If this is you, you're wired for whatever smart thermostat you want to assume the rest ... Option 4: Choose a thermostat that doesn't require a C-wire or an adapter.
It doesn't need a steady power source. ... If you want a smart thermostat but do not have a C-wire to power it, then you can run ... But if you are using Nest with some control panels that will require a C-wire, then you need one.
If you don't have a common wire and want to install a smart thermostat, you still have quite a few options. Option 1 is to buy a smart thermostat that doesn't require a C wire. The Sensi and Ecobee are two top models that don't require a common wire.