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Jonathon Pullen

Technical Services Manager Gulf Coast District

March 17, 2023

38MU/40MU 18/8 Low Voltage Wiring

This month, I will be covering how to properly wire a 38MU/40MU for 24V operations and setting up the DIP switches.


I hate to say this, but the install manual is poorly written and many people get confused on why the unit does not operate on start up. Most of the confusion is understanding the DIP switches.


Many contractors are using this equipment with an Ecobee or similar type thermostat. The system can be configured several ways. When you wire this system for 24V, wire it like any other multi stage system using 18/8 wire. The part that trips people up is properly setting the DIP switches.


The indoor unit has 2 DIP switches that will need to be set and the outdoor has one.


The indoor DIP switch SWl-1 and SWl-4 will need to be turned on. The wiring diagram shows the correct location.


The outdoor unit has 1 DIP switch which needs to be turned on, SWl-2. Turning on the DIP switches tells the unit to look for 24V inputs instead of the communications RS-485.


Set the switches correctly and you should not have any problems making the system run on 24v.

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