General Assist Technical Reference

RTPU Dual-Stage Defrost Operation

Parameter d17 - low ambient defrost configuration
Rooftop Package Units / RTPU - New Zealand and other low ambient applications
Quick answer: On a dual-stage rooftop package unit, the default configuration may allow the non-defrosting circuit to continue operating in heating while the other circuit is in defrost. For low ambient applications, parameter d17 can be changed from 2 to 1 so only one compressor is ON for the circuit in defrost.

Application

CategoryTechnical Support
ProductRooftop Package Unit / RTPU
RegionNew Zealand / Low ambient applications
KeywordsRTPU, defrost, dual stage, heat pump, low ambient, New Zealand, MCXSimulator, L2 password, d17

Question

On a dual-stage rooftop package unit, if both stages are operating in heating and one stage enters defrost, what happens to the other stage?

Does the other stage continue operating in heating?

Answer

For dual-stage rooftop package units, the default behaviour may allow the other circuit/stage to continue operating in heating while one circuit is in defrost.

In very low ambient conditions, such as New Zealand applications, this behaviour may need to be adjusted using the defrost parameter setting.

To prevent the other compressor/circuit from continuing normal heating operation while one circuit is in defrost, check and update the following parameter:

Parameter path:
Parameters > Heat Pump > Defrost > d17

Required access level:
Level 2 password: 3000

Required setting:
Change d17 from 2 to 1.

When to Apply This Setting

This parameter should be checked during normal maintenance, especially for units installed in low ambient temperature regions such as New Zealand.

Apply this setting where there are concerns about defrost performance or where dual-stage defrost operation requires the non-defrosting circuit to be managed differently.

Procedure

  1. Access the controller or MCXSimulator.
  2. Enter the required access level using the Level 2 password: 3000.
  3. Navigate to Parameters > Heat Pump > Defrost.
  4. Locate parameter d17.
  5. Check the current value.
  6. If the value is set to 2, change it to 1.
  7. Confirm and save the change.
  8. Verify unit operation during heating and defrost conditions.
RTPU defrost parameter d17 controller screen

Parameter Detail

ItemDetail
Parameterd17
DescriptionNumber of compressors ON for circuit in defrost
Default / observed value2
Recommended low ambient value1
Important: Parameter changes should only be carried out by authorised service personnel with the required access level. Confirm correct unit operation after making the change.

Notes

  • This change is generally required for very low ambient temperature applications, such as New Zealand.
  • The parameter should be checked as part of routine service or maintenance.
  • Ensure the change is performed only by authorised service personnel with the required access level.
  • Confirm correct unit operation after changing the parameter.