Sorry to cross post, but I did a similar thing wiring a Raspberry Pi, high voltage relay (I used the Sainsoft prewired stuff), and wiring directly to my thermostat. I had to use a multimeter as the thermostat went to different stages to kind of reverse engineer it.
Here is a script as an example that runs to my project dashboard. https://github.com/flores/lotomation/blob/master/bin/ghetto-nest.rb
Starting on line 22 you can see how I had to wire it for my particular thermostat. It took quite a bit of time to fiure out which pins fired and when.
It ended up looking like the pictures here: https://github.com/flores/lotomation#hvac-control
This post here talks about the temperature sensing part and has a video demo. Maybe you can rework some of that stuff for your project? http://forum.universal-devices.com/topic/13525-pi-1wire-interface-methods/?do=findComment&comment=120749
Some things I learned that I did not realize until doing it:
* It's really high voltage and potentially I am afraid about my HVAC running all day because of a bug in my code (both because of cost but more importantly fire risk!)
* the program above has some built-in safties because of the above:
** If the HVAC has been running for an hour straight, it turns off automatically and sends me a text (with a link to turn it back on if I want).
** If the power goes out and turns back on, the relays default to off mode. This way if I have a power outtage or the Raspberry-Pi crashes while the HVAC is on, it will kill itself and stay off. I had to wire the relays in reverse (default off) but this is easy with the Sainsoft stuff.
Just ideas/sharing!