What is a “C” wire from my furnace control board and can I use it?

[DPasto]

I’m trying to install a new “smart” thermostat, but it needs power, unlike the battery powered thermostat I had before. The most straightforward approach is to use a “C” wire but I don’t have one.

I have an American Standard gas furnace with central air. Looking at the control board, I do see a terminal marked “C”, used for something else. The cable to my thermostat has an unused wire: can I just connect it to the “C” terminal?

looks like the existing wire at that terminal (and the red on the other side) heads outside to the compressor. Will this terminal feed the thermostat adequately without interfering with AC? Do I need tobe concerned with multiple connections to the same terminal?

 

[DPasto]

Alright, i connected the “C” terminal to my smart thermostat with the unused orange wire. I guess it works.

I can set heat or cold by voice command or through an app

 

[piglet11]

Always a bit chancy to connect something if you don't know where it goes.
I would try and trace it back if you can.
Depending on the age of your set-up, it might have started life as a thermostat with a live, a switched live (switched by the thermostat) and a neutral if the thermostat has a heating element (resistor).
A battery powered thermostat would have two contacts, a live, and the switched live as above.
You need to check that the 'C' wire is really a neutral and not something else - 'C' is sometimes a Common terminal of a changeover switch or relay