I'm about to look for a programmable thermostat to replace a manual one. There are already a couple of good questions on thermostat replacement that make the task seem pretty straightforward for anyone who's done a bit of wiring (which I have done). The problem for me is this nagging doubt that, from a wiring perspective, all the thermostats I look at are not equivalent and only some would be able to work with the existing wiring.
So, my question is, as I'm looking for a replacement thermostat, is there anything I need to take into consideration? Do I need to read the back of the box for any specific statistic? Do I need to check the existing wiring first (number of wires, voltages, etc.)?