Daikin 8H-01 Fault Code: Causes, Fixes & Repair Costs
What does the Daikin 8H-01 fault code mean?
The 8H-01 code appears on Daikin Altherma 3 H HT F heat pumps and indicates that the system has detected dangerously high water temperatures within the mixed (blended) water circuit — the secondary loop that delivers heat to zones such as underfloor heating at a lower temperature than the primary circuit. As a protective measure, the unit shuts down to prevent damage to pipework, components, and the heating zones themselves.
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
- Faulty or stuck mixing valve Common
The mixing valve blends hot primary water with cooler return water to maintain the correct temperature in the secondary zone circuit. If it jams in the fully open (hot) position or fails to modulate correctly, the mixed circuit temperature can spike rapidly, triggering the overheating protection. This is the most frequently identified root cause of the 8H-01 code.
- Failed mixed circuit temperature sensor (thermistor) Common
The system relies on a dedicated thermistor to monitor the water temperature in the mixed circuit. If this sensor develops a fault — whether due to a degraded sensor element or a loose connector — it can report a falsely high temperature reading, causing the unit to believe overheating is occurring even when the actual water temperature is normal.
- Poor or restricted water flow through the mixed circuit Sometimes
If the circulating pump serving the mixed zone is underperforming, airlocked, or if there is significant sludge build-up restricting flow through the pipework or heat emitters, heat cannot be dissipated quickly enough. This causes genuine localised overheating in the circuit rather than a sensor misread.
- Wiring fault or poor electrical connection Sometimes
Damaged or corroded wiring between the indoor unit's control board and the mixing valve actuator or thermistor can cause erratic signals, leading the controller to misinterpret temperatures or lose control of the valve. This can result in the 8H-01 protection triggering unexpectedly.
- Blocked or fouled plate heat exchanger Rare
Accumulated limescale, magnetite sludge, or debris on the heat exchanger surfaces within the indoor unit can impair heat transfer, causing the water on the mixed circuit side to overheat. This is more common in older systems without chemical inhibitor protection or a magnetic filter.
How to fix it
- Note the fault code and check the controller display for any additional sub-codes or warnings DIY safe
Before doing anything else, take a photo or note down exactly what is shown on the controller display, including any secondary fault information. This will be useful for your engineer and can help confirm whether the fault is persistent or intermittent.
- Perform a single power-cycle from the isolator DIY safe
Switch the heat pump off at the outdoor isolator and indoor supply, wait at least five minutes, then restore power. A one-off reset can clear a transient electrical glitch. If the fault clears and does not return within a few hours, monitor the system closely. If 8H-01 returns immediately or within a short period, do not continue resetting — repeated resets will not resolve an underlying component fault and may mask useful diagnostic data.
- Check the system pressure on the expansion vessel gauge DIY safe
Low system pressure can contribute to flow-related faults. The gauge on your indoor unit should typically read between 1.0 and 2.0 bar when the system is cold. If it is below 1.0 bar, you may be able to top up via the filling loop — consult your user manual for the correct procedure. If pressure cannot be maintained or drops again quickly, report this to your engineer as it may indicate a leak.
- Inspect the outdoor unit and indoor unit for any obvious physical issues DIY safe
Check that the outdoor unit is not obstructed by leaves, garden debris, or anything restricting airflow around the casing. Indoors, check that there are no visible water leaks from the pipework around the indoor unit, and that no system isolation valves on the mixed circuit have been accidentally closed.
- Contact an MCS-certified heat pump engineer to diagnose the fault heat pump engineer
The 8H-01 fault almost always requires professional diagnosis. An engineer will test the mixing valve actuator and its wiring, check the resistance and calibration of the mixed circuit thermistor, assess water flow rates through the secondary zone circuit, inspect the plate heat exchanger for fouling, and check the main control PCB for fault history. Parts such as the mixing valve or thermistor can then be sourced and replaced. If any work on the refrigerant circuit is required during the investigation, this must be carried out by an F-Gas registered engineer.
Parts you may need
- Mixing valve / three-way valve assembly (Daikin Altherma) · from £120
- Mixed circuit thermistor / water temperature sensor · from £35
- Zone circuit circulation pump · from £95
- Main indoor unit PCB · from £320
- Magnetic system filter (if not already fitted) · from £65
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 £180–£550, depending on the underlying cause.
Frequently asked questions
Can I keep resetting the 8H-01 fault to keep my heating running?
It is not advisable. A single reset is a reasonable first step to rule out a transient glitch, but if the fault returns the system is shutting down for a genuine reason — to protect the pipework and components from overheating damage. Repeatedly resetting without fixing the underlying cause risks causing further damage to the mixing valve, the heat exchanger, or the underfloor heating circuit itself. Get an engineer booked promptly, especially in cold weather.
Is Daikin Altherma 8H-01 covered under warranty?
Daikin Altherma systems installed by a Daikin D1 or D2 registered installer carry a five-year parts and labour warranty (provided the system was registered within 30 days of installation). If your unit is still within this period, contact Daikin UK directly to log the fault before arranging any engineer visit — Daikin needs to authorise the repair first, otherwise the visit may become chargeable even if the system is under warranty.
Could a faulty mixing valve really cause such a serious fault?
Yes — the mixing valve is the component responsible for keeping the secondary zone circuit at the correct, lower temperature. If it sticks fully open to the hot primary flow and cannot blend in cooler return water, the temperature in the underfloor heating circuit (or other low-temperature zones) can rise quickly to levels that trigger the safety shutdown. On Daikin Altherma systems the 8H-01 code specifically targets this mixed circuit, making the mixing valve the first component an experienced engineer will test.
How much will it cost to fix the Daikin 8H-01 fault?
For the most common repairs — a thermistor replacement or mixing valve swap — most homeowners pay somewhere between £180 and £550 all-in, including call-out, diagnosis, parts, and labour. A straightforward sensor replacement tends to sit at the lower end, while a mixing valve or pump replacement with multiple engineer visits can push toward the higher end. In less common situations where the main control PCB turns out to be faulty, costs can rise above £600, though this is not the typical outcome for an 8H-01 fault.