Daikin 8H-02 Fault Code: Causes, Fixes & Repair Costs
What does the Daikin 8H-02 fault code mean?
The 8H-02 code means the safety thermostat within the mixed water circuit has physically tripped because the water temperature in that circuit climbed to an unsafe level. The mixed water circuit is the blended hydraulic zone found on Daikin Altherma systems that serve two heating zones — for example, underfloor heating mixed down from a higher-temperature radiator circuit. Unlike a soft sensor alarm, this is a physical safety device that has cut out to protect the system, and it cannot be cleared by a simple software reset alone.
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
- Circulation pump failure or air lock Common
If the internal circulation pump stops working or becomes air-locked, heated water stagnates in the mixed circuit rather than moving around the system. Heat builds up rapidly and the safety thermostat trips. A sudden, steep rise in flow temperature is the telltale sign. This is the most frequently identified root cause of 8H-02 faults.
- Closed or partially closed zone valve Common
Any shut-off or zone valve in the mixed circuit that is not fully open will throttle the flow of water, causing localised heat to build up. Even a valve that is 80% open can be enough to trigger overheating under certain load conditions.
- Blocked water filter or strainer Common
A clogged inline filter or strainer sharply reduces the volume of water able to flow through the mixed circuit. This is a straightforward maintenance issue but is frequently overlooked, particularly on older installations where the filter has never been cleaned.
- Low system pressure Sometimes
If the hydraulic pressure in the heating system has dropped below the required operating range, water flow becomes inadequate. Sluggish or intermittent circulation in secondary circuits can cause heat to accumulate and the safety thermostat to activate.
- Faulty or stuck mixing valve Sometimes
The mixing valve blends primary high-temperature water with return water to produce the correct temperature for the secondary circuit. If the valve sticks in a position that allows too much hot primary water through, the mixed circuit temperature can overshoot and trigger the thermostat.
- Faulty mixed water thermistor Sometimes
A failed or drifting temperature sensor in the mixed circuit can either cause incorrect temperature regulation — allowing the circuit to overheat — or in some cases report a false reading. This is less common than hydraulic causes but should be considered if flow and valve checks come back clear.
- Safety thermostat wiring or connection fault Rare
In some cases, particularly following an installation or component change, the safety thermostat may not be correctly wired into the circuit. An absent or poorly connected safety thermostat can trigger 8H-02 even when water temperatures are within normal bounds.
How to fix it
- Switch off the heat pump at the controller and allow the system to cool down DIY safe
Do not attempt to restart the unit while it is hot. The mixed circuit safety thermostat has physically tripped, and continuing to run the system risks damaging the heat exchanger or the backup heater. Allow at least 20–30 minutes for temperatures to normalise.
- Check the system pressure gauge on the indoor unit DIY safe
The pressure should typically read between 1.0 and 2.0 bar when cold (check your system documentation for the exact target). If the pressure is below 1.0 bar, you can top it up via the filling loop — but only if you are confident doing so. If pressure keeps dropping, there is likely a leak and an engineer is needed.
- Check that all visible zone valves and shut-off valves on the mixed circuit are fully open DIY safe
Walk around any accessible pipework, manifolds, and valve bodies and confirm that every lever or handle is aligned with the pipe (open position). A valve even slightly out of position can restrict flow enough to cause overheating.
- Clear any obstructions from around the outdoor unit DIY safe
While this fault relates to the water circuit rather than the refrigerant side, ensuring the outdoor unit has good airflow is always sensible before restarting. Remove any leaves, debris, or snow that may have accumulated around the unit.
- Do not attempt to locate or reset the safety thermostat yourself heat pump engineer
The physical safety thermostat that triggered this fault is typically located inside the indoor unit or hydraulic module. Accessing it requires removing panels and working near live electrical components and pressurised pipework. This must be done by a qualified engineer.
- Have an MCS-certified engineer inspect and reset the safety thermostat heat pump engineer
An engineer will locate the manual-reset button on the safety thermostat, confirm the circuit has cooled sufficiently, and reset it. They will then run the system under observation to identify whether the fault returns and where the root cause lies — checking pump operation, valve positions, the mixing valve, the thermistor, and wiring.
- Engineer to test and service the circulation pump, mixing valve, and water filter heat pump engineer
The engineer should check that the internal circulation pump is running correctly, that the mixing valve is operating across its full range, and that the inline water filter or strainer is clean. A blocked filter should be cleaned; a faulty pump or valve will need replacement.
- Engineer to test the mixed water thermistor and safety thermostat wiring heat pump engineer
If hydraulic checks are clear, the engineer should test the mixed water temperature sensor for correct resistance values and inspect the safety thermostat's wiring connections, particularly if any recent work has been carried out on the system.
Parts you may need
- Mixed water thermistor (temperature sensor) · from £45
- Internal circulation pump · from £120
- Mixing valve actuator or assembly · from £95
- Inline water filter/strainer · from £25
- Safety thermostat (manual reset) · from £35
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–£450, depending on the underlying cause.
Frequently asked questions
Can I reset the Daikin 8H-02 fault myself without calling an engineer?
Not fully. You can safely switch the system off and allow it to cool, check system pressure, and make sure all visible valves are open — but the 8H-02 fault involves a physical safety thermostat inside the unit that must be manually reset by pressing a button located behind the casing. Accessing this requires removing panels and working near live electrical components, so it should be done by an MCS-certified engineer. Repeatedly forcing a software restart without addressing the underlying cause can also lead to more serious damage.
Why does my Daikin Altherma keep showing 8H-02 after it has been reset?
A recurring 8H-02 fault almost always means the underlying cause has not been resolved. The most common culprits are a circulation pump that is starting to fail, a partially closed valve that was missed during initial checks, or a blocked water filter that was not cleaned. Less commonly, a faulty mixing valve or a drifting temperature sensor can cause repeated overheating. Each reset without a fix risks damage to the heat exchanger or internal components, so a thorough diagnostic inspection is strongly recommended before any further resets.
Is my Daikin Altherma under warranty and will the 8H-02 repair be covered?
Daikin Altherma systems installed by a Daikin D1 or D2 registered installer carry a 5-year parts and labour warranty, provided the system was registered within 30 days of installation at daikin.co.uk/warranty. If your system is within that period and was properly registered, the safety thermostat reset and most component replacements would typically be covered at no cost. Check your installation paperwork or contact your original installer to confirm your warranty status before booking an independent engineer.
Could the 8H-02 fault cause permanent damage to my heat pump?
If the system is shut down promptly when the fault appears — which is exactly what the safety thermostat is designed to trigger — the risk of lasting damage is low. However, if the fault is repeatedly reset and the unit is allowed to operate with restricted water flow or an overheating mixed circuit, there is a real risk of damaging the plate heat exchanger or the backup immersion heater over time. In extreme cases, persistent overheating stress can shorten the life of the compressor. The safest approach is to leave the system off and have an engineer investigate before restarting.