Daikin EC-00 Fault Code: Causes, Fixes & Repair Costs
What does the Daikin EC-00 fault code mean?
The EC-00 fault code indicates that the domestic hot water (DHW) tank thermistor has detected a water temperature exceeding 89°C. This is a safety-level alarm on Daikin Altherma air-to-water heat pumps. At such temperatures there is a genuine scalding risk, so the system either suspends DHW operation or shuts down entirely depending on the model configuration. The fault may reflect a real over-temperature condition in the tank or a false reading caused by a faulty sensor or control board.
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 DHW tank thermistor (R5T) Common
The tank temperature sensor can fail or drift, causing it to report an erroneously high temperature even when the water is at a safe level. This is the most frequently diagnosed root cause of EC-00 in UK installations and is typically confirmed by testing the sensor's resistance against the manufacturer's temperature-resistance table.
- External heat source raising tank temperature Sometimes
If a solar thermal panel or immersion heater is connected to the same DHW cylinder, it can genuinely push the water temperature above the 89°C threshold, triggering the fault legitimately rather than due to a component failure. Worth checking if your system includes solar or an auxiliary heat source.
- Faulty hydro PCB (A1P) Sometimes
The main controller board inside the hydronic (indoor water) unit processes all sensor readings. If the PCB has developed a fault it may misinterpret or corrupt the thermistor signal, producing a false EC-00 alarm even when both the tank temperature and the sensor itself are within normal range.
How to fix it
- Do not ignore or attempt to bypass the fault DIY safe
A genuine tank temperature above 89°C presents a serious scalding hazard. Before doing anything else, confirm whether your DHW cylinder feels unusually hot to the touch on the outer casing. If it does, switch off any auxiliary heat sources (solar pump, immersion heater) immediately and do not draw hot water until the system has been inspected.
- Switch off the heat pump at the controller and check for any tripped MCBs DIY safe
Turn the unit off via the user interface or wall controller. Check your consumer unit for any tripped circuit breakers associated with the heat pump or immersion heater. Reset any tripped MCB once only — if it trips again, leave it off and call an engineer.
- Check and remove any external heat source from the equation temporarily DIY safe
If your installation includes a solar thermal system or a separately controlled immersion heater feeding the same cylinder, switch those off at their own controls or isolator. Wait 30 minutes, then attempt a manual reset via the Daikin user interface. If the fault clears and does not return, the external heat source is the likely culprit and system controls may need rebalancing by your installer.
- Have an engineer test the DHW thermistor (R5T) heat pump engineer
An MCS-certified Daikin engineer will disconnect the R5T sensor from socket X9A on the hydro PCB and test its resistance at a known water temperature. If the reading falls outside the expected range in the manufacturer's resistance table, the sensor requires replacement. This is the most common fix for EC-00 and is relatively straightforward for a qualified engineer.
- Have the hydro PCB (A1P) inspected and tested heat pump engineer
If the thermistor tests as healthy, the engineer will check whether the hydro PCB (A1P) is receiving correct supply voltage and is processing sensor data accurately. A confirmed faulty PCB will need to be replaced with the correct Daikin spare part — using an incorrect part number can reintroduce the fault.
- Confirm the repair and verify the system resets cleanly heat pump engineer
Once the root cause has been addressed, EC-00 should auto-reset. If not, the engineer can perform a manual reset via the service interface and monitor the DHW temperature through a test cycle to confirm normal operation has resumed.
- If in doubt, contact an MCS-certified heat pump engineer heat pump engineer
All internal diagnostics on a Daikin Altherma — including sensor testing, PCB work and any refrigerant-adjacent components — must be carried out by a suitably qualified, MCS-certified engineer. Refrigerant circuit work additionally requires an F-Gas registered engineer. Do not attempt internal repairs yourself.
Parts you may need
- DHW tank thermistor R5T · from £45
- Hydro PCB A1P · from £320
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–£400, depending on the underlying cause.
Frequently asked questions
Is it safe to keep using hot water while EC-00 is showing?
No — you should not draw hot water from the tap until the fault has been investigated. If the tank has genuinely reached above 89°C, the water presents a serious scalding risk. Switch off any auxiliary heat sources and avoid using the DHW circuit until an engineer has confirmed the cause and cleared the fault.
Could my solar panels be causing the EC-00 fault on my Daikin Altherma?
Yes, this is a recognised cause. If a solar thermal collector is connected to the same cylinder and is not properly controlled, it can push the tank temperature above the 89°C safety threshold that triggers EC-00. Temporarily disabling the solar pump and resetting the heat pump can help identify whether this is the cause. Your installer may need to adjust the system controls or fit additional over-temperature protection on the solar circuit.
How much does it cost to fix a Daikin EC-00 fault in the UK?
The most common repair — replacing the DHW thermistor (R5T) — typically costs between £180 and £350 including parts and a call-out. If the hydro PCB (A1P) turns out to be the faulty component, costs are considerably higher, often in the range of £450 to £900, as the board itself is an expensive part. If your unit is within Daikin's three-year warranty period and has been serviced annually, the repair may be covered — contact your installer and retain all service records, as Daikin may decline warranty claims without them.
Will the Daikin Altherma reset itself after an EC-00 fault?
The system is designed to auto-reset once the underlying fault condition has been resolved. However, if the thermistor or PCB has failed, the fault will simply reappear after reset because the root cause is still present. A manual reset via the user interface can be attempted, but if EC-00 returns promptly, an engineer needs to carry out a proper diagnosis rather than repeatedly resetting the unit.