Daikin H8-00 Fault Code: Causes, Fixes & Repair Costs
What does the Daikin H8-00 fault code mean?
The H8-00 fault code indicates that the outdoor unit has detected an abnormality in the compressor input current transformer (CT) system. The heat pump continuously monitors the electrical current drawn by the compressor inverter circuit and compares it against expected values for the running frequency. When the measured current readings are inconsistent or fall outside acceptable limits, the unit triggers H8-00 and shuts down to prevent further damage. This is a serious outdoor unit fault that almost always requires a qualified engineer to diagnose and resolve.
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 inverter board or power transistor Common
The inverter board in the outdoor unit controls compressor speed and current. If the power transistors on this board are damaged or degraded, the current readings detected by the CT will be erratic or absent, triggering H8-00. This is among the most frequent causes engineers find when diagnosing this code.
- Defective reactor (DC choke) Common
The reactor, or DC choke, smooths the electrical current supplied to the inverter circuit. When it develops a fault, current flow becomes unstable, causing the CT sensor to report abnormal values. This component is inside the outdoor unit and requires specialist testing equipment to assess.
- Loose or damaged inverter wiring Sometimes
Wiring connections between the inverter board, CT sensor, and compressor can work loose over time due to vibration, or may have been disturbed during previous servicing. A poor connection can produce intermittent or incorrect current readings and trigger H8-00, sometimes only under load.
- Failing or seized compressor Sometimes
If the compressor motor windings are breaking down, or the compressor has mechanically seized, it will draw abnormal current that the CT system flags as a fault. This is the most serious and costly underlying cause, though it tends to be accompanied by other symptoms such as the unit cutting out shortly after starting.
- Failed CT sensor on the outdoor PCB Rare
The current transformer sensor itself may be defective, causing it to report incorrect data to the control board even when the compressor and inverter circuit are functioning correctly. This is a less common cause but can lead to a misdiagnosis if not properly tested.
How to fix it
- Turn off the heat pump from the controller and wait 5–10 minutes before restarting DIY safe
A transient electrical disturbance can occasionally cause a one-off H8-00 event. Powering the system down fully — including at the indoor controller — and waiting at least five minutes before restarting allows the inverter capacitors to discharge and the control logic to reset. If the fault clears and does not return, monitor the system closely over the next 24–48 hours. If H8-00 reappears, do not keep resetting repeatedly as this can stress already-strained components.
- Check the isolator and consumer unit for any tripped breakers DIY safe
Locate the outdoor unit isolator (usually mounted on an external wall near the unit) and the heat pump circuit breaker in your consumer unit. Ensure both are in the ON position and neither has tripped. An undervoltage or supply-side issue can sometimes provoke inverter faults. If a breaker has tripped and trips again on reset, stop and call an engineer — do not attempt to force it back on.
- Inspect the area around the outdoor unit and clear any obstructions DIY safe
Walk around the outdoor unit and check that the air inlet and outlet are clear of leaves, debris, or any objects that may have accumulated. Ensure nothing is restricting airflow to the fan. Poor airflow can cause the unit to work harder, increasing compressor current draw. Do not reach into the unit or touch any internal components.
- Check system water pressure on the heat pump display or pressure gauge DIY safe
Low system pressure can cause related operational faults. Check that the heating circuit pressure is within the range specified on your commissioning sheet or user manual — typically between 1.0 and 2.0 bar for most UK installations. If pressure is low, top up via the filling loop in line with your user manual instructions. Note that H8-00 is primarily an electrical/inverter fault, so pressure is unlikely to be the direct cause, but it is worth ruling out.
- Arrange for an MCS-certified heat pump engineer to carry out a full electrical diagnosis of the outdoor unit heat pump engineer
A qualified engineer will use specialist tools — including an inverter checker and insulation resistance tester — to assess the condition of the power transistors, reactor, CT sensor, and outdoor PCB. They will check wiring continuity across the inverter circuit and test compressor winding resistance and insulation. This is the only reliable way to identify which component is at fault. Do not attempt to open the outdoor unit electrical enclosure yourself.
- Have the faulty component replaced by a qualified engineer (inverter board, reactor, CT sensor, or compressor as indicated by diagnosis) heat pump engineer
Depending on the diagnostic findings, the engineer will replace the defective part. Inverter board and reactor replacements are relatively contained jobs, while compressor replacement is a major undertaking that may involve handling refrigerant, which must only be carried out by an F-Gas registered engineer. Ensure any repair is documented and that you receive a job report, particularly if the system is under warranty.
Parts you may need
- Outdoor unit inverter PCB (power board) · from £320
- DC reactor / choke · from £95
- CT sensor / current transformer · from £45
- Power transistor module (IPM) · from £150
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 £280–£650, depending on the underlying cause.
Frequently asked questions
Is the Daikin H8-00 fault serious, or can I just reset it and carry on?
H8-00 is a serious fault that indicates a genuine electrical malfunction in the compressor inverter circuit. A single reset is worth trying, as a momentary disturbance can occasionally trigger it. However, if the code returns — especially quickly after restarting — you should stop resetting the unit and call an engineer. Repeatedly forcing the unit to run while this fault is active risks causing further damage to the inverter board or, in the worst case, to the compressor itself.
How much does it typically cost to fix an H8-00 fault in the UK?
For the most common causes — an inverter board, reactor, or wiring fault — most UK homeowners pay somewhere in the range of £280 to £650 including parts, labour, and the callout. If the diagnosis points to a failing compressor, costs rise significantly: compressor repair is typically £800–£2,500 and full replacement can be £1,500–£3,500 or more depending on system size. It is worth checking your warranty status with Daikin before authorising any repair, as parts and labour may be covered.
Do I need an F-Gas engineer for an H8-00 repair?
Not necessarily for every repair under this fault code. If the engineer establishes that the cause is an inverter board, reactor, wiring connection, or CT sensor, those can be replaced without touching the refrigerant circuit. However, if the compressor itself needs to be replaced or removed, the refrigerant must be recovered and recharged by an F-Gas registered engineer. Any reputable MCS-certified heat pump engineer will hold the appropriate F-Gas qualification.
My Daikin heat pump may still be under warranty — will H8-00 be covered?
Daikin UK typically offers a 5–7 year warranty on parts and labour when the system was installed by a certified installer and registered correctly. If the fault is due to a component defect (such as a failed inverter board or reactor), it is likely to be covered. Contact Daikin UK or your original installing company to log the fault before authorising any third-party repair, as using an unauthorised engineer can invalidate the warranty terms.