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.

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.

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.

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.

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?

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!

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.

G green is for the fan.

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.

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

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?

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?

{{!gwi}}

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.

I climb up the attic open the door at furnace and find out there is a transformer that step down 120 v to 24 v . and two terminals at the second end (24v) red terminal is power 24v and the other terminal is use to connect to frame or chassis and serve as common ground so a c wire is any unused wire you can pick any color you want in the same bundle that goes from the furnace to the thermostat in my case those extra wires are trim flush to the bundle jacket on both end so I need to peel that jacket off about 2 inches on both end and connect a single wire about 2 foot long from the frame to the wire you selected (let say black) on the furnace end then the black wire on the thermostat end will be "C" wire and connect it to the "C" terminal on the new wifi fancy Honeywell or Nest. If I say anything wrong please correct me. I will do this at some other time. for now I just thinking. I signed up for smart (something) at gapower.com and they will sent me a nest gen.3 for free. I think this nest also will need "C" wire. I wait till I get the Nest then I will install both the Nest and the Honeywell at same time I will put the Nest down stair and Honeywell up stair.

My furnace doesn't have a C terminal. But there are plenty of unused wires. So I just connect one of them to the chassis of the furnace for the C wire?

The chassis of the furnace may be isolated from ground. Look for a ground connection on the control board on the 24V side of the transformer. That is where the C wire should be connected.

wires go into this box

I opened the junction box. I am pretty sure the green wire on the second picture is the ground wire from the power supply. Unfortunately I can't find any controller board. So I assume I can use that as C-wire. TIA.

Hi did you get around this issue? I have a very similar setup and trying to understand where should i connect my c wire from thermostat to.

raj bn: This is an old thread from 2013. I returned the Honeywell and bought a Nest, which doesn't require a Common wire. However, I recently upgraded to an ecobee, which comes with an "accessory" part to attach directly to your furnace electronic board if you lack a C. It took a bit of effort (my furnace is installed in the attic crawl space and it's very difficult to access), but I was able to connect the accessory part and now my ecobee works.