800 111 999.
What does the Daikin HJ-08 fault code mean?
The HJ-08 code appears on Daikin boilers when the heating system's water pressure has climbed above the safe operating threshold. The boiler detects this excessive pressure and triggers a safety lockout to protect internal components from damage. You'll typically need to bring the system pressure back down to around 0.8–1 bar before the boiler will restart. Until the underlying cause is identified and resolved, the fault is likely to keep returning.
Common causes
- Faulty or stuck expansion vessel Common
The expansion vessel absorbs the increase in water volume as your heating system warms up. If its internal diaphragm has ruptured or the vessel has lost its air pre-charge, it can no longer cushion pressure changes. The result is that system pressure climbs sharply every time the boiler fires, often causing the pressure relief valve (PRV) to dump water — you might notice a trickle from a pipe outside the house. This is the most common root cause of recurring high-pressure faults.
- System overfilled during maintenance or bleeding Common
If the system was recently topped up via the filling loop — perhaps after bleeding radiators — it may have been filled beyond the recommended 1–1.5 bar cold pressure. Once the heating warms up and water expands, pressure rises further, pushing the system into HJ-08 territory. This is a straightforward fix once identified.
- Faulty pressure relief valve (PRV) Sometimes
The PRV is a safety device designed to release water when pressure gets dangerously high. If it sticks in the closed position, it cannot perform this function and pressure builds unchecked. Conversely, a PRV that has been manually opened and not fully reseated may let pressure drop too quickly or continue to weep. Either way, a compromised PRV will contribute to pressure instability.
How to fix it
- Check the pressure gauge on your boiler DIY safe
Look at the pressure gauge on the boiler's front panel. A reading above 2.5–3 bar when the system is warm (or above 2 bar when cold) confirms excess pressure. Note the reading so you can report it accurately to an engineer.
- Check whether the system was recently bled or topped up DIY safe
If radiators were bled recently and the filling loop was used to re-pressurise the system, it's possible the system was overfilled. This is worth mentioning to your engineer as it points to a simple resolution rather than a component fault.
- Do not attempt to open the filling loop or add more water DIY safe
It may seem counterintuitive, but adding water will make the situation worse. The system already has too much pressure — do not touch the filling loop while HJ-08 is active.
- Bleed pressure via the pressure relief valve — with caution Gas Safe engineer
The PRV can be used to release a small amount of water and reduce system pressure. However, this involves handling a pressurised valve and directing hot water safely away from people and property. Incorrectly opening the PRV can cause scalding or damage the valve's reseating. This task is best left to a Gas Safe engineer unless you are confident in the procedure and have a suitable drain hose and safe discharge point.
- Have a Gas Safe engineer inspect the expansion vessel Gas Safe engineer
If pressure keeps rising each time the boiler fires, the expansion vessel is the most likely culprit. An engineer will check the vessel's air pre-charge pressure and inspect the diaphragm. A depleted charge can sometimes be restored; a ruptured diaphragm requires full vessel replacement. This is not a DIY task — sealed heating systems must be depressurised safely before any component work.
- Have a Gas Safe engineer assess or replace the PRV if needed Gas Safe engineer
If the PRV is leaking continuously or failing to hold pressure after being operated, it needs replacing. A faulty PRV cannot protect your system effectively and should be dealt with promptly. Replacement involves draining sections of the system and fitting a new valve — Gas Safe work throughout.
- Reset the boiler after pressure is normalised DIY safe
Once a Gas Safe engineer has brought the system pressure back into the correct range (typically 1–1.5 bar cold), press and hold the reset button as described in your Daikin boiler's user guide. If the fault reappears within a short time, do not keep resetting — call your engineer back, as the underlying cause has not been resolved.
- If the fault persists, call a Gas Safe registered engineer Gas Safe engineer
Recurring HJ-08 codes after an initial repair attempt indicate an ongoing pressure management problem. A Gas Safe registered engineer can carry out a full system pressure test, inspect all related components, and carry out any necessary repairs or replacements safely and in compliance with UK Gas Safe regulations.
Parts you may need
- Expansion vessel (8–12 litre, standard combi size) · from £45
- Pressure relief valve (PRV) · from £25
- Expansion vessel Schrader valve (air charge valve) · from £5
- System pressure gauge (replacement) · from £18
The exact spare depends on your boiler's GC number (on the data badge). Check this against the part before buying.
Typical repair cost
Expect to pay roughly £120–£400, depending on the underlying cause.
Frequently asked questions
Can I fix a Daikin HJ-08 fault myself?
There is limited DIY scope here. You can check the pressure gauge reading, confirm you haven't recently overfilled the system, and avoid adding more water. Resetting the boiler once or twice is also fine. However, releasing pressure via the PRV, recharging or replacing the expansion vessel, and replacing a faulty PRV all require a Gas Safe registered engineer. Attempting these tasks yourself could void your warranty, invalidate home insurance, or cause injury from pressurised hot water.
Why does my Daikin boiler keep showing HJ-08 every time it heats up?
If the fault returns each time the heating comes on, the most likely explanation is a failed expansion vessel. When the vessel's diaphragm ruptures or its air charge is lost, there is nowhere for the expanding hot water to go, so pressure spikes with every heating cycle. You may also notice water dripping from a pipe outside your home — that's the pressure relief valve doing its job by dumping the excess. An engineer will need to recharge or replace the vessel to stop the cycle.
How much does it cost to fix a Daikin HJ-08 fault in the UK?
Most homeowners pay between £120 and £400 for repairs related to HJ-08. A PRV replacement typically falls in the £100–£200 range including labour, while expansion vessel replacement usually costs £180–£350 for straightforward access. If the vessel is in a difficult location or the boiler needs significant stripping down, costs can rise beyond £450. A standard diagnostic call-out is typically £85–£150. Note that Daikin is a less common brand in the UK than Worcester Bosch or Viessmann, so it's worth confirming parts availability with your engineer before committing to a major repair on an older boiler.
Is a Daikin boiler over 10 years old worth repairing if it shows HJ-08?
For a single, well-defined fault like a failed expansion vessel, repair is usually worthwhile even on a boiler approaching 10 years old, provided the rest of the system is in good condition. However, if the boiler is over 12 years old and has had multiple fault codes or repeated breakdowns, the cost-versus-replacement calculation becomes less clear-cut. A Gas Safe engineer who services Daikin units can give you an honest appraisal of the boiler's overall condition alongside the repair quote.