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Riello NXHM fault PB

Anti-freeze mode

Heat pump

What it means

Unit enters anti-freeze protection mode.

Likely causes

  1. Low ambient or water temperature triggering anti-freeze protection
  2. Temperature setting is not correct
  3. Water flow is too low
  4. Shut off valves of the water circuit not in correct position
  5. Water filter is plugged
  6. Air in the water system
  7. Water pressure too low
  8. Expansion vessel broken
  9. Resistance in water circuit too high for the pump
  10. Water volume in installation too low

Where to start

  1. Check the parameters T4HMAX, T4HMIN in heat mode
  2. Check the parameters T4CMAX, T4CMIN in cool mode
  3. Check the parameters T4DHWMAX, T4DHWMIN in DHW mode

The full answer, free

  • 7 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 65), right under the answer
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Safety: Isolate the heat pump 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 Riello manual for the NXHM. Check the model on the data plate before you order parts.

Other Riello NXHM fault codes