Daikin HJ-09 Fault Code: Causes, Fixes & Repair Costs
What does the Daikin HJ-09 fault code mean?
The HJ-09 code appears on Daikin Altherma heat pumps when the water pressure in the heating circuit drops below the minimum threshold required for safe, efficient operation. The system monitors hydraulic pressure continuously, and when it detects a reading too low to circulate water properly through the heat pump and heat emitters, it triggers this alert and may suspend heating until pressure is restored. In most cases this is a straightforward issue that a homeowner can resolve by topping up the system via the filling loop, though recurring pressure loss always warrants investigation by a qualified engineer.
General guidance only — not a substitute for professional advice, and not certified by an heat pump engineer. Always have your specific appliance assessed by a qualified professional. Any heat pump work must be carried out by an MCS-certified heat pump engineer. If you notice a refrigerant leak, a burning smell or water leaking from the unit, switch it off at the isolator and call an MCS-certified heat pump engineer. Refrigerant work must be carried out by an F-Gas registered engineer.
Common causes
- Gradual pressure loss over time Common
Sealed heating systems naturally lose a small amount of water pressure over months of normal use. If this is the first or an infrequent occurrence and there are no visible wet patches anywhere, a simple top-up via the filling loop is usually all that is needed.
- Air trapped in the system Common
Air pockets can accumulate in radiators or pipework, displacing water and reducing the overall hydraulic pressure registered by the system. Bleeding radiators to release trapped air can lower the pressure reading and trigger HJ-09.
- Weeping or degraded seals and O-rings Sometimes
Rubber seals and O-rings throughout the pipework and heat pump connections perish with age and heat cycling, typically after eight to ten years of use. Even a very slow weep from a deteriorating seal can cause pressure to drop gradually, often leaving only faint limescale or rust staining as evidence.
- Pressure relief valve (PRV) weeping or discharging Sometimes
The pressure relief valve is a safety device designed to release water if pressure rises too high. Over time these valves can stick slightly open, allowing water to escape slowly to the condensate or drain pipe. You may notice occasional drips from the discharge pipe outside or from the unit itself.
- Internal corrosion or a leak at a joint Rare
In systems where chemical inhibitor levels have not been maintained, internal corrosion of pipework, radiators or the heat exchanger can develop pinhole leaks. These may be hidden behind walls or under floors, making them harder to locate without a pressure test.
How to fix it
- Check the pressure gauge on the heat pump controller or system display DIY safe
Most Daikin Altherma units display live system pressure on the controller screen or on an analogue gauge on the indoor unit. A healthy reading is typically between 1.0 and 2.0 bar when the system is cold; consult your model's user guide to confirm the recommended range. If the gauge reads below 0.8 bar, low pressure is confirmed as the likely cause of HJ-09.
- Bleed any radiators or fan coil units to release trapped air DIY safe
Before topping up, bleed each radiator from the top floor downwards using a radiator key. Hold a cloth under the bleed valve and open it slowly until water (not air) runs out, then close it. This ensures you are not adding water to compensate for an air pocket that can be removed instead.
- Repressurise the system using the filling loop DIY safe
Locate the filling loop, which is usually a braided hose with one or two isolation valves connecting the mains cold water supply to the heating circuit — often found near the indoor unit or a cylinder. Slowly open the valve(s) and watch the pressure gauge rise. Stop when the pressure reaches the target level for your system (commonly around 1.2–1.5 bar cold). Close the valve(s) firmly. Never leave the filling loop open and unattended, and do not overpressurise beyond 2.0 bar.
- Reset the unit and check whether HJ-09 clears DIY safe
Once pressure is restored, follow the reset procedure on your Daikin controller — usually holding the reset or return button for a few seconds, or cycling power at the local isolator. The fault should clear and the heat pump should resume normal operation within a few minutes.
- Monitor pressure over the following days DIY safe
Check the pressure gauge daily for at least a week. A one-off top-up that holds steady is reassuring. If pressure drops again within days or weeks, there is an underlying cause — a leak, a weeping PRV, or corroded pipework — that requires professional investigation. Do not keep repeatedly topping up without finding the root cause.
- Inspect visible pipework and the PRV discharge pipe for signs of leakage DIY safe
Walk around the indoor unit, any visible pipework and radiators looking for damp patches, rust staining, limescale deposits or wet spots. Check whether the PRV discharge pipe (which typically exits to outside or to a drain) shows signs of dripping. Report anything you find to the engineer.
- Contact an MCS-certified heat pump engineer if pressure loss recurs or a leak is suspected heat pump engineer
If pressure drops repeatedly, if you cannot locate the filling loop, or if you suspect a leak anywhere in the system, arrange a visit from an MCS-certified heat pump engineer. They can carry out a full system pressure test to locate hidden leaks, service or replace a weeping PRV, and check inhibitor levels to protect against further corrosion. Any work on the refrigerant circuit requires an F-Gas registered engineer.
Parts you may need
- Pressure relief valve (PRV) · from £25
- Filling loop assembly · from £18
- System pressure gauge · from £15
- Automatic air vent · from £10
- Radiator bleed valve · from £5
- Central heating inhibitor (1 litre) · from £12
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 £100–£380, depending on the underlying cause.
Frequently asked questions
Can I top up my Daikin Altherma heat pump pressure myself?
Yes — repressurising via the filling loop is one of the few tasks that is safe and straightforward for a homeowner to carry out. Locate the filling loop (a braided hose near the indoor unit), slowly open the valve until the gauge reaches the recommended pressure (usually around 1.2–1.5 bar cold), then close it firmly. If you are unsure where the filling loop is or the fault returns quickly after topping up, call an MCS-certified engineer rather than continuing to add water.
Why does my heat pump keep losing pressure after I top it up?
Recurring pressure loss almost always means water is escaping from the system somewhere. Common culprits include a weeping pressure relief valve (look for drips from the discharge pipe), a deteriorating seal or O-ring at a joint, corroded pipework, or a pinhole leak in a radiator. A qualified heat pump engineer can carry out a pressure test to locate even hidden leaks. Do not ignore repeated pressure drops — ongoing low pressure stresses the pump and can cause wider damage over time.
What pressure should a Daikin Altherma heat pump run at?
Most Daikin Altherma systems are designed to operate between 1.0 and 2.0 bar when the system is cold, with around 1.2–1.5 bar being a typical target. Pressure rises slightly when the system heats up, which is normal. Always check your specific model's user manual to confirm the correct range, as it can vary slightly between Altherma models. If pressure regularly climbs above 2.5 bar, that is equally a concern and should be checked by an engineer.
How much does it cost to fix a heat pump that keeps losing pressure?
If you can resolve it yourself with a one-off top-up, there is no cost beyond a few minutes of your time. When an engineer is needed — for a weeping PRV, a failed seal or a hidden leak — most UK homeowners pay between £100 and £380 including labour and parts. A straightforward PRV or seal replacement sits towards the lower end; locating and repairing a hidden leak in pipework can cost more due to the time involved. In rare cases where internal corrosion has damaged a heat exchanger, repair costs can rise significantly — at that point it is worth discussing the repair-versus-replace economics with your engineer.