It has nothing at all to do with programmable Vs. non-programmable. All of the homes I mentioned above, including ours, have a programmable thermostat. You don

It has nothing at all to do with programmable Vs. non-programmable. All of the homes I mentioned above, including ours, have a programmable thermostat. You don't need a C wire for a programmable thermostat. The other channels provide power, the C wire just provides more. It is useful if you want to run a color LCD screen or WiFi. If you have a thermostat with a black & white calculator-like LCD, it likely doesn't have a C wire running to it. > I've actually more commonly heard of some control boards that mess with the fan if you use the C wire rather than the power extension kit, since the control board basically fakes the C wire using the G wire as the Nest does if you don't have a C wire. What? That would burn out thermostats. How would the control board know to pass 24Vac through a low voltage line? Can you link anything to this "common thing" that control boards do? Sorry but I'm extremely skeptical of that, it doesn't even make sense from a thermostat signal point of view, since they'd just assume you would run C if C was required during installation rather than doing the hack you're suggesting here. The reality is that C isn't required on almost any included thermostat, programmable or simple. Good installers run the C but terminate it, bad installers just don't run the unneeded C. I've never heard or read about a first party control board which acts like a bypass kit out of the box, and I don't even understand the point.