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. 
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Thanks! So there will be three wires sharing the Com terminal- and that is not a problem, is it? And should I pay attention to something else? Like that they don't touch each other... or?– user46790Commented Dec 18, 2015 at 18:54
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The answer below from @Tester101 is certainly more complete, but since they are electrically connected under the screw, it doesn't matter if they touch each other. You just want that metal clamping plate to be as even as possible, and touching all wires.– JPhi1618Commented Dec 18, 2015 at 18:56
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1 Answer
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).
answered Dec 18, 2015 at 18:50
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I assumed the type of terminals pictured were "made" to have 1 wire on each side if needed like on a backwire electrical outlet since the screw is clamping the wire between two plates.– JPhi1618Commented Dec 18, 2015 at 18:53
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@JPhi1618 If I remember correctly, NEC says one per terminal, unless the terminal specifically states that more are allowed (110.14(A)). At any rate, we're talking about 3 wires, not 2. Commented Dec 18, 2015 at 18:56
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Yes, I'm asking if I can use the C terminal with three wires. Most of the furnace terminals are already sharing two wires... as you can see from the picture (Com, Y and R). Commented Dec 18, 2015 at 18:59
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Ah... and those were professionally installed, so I would assume that more wires are allowed, not just one. But is 3 too much?? Commented Dec 18, 2015 at 19:03
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Don't assume. If the terminals are allowed to have more than one wire, it will say so on the terminal block or in the manufacturer's documentation. Two wires per terminal in HVAC work is common (though not usually correct), while 3 or more will tend to use a pigtail. Commented Dec 18, 2015 at 19:10