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New wifi thermostat and wiring issues with C wire

Posted by smiles33 (My Page) on
Mon, Nov 11, 13 at 12:27

I'm befuddled and have spent far too much time on this already so I'm hoping someone here can help. We recently bought a home in a very rural area so it's difficult to get vendors/technicians to head out here.

I want to replace an older (12 years?) battery-operated thermostat with a wifi thermostat (Honeywell's 7 day programmable wifi thermostat, model YRTH6580WF1007 purchased from Costco). The house has both heating (propane gas powered furnace) and AC.

The existing thermostat had Y, W, R, G wires connected and THREE extra wires (brown, orange and blue). There was a jumper cable from R to Rc.

I climbed up into our attic crawl space to get to the furnace. The former owner/builder put the Bryant furnace up there on its side under the eaves so it's a pain to get to. There is clearly a blue wire connected to the C wire on the circuit. See photo:

Note that I also saw a second bundle of wires on the circuit just above this set of wires that don't seem to be connected to anything. See the second brown bundle of wires directly above the existing set in the photo above. You can see that the wires are just wrapped around the brown bundle but aren't inserted anywhere. I have no idea what those are for.

Yet when I connect the blue wire to the C terminal on the thermostat, I'm not getting any power to the thermostat. I have disconnected every wire and re-connected in case I didn't seat them well. I even had my husband come and do it once as well. I also tried switching to the brown and orange extra wires to serve as the C wire but switched back to blue as it just didn't make sense. The electrical panel breaker was turned back on. I made sure the furnace was turned back on, too.

Help! Is there an easy solution I'm missing here or do I need to just bite the bullet and find an electrician willing to drive out here (for the right price, of course)?

Thanks in advance!


Follow-Up Postings:

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RE: New wifi thermostat and wiring issues with C wire

Do you have a voltmeter? If you do check to see if there is 24 volts between the R and C wires. The breaker and furnace have to be on.

The thermostat may be defective. It is unlikely, but there is always a chance.

Can you trace the wires from the furnace to the thermostat? There may have installed new wires with the furnace and only connected the wires needed for the old thermostat. See if you can find a splice point in the attic.


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RE: New wifi thermostat and wiring issues with C wire

Hi Mike,

Thanks for replying. I don't have a voltmeter, but I could buy one. No, I'm not able to trace the wires from the furnace to the thermostat--they are about 100+ feet apart and many walls/obstructions in the way. I think it may be time to call in a professional.

Thanks again.


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RE: New wifi thermostat and wiring issues with C wire

Just wanted to update that my dad took a look and pointed out that the C wire only has 24 volts, not 120. It's written on the circuit board itself and when he tested it with a voltmeter, there wasn't much electricity. So I just returned the Honeywell to Costco and put the old battery-operated thermostat back on. It's not worth upgrading my furnace just to get the remote access function.


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RE: New wifi thermostat and wiring issues with C wire

The control board and the thermostat operate on 24 volts, not 120. The C wire should be the ground (0 volts). The red wire should have 24 volts. It is hard to tell from the picture but I think the "Com" connections is labeled 24 volts. There should be a connection on the board labeled ground or "Gnd".

You don't have to upgrade your furnace. You just need to supply 24 volts and a ground from the circuit board for the thermostat for it to work.


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RE: New wifi thermostat and wiring issues with C wire

The 'COM 24v' designation means that this is the 'common' for the 24vac power. It may or may not be ground, but I suspect it should be.

The 2nd set of brown MAY be the cable that goes to the AC unit outside. The AC unit only needs like two wires, so the rest are unused. (Such a large cable is used because the outside unit might be a heat pump and would need a few more wires, or it might be multistaged.)

There is an Rh and Rc because sometimes systems are split into separate heating and cooling. In that case, there would be two separate 24vac systems that would be connected separately. When there is only one system, then the two are jumpered together.

Of those 3 other wires (brown, blue, orange), was only the blue connected?


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RE: New wifi thermostat and wiring issues with C wire

Thank you both for posting and clarifying. I think I'm a bit more confused. I thought the green wire (labeled G) was the ground. Interesting. I already returned the fancy new thermostat to Costco, but is there an easy way to supply 24 volts and a ground from the circuit board, mike_home?

Weedmeister: yes, only the blue was connected. The brown and orange were wrapped loosely around one of the brown bundles. I think you're right, the other thick brown group of wires must be for the AC unit (which is outside). Thanks for clarifying that mystery!

Since we're 45 minutes from the next nearest town, any service provider I get out here is going to charge quite a bit. Not sure if it's worth doing it just to get the remote access to a thermostat.

Thanks again!


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RE: New wifi thermostat and wiring issues with C wire

The R wire is the 24 volts. I believe you have this connection. You need to look for a ground connection and connect it to one of the unused wires. It is probably hidden under the wires on the board. Just be careful not to short the 24 volts to ground.


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RE: New wifi thermostat and wiring issues with C wire

G green is for the fan.


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RE: New wifi thermostat and wiring issues with C wire

It is really hard to tell from your pic but there is only 1 wire on the com terminal, that is required for the outside unit. If you trace the wires you are probably gonna find that the blue wrapped around the wire on top in the pic, is the one going to the thermostat. you need it connected to the com terminal in the furnace. Think you did the right thing with the fancy thermostat if you are not comfortable with wiring though.


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RE: New wifi thermostat and wiring issues with C wire

Thanks all. I decided I'm not going to mess with it. If we have bigger electrical issues in the future and have to get someone out here, I'll ask the electrician to hook it up so it's ready for when I upgrade the thermostat. Thanks again!


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RE: New wifi thermostat and wiring issues with C wire

Forget about C wires. Get a Nest thermostat on sale now.


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RE: New wifi thermostat and wiring issues with C wire

I'm in the exact same boat, same thermostat but older furnace, 1982. I figured out the wires for the thermostat, thanks to this thread, except

the transformer 24VAC side for the C-wire is connected to a ground wire to the frame AND a yellow wire to the fan. There is a separate wire, white which the wiring diagram says is the G for the thermostat. The white for G is currently not connected to anything = 2-wire thermostat. I assume the white (G) is a relay to turn the fan on? So, to make a short story long:

Do I leave the ground AND the fan wire AND the transformer / C-wire ALL connected together with the 5 wire thermostat setup?


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RE: New wifi thermostat and wiring issues with C wire

I'm in the exact same boat, same thermostat but older furnace, 1982. I figured out the wires for the thermostat, thanks to this thread, except

the transformer 24VAC side for the C-wire is connected to a ground wire to the frame AND a yellow wire to the fan. There is a separate wire, white which the wiring diagram says is the G for the thermostat. The white for G is currently not connected to anything = 2-wire thermostat. I assume the white (G) is a relay to turn the fan on? So, to make a short story long:

Do I leave the ground AND the fan wire AND the transformer / C-wire ALL connected together with the 5 wire thermostat setup?


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RE: New wifi thermostat and wiring issues with C wire

That's great you figured out most of it mtnmankevin. I ended up just buying a Nest, since we have 2 at our main house and I knew it was an easy install. Now I can have the weekend house warm before we arrive.


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