Yea, I was referring to the issue with not having common for voltage, which is 'common' in retrofits, But yes, if a Cat5 is pulled it should be fairly easy.
So then, whatever HA controller I get later, it SHOULD just be easy to remove the old thermostat, and plug in the new one along with a CAT5 cable, and be good to go? I just wanted to make sure I didn't have to tell my hvac guy that he needed to make some special kind of connections or something.
As for controlling, I'm happy so long as it has a protocol I can write a CQC driver for. An elk will be in the future somewhere.
Depending on what your HVAC system is, you may not need a "C" common wire. For my RC80B, the "C" is optional, but the cool blue backlight doesn't work without it.
The key's here:
1) Regardless of what system you go with, you're going to need either a cat5 to each thermostat and zone controller, or to the HVAC unit. These need to be homerun to your wire closet. To be safe, if you can, make sure you do both. Cat5 is cheap, especially compared to the "time" cost of having to pull extra cable.
2) Make sure that your HVAC controller leaves you enough thermostat wire. They have a habit of cutting the thermostat wire to "just barely" reach whatever terminal block they install for you. I can't tell you how annoying that is. Ask them to push some extra thermostat wire behind the wall and use a clamp behind the tstat. That way, if you need more play (and you will), you can just pull some more through. It might also be smart to ask them to NOT clip the unused wire (bend it back into the hole). That way, you have extra connectors.
3) It's a good thing to ask for a "C" wire. Yes, this is easy enough to do on your own (standard is "blue" and on your unit, you'll see a "C" on the wire harness inside the HVAC unit once you remove the cover), but its much easier for the HVAC guy to do it. Tell them that you have a fancy tstat that needs common.
So, in a simple one tstat, one HVAC unit, unzoned system, speaking to an automation controller, HAI is a very easy and inexpensive purchase. If you never plan on an automation controller, or you have zones, or you have a complex HVAC setup, there might be better options.
I'm also assuming that you aren't getting anything exotic (eg: simple gas furnace + AC or simple 2 stage heat pump). Model #s are RC80b and RC100B respectively.