I'm upgrading the thermostat in our new old house and have the following situation -

W/Y/R/G wires are all clearly identifiable at the thermostat (an old Goodman CHT18-60), so I'm going to ignore everything that's not directly related to my question...

There are two cables connected to the control board - a skinny one and a fat one. Both have more wires than are being used. The skinny one is the only one with a wire (blue) connected to the COM terminal. (The only other wire involved in the skinny cable is a pigtail to the Yellow wire in the fat cable.) Skinny wire proceeds to run straight outside to the AC. The fat cable has wires connected to W/G/Y/R, but no COM. There's obviously no COM at the thermostat.

Question - since I need power for the new thermostat and I really (really) don't want to use one of those "add-a-wire" jobs (which means no independent fan control) - can I just use one of the unused wires in the fat cable for COM?

Assuming yes, since it's low voltage, is it safe to just add it under the terminal with the blue COM wire in the skinny cable, or should I pigtail it?

Let me know if more information is needed.

Thanks.

-Rob

Do you have an extra conductor in the wire to the furnace? Is there a common hooked up already in the furnace? If there is a wire hooked to common, then that's your C. If there is not then this is where you'd need that extra wire available.

If your AC control wire runs to the thermostat instead of the furnace, there has to be a common present or else the AC would not turn on.

PS, not an expert.

quote:

Originally posted by Skins2881:
Do you have an extra conductor in the wire to the furnace?

Yes - the fat cable running from the control board to the thermostat is full of unused wires.

quote:

Is there a common hooked up already in the furnace? If there is a wire hooked to common, then that's your C. If there is not then this is where you'd need that extra wire available.

Right.... so the blue wire in the skinny cable is hooked up to the C terminal, but the skinny cable doesn't run to the thermostat at all - just outside to the AC.

quote:

If your AC control wire runs to the thermostat instead of the furnace, there has to be a common present or else the AC would not turn on.

Of course. The AC control wire runs to the furnace - connects directly to the C terminal (blue) and yellow wire is pigtailed to yellow wire in fat cable.

So the question was, since I need a C wire running to the thermostat, can I just use one of the unused wires in the fat cable and attach it to the C terminal on the control board along with the C-wire running out to the AC?

-Rob

I misread, the two wires go to furnace and only one larger wire at t-stat?

Then all you need is the common to t-stat, and you have an extra conductor. Problem solved. Blue is most commonly used for C, so if there is a unused blue, use that.

I would pigtail the existing C from the AC to your new C for the T-stat, but I often just see both wires under The same terminal.

ETA listen to Stlhead, disconnect power at furnace disconnect switch, if not present, at electrical panel and confirm with tester that the breaker labeled furnace actually goes to the furnace.

If you must do it yourself here is what you could do, but I would recommend you hire a pro.

Take an unused wire from the "big cable" (wire that goes to the thermostat) Brown or black preferably, but not critical, connect it to the C terminal on the furnace (the one with the blue wire going to the outdoor unit). Use the same wire that you connected to the C terminal at the furnace to common terminal on the new stat. Also please remove power from your furnace before messing with the wires, otherwise you will blow the fuse, pop the control breaker if yo are lucky (if there is one), or even possibly let the smoke out of the transformer.

Again, if you have to ask anonymous jerk offs like us how to tackle an electrical issue it might be best to call a pro who may not be any smarter, but hopefully is insured.

Cool.

All of the unused wires in the fat thermostat cable are wrapped around the outer sheathing at both ends, so easy-peasy.

I'll pigtail the C's to the control board and call it a day.

Thanks, guys!

-Rob

ETA - LOL @ Stlhead. Dad taught me when I was young how not to kill myself with electricity. There's not too much basic stuff I'm not comfortable doing. No worries.

Use any of the unused wires and put it under the com screw with the blue wire thats already there.
Or pigtail them if you prefer.