I'd like install a new Wi-Fi thermostat. The old one didn't have a C-wire but luckily there is an unused wire on both sides (one coming out of the wall and one at the furnace not connected yet), so I am going to use this unused wire as the C-wire. My only concern is that the C terminal at the furnace is already occupied with other two wires (that have nothing to do with the thermostat, but are connected one to the humidifier and the other one not sure to what, probably the air purifier). The question is: can I add a third wire to the C board of the furnace and use it as C-wire for the wi-fi thermostat? See picture: yellow is the unused wire.
Technically, there should only be one wire connected to the screw terminal. However, it's common practice in the HVAC industry to connect more.
If you want to do it the proper way, you should use a twist-on wire connector (or other connector), and a length of scrap wire to make a pigtail.
- Disconnect the wires that are currently attached to the terminal.
- Group together those two wires with the wire you want to add, and a short bit of scrap wire of about the same gauge.
- Use the twist-on wire connector (or other connector) to connect all the wires together.
- Attach the other end of the scrap piece of wire to the screw terminal.
You may also want to consider why there are already two wires. It's common in a furnace to have the A/C contactor coil terminated there, but you'll want to figure out where the other wire goes. The concern here, is that you could overload the transformer. If the transformer is operating at or near maximum capacity, the additional load of the thermostat could push it beyond the limit. Best case here would be a blown fuse, worst case being a blown transformer.
If the transformer is powering a humidifier or air cleaner, you'll want to make sure it can handle the additional load of the thermostat (though thermostats are typically a small load).