Thought I'd document this and share for the other DIYers in there who have Fujitsu units, so that hopefully it helps out some people. After a bunch of research and building on what others have posted, I was able to get my Nest 3rd Generation thermostat working with my Fujitsu heat pump system (ARU12RLF) using Fujitsu's UTY-TTRX third party adapter. What follows are the steps I took, and a couple issues I encountered along the way.
This was my previous thermostat, which Fujitsu calls rather ineloquently a "wired remote controller". Looks like it was designed in 1989 and never changed (mine was an AR-WAE1U, but other models seem to look and act nearly identically). I call it a thermostat, but it doesn't even have a temperature sensor - it was reading the temperature from the condenser, which was in the attic, and a 10 degree temperature difference from the room!
From what I can tell, this thing works with nearly all (maybe all?) residential Fujitsu heating & cooling units, and though my system's model number didn't appear in the UTY-TTRX compatibility lists I looked at, it was an easy guess that since this thermostat was already working, I could swap it out for a UTY-TTRX + Nest solution.
When I unscrewed the old thermostat from the wall, I saw I had 3 wires, labelled R, W, and B (abbreviations of their colors). I had to search a little to find out these had different names going into the UTY-TTRX (see diagram and photo below):
- Red = Y1
- White = Y2
- Black = Y3
I bought a 24V transformer online, the same type I used to wire my Nest Hello. (in fact I ended up using some leftover 18 AWG copper wire to go from the transformer to the UTY-TTRX). At first I plugged the transformer into an outlet (shown below) in order to test the setup, although I eventually was able to tap into the outlet from inside the wall.
The biggest gotcha: the UTY-TTRX has a bunch of dip switches on the inside of the unit, and unless they are set correctly the whole setup will silently fail. Actually, the UTY-TTRX has LED status lights which will report errors, but for some strange reason they can only be seen if you unscrew its cover.
I left all the dip switches in their default position except one (shown below with a red box). I had to shift the single "SW1" switch to the left, to enable "3 wire" mode (pretty straightforward I suppose since I have 3 wires going into it, but only after I knew where to look).
Then connecting the UTY-TTRX to the Nest was pretty straightforward, and matched what the Nest app suggested:
After everything was connected, I was able to flip everything on, and was able to test heat & cooling successfully. But in addition to the Nest, I had a couple devices and a whole bunch of wires. I ended up opening up the wall and putting them in there, behind the Nest.
Second big gotcha: don't put your transformer too close to the Nest! The transformer doesn't generate a lot of heat, but I initially bolted it to the underside of my PVC electrical box, and when I got everything in place and running I found out that the Nest's temperature reading was 4 degrees F higher than it was supposed to be!
I ended up dropping the transformer deeper down into the wall (about 2 feet), so that the heat had space to dissipate, and the Nest was again accurate within 1 degree.
Still waiting for my decorative wall plate to arrive to show the finished work, but that's the gist of it!
