What it means
Daikin's guides call it a "Leaving water temperature sensor problem". The service manual calls it the outlet water thermistor (after the backup heater) and says the code resets automatically. Other 81 sub codes cover other water sensors, for example 81-01 for the mixed water thermistor.
What to check, in order
- Read the help text
On the controller, open the main menu and choose Malfunctioning to see the code and a short description. The help button gives a longer one. Note the code for your installer.
- Use emergency heating if you need it
Daikin's user reference guide says the backup heater can take over the heating when the heat pump fails. If emergency mode is set to Manual, you confirm this on the Malfunctioning screen. Expect higher electricity bills while it runs.
When to call your installer
Call your installer. Daikin's service manual lists a faulty outlet water thermistor or a faulty hydro PCB as the causes.
A repair visit will usually check the sensor's reading and its wiring.
- Note the model and serial number. They're on a label on the rating plate on the outdoor unit, and on the indoor unit or cylinder.
- Write down the exact code, and what the appliance was doing when it appeared.
- Check whether it's still under warranty. If it is, the repair may cost you nothing.
Quick answers
What does the Daikin Altherma 81-00 error code mean?
The sensor that measures the water leaving the heat pump for your radiators or underfloor heating is giving a reading the heat pump can't use, so it has stopped. Daikin's guides call it a "Leaving water temperature sensor problem". The service manual calls it the outlet water thermistor (after the backup heater) and says the code resets automatically. Other 81 sub codes cover other water sensors, for example 81-01 for the mixed water thermistor.
Can I fix Daikin Altherma 81-00 myself?
Engineer only (engineer visit). Start with: read the help text; use emergency heating if you need it. Call your installer. Daikin's service manual lists a faulty outlet water thermistor or a faulty hydro PCB as the causes.
Is Daikin Altherma 81-00 worth repairing?
Worth fixing. Replacing a sensor is a fairly small job. If the hydro PCB is at fault it costs more, so ask for a quote.
Checked against
- Daikin Altherma 3 R W installer reference guide 4P496758-1B, error codes, p. 90 (ManualsLib copy)
- Daikin Altherma 3 R ECH2O installer reference guide, error codes overview, p. 274 (ManualsLib copy)
- Daikin service manual ESIE20-06A, Daikin Altherma 3 M (EBLA/EDLA), PDF (copy hosted on the OpenEnergyMonitor forum)
- Daikin Altherma low temperature split user reference guide 4P499574-1A (EHVH04/08S23DAV), PDF
Last reviewed October 2026. Manufacturers update their pages, and your manual is the final word. Spotted a mistake? Email contact@appliance-error-codes.com.