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Jesse Van Atta

VRF Quality Assurance Manager Gulf Coast District

Crossover and Zoning

Carrier has approved the use of Crossover equipment with zoning applications. The instructions for this will be released in several phases during 2026. Release dates for the different phases of instructions are subject to change. 

First phase:  

  • Instructs using a residential (non-Crossover) fan coil with a 37MURA or 37MUHA outdoor unit and ZONECC4ZCN01 zone controller. (May 6, 2026) 

Second phase: 

  • Instructs using a 45MUAA or 45MUHA matched with a 37MURA or 37MUHA, communicating on S1/S2. (May 21, 2026) 

Third phase:  

  • Instructs using a residential furnace and coil with a 37MURA or 37MUHA outdoor unit in a dual fuel application. (May 30, 2026) 

*See product bulletin 101-26-13 (Carrier) or BY-103-26-13 (Bryant) for complete instructions.

Important things to consider for zoning when using Crossover equipment: 

1. Static pressure. The blowers provide a constant air volume based on the mode of operation. The 45MUAA fan coils can handle up to .8” static, above which CFM performance falls off dramatically. The 45MUHA 18-36K are rated up to 1” static. 45MUHA 48-60K are rated at .8”.  

2. Bypass. Because of the constant air volume motor, the blower is likely to hunt if static pressure changes significantly while running. It is also necessary to use the dump zone method vs bypassing supply air directly into the return. When using the standard supply-to-return bypass method, the T1 (return air) thermistor will be deceived any time the bypass damper opens due to direct supply air on the sensor. See below for an example of a dump zone design.

3. The Algorithm. Capacity request (CR) and target evaporator setpoint (TES) are at the core of logic driving the inverter frequency. This is a very important difference from a standard or Infinity/Evolution zone system. CR is calculated from indoor temperature vs setpoint. TES is based on mode of operation (heat or cool) and Y1 vs Y2 demand. But how does the logic determine what these temps are?

  • When the outdoor unit is matched to a non-communicating fan coil, the CR and TES are derived from a refrigerant pressure transducer (reading suction in cooling or discharge in heating) combined with a thermistor (reading dedicated suction temperature). From these components, it determines TES and CR, building a memory of the sensor readings when the system thermals on and off (setpoint).\

 

  • When the outdoor unit communicates with the indoor unit, CR is derived from the T1 thermistor and learned setpoints from building a memory of the T1 temperature when the system thermals on and off. TES is monitored from the T2 (Indoor coil).

 

  • The 45MUAA and 45MUHA fan coils also have logic for TES based on T2A and T2B thermistors (indoor coil refrigerant entering and leaving temperatures) which direct the EEV to open or close as needed during the cooling cycle.

 

  • Other factors in the algorithm for inverter frequency are outdoor ambient, outdoor coil, compressor discharge, and IPM temperatures.


Be aware Carrier has not validated any third-party zoning for use with the Crossover equipment. The instructions will be revised and re-issued at times. Always refer to the most up-to-date literature which can be found on HVACPartners.


Addendum (6/23/26):
Carrier’s compliance with CVP (Control Verification Procedure) requires that the Crossover outdoor units be wired for communication only. It will be necessary to add a 24-volt interface (KSAIC0701230) with any non-communicating indoor unit, and that when a Crossover fan coil is used, it must be wired for communication.
*The built-in interface on the Crossover outdoor unit will remain for emergency use only.

References:
• Product Bulletin: 101-26-13
• Product Bulletin: BY-103-26-13
• CA-Crossover-Zoning
• BR-Crossover-Zoning
• 37MURA-02SI.pdf
• 37MUHA-02SI.pdf
• 45MUAA-02SI.fm
• 45MUHA-03SI.fm
• ZONE4CN-02SISP.pdf

Disclaimer: The technical statements, information and recommendations contained herein are believed to be accurate as of the date hereof, but Mingledorff’s does not make representations or warranties, express or implied, as to its accuracy, its completeness, or the results to be obtained. The information is being provided for informational purposes only and is intended for use by persons having adequate skill and expertise regarding the proper selection, use and application of the products and recommendations and at their own risk and discretion. 

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