Hi Justin, great project.  I have a couple of questions: 

1) Your schematic shows a resistor to each of the 4 GPIO points being used on the ESP but the photo seems to have 6. (4 at the controller and 2 at the ESP) Is there a change not shown in the schematic?

2) Are the diodes that are shown on the schematic actually part of the controller circuit board or were these part of your add-on circuit at some stage? I don't see diodes in your photo.

I want to recreate this myself having just moved into a house with the same controller.  I want to get it integrated to Home Assistant.

Cheers,

Dave

Hi Dave, the diodes can just be seen attached to the ESP and are used as a Wired AND circuit. 

The other resistors are part of the temperature sensing circuit I added later. not shown in the photo

Overtime the controller  behaved eratically.  While in cool mode it would randomly stop cooling then restart.  The ESP8266 always appeared to be online so I am guessing that the sudden power demands the ESP was placing on the supply was messing with the controller.

I experimented with various configurations some of which involved simply placing a single diode in line with the ESP8266.  This worked well for a time but I could see at time the ESP was getting tortured with high supply rails but continued to work ok.  Over time this also began to cause issues with the swampy controller.

Tests showed that the supply to the controller would droop randomly.  It is fed from the external controller box on the roof over relatively light gauge wire and guessing it can not delivery the peaks demanded by the ESP.

The latest configuration uses a cheap $1 mini buck converter (replacing the original LT1117-3.3 linear regulator) set to 3.0v on its output with a large 220uF cap on the input of the regulator.  I set low at 3.0V to give the buck regulator as much head room as possible. 

The ESP appears to be rock solid with a 3.0V supply (should be 3.3V but the buck failed to start) and the controller appears to be stable.  Time will tell.