C wire for thermostat

[Hall]

Right. A “Dumb” thermostat can get by with internal batteries. But if you want a “Smart” thermostat you can control or monitor via wifi, a C wire is typically needed. Nest thermostats suggest they can get by without one but reviews on that claim are very mixed.

I use an ecobee and they pretty much don't work without power (C-wire). At least with them, they provide/include a C-wire adapter for installations that don't have add'l conductors.

 

[SHOZ]

Many thermostats can use batteries or "power stealing" for their power. It's best to connect the C wire if one is available.

Had mine hooked up like that for years until I got a new furnace and they wanted to use the C as common for the 24v. The batteries use to last so long I never though it was running straight off that.