38MU/40MU 18/8 Low Voltage Wiring

249 VAN Launch Kit 249 VAN Consumer Brochure Smart Thermostat Sell Sheet 249 VAN Launch Kit

Unlock the Full Potential

This installation was on a 4-story home, and the tech had not yet gone to the basement to troubleshoot the “infinity” water source heat pump, as he was told he by the homeowner. He was in the attic at the infinity furnace above the 4th floor. He was getting his dcv in the proper range on his A and B green plug on the air handler, 3 to 4 vdc. His indoor board ohm reading was in range, 28k to 32 k. The tech ohmed out the A and B thermostat wires on the water source heat pump from the upstairs. His ohm reading was not in range of a normal infinity board. His reading was around 64.4k. The tech then made the long walk to the basement where the “geothermal heat pump” was located. Bingo.

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.