As I read many of these threads I haven't seen any suggestions to the best way of working on these systems, especially thermostats. As I am still in the learning process of HVAC workings, I found the very best way to begin and even end work on them is to take pictures. My daughter gave me an old Casio Exilim camera in which I can take pictures of thermostats, wiring, inside of heat pump, air handler, zone control and everything in between. You can then put the pics on your computer and even zoom in to the size of a flea. This is great when working with the actual pics and a wiring diagram next to your computer. Thought this might help others as it certainly did me! You can even read a helpful thread and then click to your picture to see what
you have instead of going in the attic, crawl space, etc.