Daikin Altherma fault UA-21

What it means

Communication abnormality between hydro PCB and bizone PCB after communication was already made.

Likely causes

1. Faulty hydro PCB
2. Faulty bizone PCB
3. Faulty current loop PCB
4. Applicable ESV (Engineering Software Version) issue
5. Backup heater wiring is NOT correct
6. User interface is replaced or reset to factory settings; previous measurements are lost
7. Hydro PCB is replaced or reset to factory settings; previous settings are lost
8. Backup heater is allowed in emergency mode, setting [4-00]
9. No power supply
10. Faulty transformer
11. PCB malfunction
12. Faulty room thermistor
13. Deviation of thermistor (lower temperature measured than actual)
14. Bad contact between sensor and pipe
15. Loose or disconnected wires
16. Damaged connectors or wires
17. Deviation of outlet water after backup heater thermistor (lower temperature measured than actual)
18. Incorrect or missing software on the PCB
19. Communication error between bizone PCB and current loop PCB
20. LD1 not flashing: no operation of LAN adapter
21. LD2 flashing: TCP/IP communication problem with router
22. LD3 flashing: communication problem between LAN adapter and indoor unit

Where to start

1. Perform a check of the hydro PCB
2. Perform a check of the bizone PCB
3. Perform a check of the current loop PCB