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Daikin Altherma fault UA-21

Bizone PCB / hydro PCB mismatch error

Heat pump

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. No power supply
  6. Faulty transformer
  7. PCB malfunction
  8. Faulty room thermistor
  9. Deviation of thermistor (lower temperature measured than actual)
  10. Bad contact between sensor and pipe
  11. Loose or disconnected wires
  12. Damaged connectors or wires
  13. Incorrect or missing software on the PCB
  14. Communication error between bizone PCB and current loop PCB

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

The full answer, free

  • 22 more checks in order
  • The readings and component tests from the manual
  • How to put it right, step by step
  • The manual page for this fault (page 64), right under the answer
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Safety: Isolate the outdoor unit and any backup heater, and give the inverter capacitors time to discharge before you open the electrics. Anything on the refrigerant circuit needs F-gas certification.

From the Daikin manual for the Altherma. Check the model on the data plate before you order parts.

Other Daikin Altherma fault codes