Originally Posted by MrG
Butch is correct - neither of my thermostats currently has a C wire.
I did a bunch of research and ended up just going with a programmable Honeywell without the WiFi and all that. I couldn't find a WiFi thermostat that didn't collect data that I'm not thrilled about giving out in exchange for a marginal increase in convenience. Since it doesn't need a bunch of power for bells and whistles, the one I got can run on batteries or a C wire, so I'm going to give batteries a shot first and see how it goes.
Gotcha - sorry I missed the first part of the thread. I did a TON of research into this myself when I got obsessed with my HVAC system a couple of years ago. In my case, I found out that there was a C wire connection on the HVAC board, but my installer was too lazy (or too old school to hook it up). I was able to pull wire from the wall back to the furnace to hook this up. I was going to go with a snazzy thermo, but cheeped out and just got a fancy, but dumb Honeywell. I also hooked it up so it gets outside temp, humidity, etc..
What I found, and what I was going to recommend, is using a Honeywell IAQ/Redlink System with an EIM (interface module). Basically you mount a module at the furnace that allows you to make all the connection down there (think a hub), and you only need 2 or 3 wires to the thermostat, and it gives you insane control (multi stages, all kinds of add on sensors, controls, etc...), but uses only those couple of wires instead of having to run 8-10 wires to the thermostat to make it work. This must be a response to the fact that most older homes have limited wires, but most new HVAC systems have a ton of options that no one actually takes advantage of. I was going to use this in my parents house where they have a new multi stage high efficiency unit but a themostat from the 80's that doesn't have the ability to control.