Thanks to the previous poster who shared details of how to wire up a Nest thermostat to a Fujitsu mini-split system. I have three ducted units and converted them using the UTY-TTRXZ1 adapter. However, I wanted to share some details on the Google Nest basic thermostat that I think will help others who are looking to pursue this type of conversion.
I purchased three Google Nest (the standard with the mirrored look, not the "learning" version) and wired up the G, C, W, Y, and R wires. Unfortunately what seems to happen is that the voltages are not within the tolerances required for the UTY-TTRX adapter to properly operate. It seems that when the Nest is on "COOL" mode it applies a 24VAC signal on the Y and G wires, but the W wire registers a 3.3VAC signal when it should be closer to 0VAC, which causes the UTY-TTRX to fail in sending a command to the indoor unit to turn on the AC. Conversely, when I turn the Nest to "HEAT" mode, W and G register 24VAC but Y registers 3.3VAC when it should be 0VAC. Similarly, the system will fail to start the compressor and run the unit in either of these modes. After struggling for a while and finding another thermostat to try out, I found that I did not have the same issue as the Nest system and my system worked flawlessly.
The lesson--it appears that the base version of the Google Nest thermostat may not be compatible with the UTY-TTRX/TTRXZ1 box because the box expects signal voltages that follow a typical expected pattern for HVAC units (e.g. when Y is live at 24VAC signaling air conditioning to turn off, W should be at 0VAC as heat must be off for air conditioning to be on, and vice versa). This may be tolerable for other systems but unfortunately the UTY-TTRX seems to require voltages for "off" modes that the Nest doesn't seem to be able to provide.
The previous poster looked like they were able to successfully make their Nest work but it looks like their wire bracket was for the Nest learning thermostat, not the base version with the mirror display.
