Changing a furnace "common" or "fan" wire - how to?

June 18, 2021 7:01 AM

I had a new furnace installed and the installer changed the wiring to the thermostat in an unhelpful way. The green wire, which used to be "fan," is now "common." I don't really want that. Is there an easy fix?

I should probably just call the company and have them come out and change it, but there is a hassle factor because we would need to be home at the right time, etc.

What happened is this: I have a Nest thermostat and we like to use the "fan only" function to circulate air in the house all day -- i.e., 15 or 30 min an hour. With the old setup, I used the green wire from the furnace and plugged it into the "G" port on the Nest, which allowed it to run the fan. We did not have a "C" wire (common), but this was not a problem because the Nest could charge when the fan was running.

The new furnace guy changed the green wire to a "common wire," which has much higher voltage than the old "fan" wire. He plugged the green wire into the "C" port on the Nest rather than the "G" port. Now I no longer have the "fan only" option because the Nest no longer has a "G" wire to operate the fan. He did this so my Nest could charge through the common wire, but I didn't really need that because of the way we use the fan.

I stupidly tried plugging the green wire into the G port, but blew a fuse in the furnace, which I replaced. I put the wire back into C and it is working fine, except no fan control.

Is this hard to switch over so I have a "fan" wire rather than a "common" wire on the green wire? Or should I just call the guy?

posted by Mid to Home & Garden (16 answers total)

I'm not a specialist, but I'm in sort of the same boat - which wiring do you have currently? Just two wires, like an old fashioned "close this circuit between W and R and the furnace starts" thermostat? If your furnace has the whole suite of Y/G/R/W/C terminals, your best bet is to probably have a handyman or the furnace dude fish a new 5 wire circuit from your Nest to the furnace and do it the proper way.

Otherwise it'll be helpful to know how many wires you have in the walls and what terminals you have on your furnace.
posted by Kyol at 7:24 AM on June 18, 2021 [1 favorite]

Response by poster: I have 4 wires. Red to Rh, yellow to Y1, white to W1, and green used to be G but is now C.
posted by Mid at 7:34 AM on June 18, 2021

So in theory you just need to move the wire on both ends, but it’s not likely to be obvious where you need to move the wire on your furnace. You can have a look to see where the Green wire ends up on the furnace control board and see if there’s a really obvious connector for fan control. But calling the guy back is likely to be more successful.
posted by GuyZero at 8:08 AM on June 18, 2021

And yeah, getting a new wire run is the real solution. But that’ll depend a lot on how far the furnace is from the thermostat.
posted by GuyZero at 8:09 AM on June 18, 2021

The wire for a furnace is very straightforward. The colors don't matter a ton, what matters is what they are hooked into. If you are comfortable changing a fuse, you can probably also look at where the wires are connected color wise.

Some of those wires already run have spares. Some don't. I've had to really figure out how to configure it for my needs, and having the fan run alone may depend on your furnace. Some just won't do that no matter what.

If you need to re-run additional wire, you can, and it's not expensive, but if someone stapled what you have halfway up it can be pretty annoying.

Last, while nest claims you can run the nest without the power wire (common) I've never had it work long term.
posted by bbqturtle at 8:27 AM on June 18, 2021

And yeah, like GuyZero said, while IANAHVACT and IANYHVACT, I'd give a shot at moving the off-season wire (yellow / y1 "compressor" in the winter, white / w1 "heating" in the summer) to the green fan relay terminals on both ends and just accepting that you won't have heat in the summer or cooling in the winter and being mildly confused in the switchover months until you swap the wires around.

Or pull a 5th wire for fan control. That reminds me that that's been on my summer project list for 2 years....
posted by Kyol at 9:07 AM on June 18, 2021

Consider an add a wire assembly. I’ve never used one myself, but the theory of operation is sound and I’d expect it would work.
posted by doomsey at 9:10 AM on June 18, 2021

Response by poster: Yeah, I guess what I am asking is how hard would it be to move the green wire at the furnace end so that it becomes a “fan” wire rather than common? I know how to move it at the Nest. Is it just like unplugging a jumper and plugging it somewhere else? I don’t think I need a common wire because the Nest charges fine when the fan runs every hour. I had no common line before this and it all worked fine - so how do I get back to that? Is it soldering and voltage testing, or just like plug and play? This is a new carrier furnace. Thanks
posted by Mid at 9:44 AM on June 18, 2021 [1 favorite]

It's usually plug and play on new ones, usually a screw you unscrew, pull the wire off of the one spot, then screw in the screw again. Sometimes there's two wires under one screw, or a little jumper, but it should be easy enough to move 1 wire to one spot and another wire to another spot. 1/10 difficult!

Putting the wire in the correct space should also be easy - it's pretty well labeled.

But... even if it was soldering, soldering is a LOT easier than you'd think. 2/10 difficult!
posted by bbqturtle at 10:32 AM on June 18, 2021 [1 favorite]

You should find that there is a screw terminal block inside the furnace cover labelled with letters “Y” “G” “W” “C” and maybe some others.

Fan is “G”. Yes, they’re named after colors.

You should see the green wire go to “C” right now. Move it to “G”. Move it on the Nest as well. Done!


(I’m with your installer, though. Nests need C. Hence the add-a-wire recommendation.)
posted by doomsey at 10:41 AM on June 18, 2021 [1 favorite]

(I also agree that Nests need C. I've installed 3 without C [friends and families, my own], and had to add the C again each time. It's always a hassle. AND, most furnaces can't just run the fan. So don't get your hopes up too much more!)
posted by bbqturtle at 10:43 AM on June 18, 2021

If you have an outlet nearby you can also buy 24V DC adapters that wire into the C wire in the backplate and provide power that way.
posted by GuyZero at 10:44 AM on June 18, 2021

Response by poster: Thanks, that's great! I have been totally fine for ~2-3 years with two Nests without a "C," but we run the fan 15 minutes every hour (using the Nest option) and I think that charges it fine.
posted by Mid at 11:21 AM on June 18, 2021

Yes, Nests NEED the common wire. It may appear to work but it can seriously break your logic board over time. Ask me how I know.

However, there is a solution: a common maker. This should let you use the green wire as Fan and still have a common wire.
posted by kdar at 11:22 AM on June 18, 2021 [3 favorites]

An alternative is to add a fresh air ventilation controller to the system to do the same thing as what you were using the Nest for. My house came with one of these that has a duct to outside air and a damper. It turns the HVAC fan on periodically and sucks outside air through the return ducting and filter. You could probably set one up without the outside air inlet.
posted by ArgentCorvid at 9:53 AM on June 21, 2021

Response by poster: I took the advice in this thread and added a Venstar Add-a-Wire. Works perfectly!
posted by Mid at 10:26 AM on July 11, 2021 [1 favorite]