Thermostatic Controls - Help Please: Blue Wire = Common Wire?

My current thermostat has 4 wires hooked up: one to Y, one wire to G, one wire to W, and one wire to RH with a jumper between RH and RC.

I thought there were only 4 wires but upon further inspection, there is a blue wire that is not used. Could this be the C wire (common wire?)? Is there a way to test if it is? Or, can I try hooking it up without any bad consequences?

I want to buy a new thermostat and the one I'm interested in needs a C wire connected in order to have the backlight stay on continuously.

Any help would be greatly appreciated.

The blue is probably an unused common. If you have a volt meter, test between the blue and the red wires if you get a reading, which should be about 24 volts AC, the blue is a common. Poor man test, and I'll catch h e double hockey sticks for telling what I would do, is to briefly tap the blue wire bare tip against the red wire bare tip. If you see a tiny spark, the blue is common. Don't leave the wires touch for more than a fraction of a second. I don't want you to burn out the transformer.

Goldstar, thanks so much for the reply. To do your test (I won't tell :), I have the power off so would I have to turn the power back on? I'm not sure if I have a volt meter but I have to run out to the hardware store for some plastic anchors and maybe I'll pick one up.

That would be great if the blue wire is a common wire. If it is, would I still want to jumper the RH and RC. It looks like from the manual, you would hook it up the same way I had it but just add in the common wire on C.

I just bought a "16-Range Electric Multi-Tester". That was the only thing they had. I've never used one of these before and I'd really, really appreciate some help.

It looks like this one:
Amazon.com: 16-Range Electric Multi-Tester: Home Improvement (http://www.amazon.com/dp/B000NPT6G6/ref=asc_df_B000NPT6G6922997/?tag=prontocom92541-20&creative=380333&creativeASIN=B000NPT6G6&linkCode=asn)

I tried touching one lead to the thermostat's red wire and one to the blue wire, but nothing happened. Would the power need to be on for the furnace in order to do the test? I feel so lost :(

Yes, the power needs to be on..

Easy thing to do is go to the furnace, and find the t-stat wire, and see what the blue wire is hooked on the board.. If the blue wire is not hooked up, then strip it back and wire to C on the board, and take the blue wire at the t-stat and wire that to C.

Yes, the power needs to be on..

Easy thing to do is go to the furnace, and find the t-stat wire, and see what the blue wire is hooked on the board.. If the blue wire is not hooked up, then strip it back and wire to C on the board, and take the blue wire at the t-stat and wire that to C.

Thanks, I appreciate it. I did try the voltmeter with the power on and nothing happened so I think it's probably not hooked up on the board. I've never opened up my furnace but I am going to assume the board is in plain site?

Most of the time they are right there in the blower area, can't miss it. Just follow the t-stat wire into the system, and you can see where it hooks up to.

Yes, the power needs to be on..

Easy thing to do is go to the furnace, and find the t-stat wire, and see what the blue wire is hooked on the board.. If the blue wire is not hooked up, then strip it back and wire to C on the board, and take the blue wire at the t-stat and wire that to C.

Thanks very much, Jay. I looked at the furnace and can't figure out which are the wires from the thermostat. I don't see a circuit board in the furnace. Am I looking in the wrong place? At the t-stat, the wire colors are red, white, green, yellow and the unused blue. Here are pics I took of inside the furnace if you can make out where that blue wire would hook up to:

Furnace - Windows Live (http://cid-37bb3fd7c3261efa.skydrive.live.com/browse.aspx/Furnace?authkey=hB3noT1QFs0%24)

EDIT: Looks like the board is in the section below the section I had opened up. This is my furnance:

http://www.xpedio.carrier.com/idc/groups/public/documents/techlit/58ss-2p~1.pdf

Yes, item 14 in the lower (blower) unit is the spot. You should see a cable with your five wires ( red, yellow, green, white, and blue). See if the blue is connected or folded back out of the way.

Ok, this is what I have:

Green wire to G
Yellow wire to Y
Red wire to R
White wire to W
The Blue wire was not connected to anything.

Then, there is a completely separate wire with 2 wires in it, one is red and the other is white. The White wire goes to Y and the Red wire goes to C

Here is a picture of it:

FurnaceCircuitBoard - Windows Live (http://cid-37bb3fd7c3261efa.skydrive.live.com/browse.aspx/FurnaceCircuitBoard?authkey=BuqLf5Pol18%24)

Just hook up the blue wire to C.

The other two wires is going to the unit outside for your A/C.

Just hook up the blue wire to C.

The other two wires is going to the unit outside for your A/C.

Thanks so much, Jay. Just so I'm sure about this, all I have to do is leave that wire that's already connected to C connected, and then also connect to C my blue wire, correct?

This would be great because then I can get the thermostat I want which can stay backlit all the time. On that note, do you have any opinions as to either the Honeywell Vision Pro 8000, or have you ever heard of a Totaline touch screen thermostat?

Thanks again, I really appreciate the help.

Thanks so much, Jay. Just so I'm sure about this, all I have to do is leave that wire that's already connected to C connected, and then also connect to C my blue wire, correct?
Correct.


On that note, do you have any opinions as to either the Honeywell Vision Pro 8000, or have you ever heard of a Totaline touch screen thermostat?
The VisionPro is a great t-stat, I have one (IAQ model) and am very pleased with the results of it on my two stage system, the house is always comfortlable.

Totalline is not a very popular model, it seem to be a good stat as well, but the VisionPro is by far the best.

When do get it, set the Cycle per hour (CPH) to 3.

Thanks so much, Jay. I'm glad I can connect the blue wire to the C, along with the red wire that's already there. I just wanted to make sure there was no problem with doing it that way.

This is the Totaline thermostat I was looking at. I like that the background is blue and the numbers appear larger than on the Vision Pro. I think I'll probably go with the Vision Pro since it seems there really is no support for the Totaline. I called the phone number they had listed in their manual and was told they no longer support it. So no warranty, I guess. They do go pretty cheap on Ebay.



Thanks again, I really appreciate it.

Just an update. I just ordered the Honeywell Vision Pro 8000 for 1 heating/1 cooling. Thanks again for all your help, I do appreciate it.

Glad to help, I know you will be happy with it.

Just wanted to say thanks again. I installed my Vision Pro 8000 and it works great. I connected the blue wire just like you said Jay and now the backlight stays on all the time like I wanted. My husband was at work when I was installing, so I had to drill the holes myself, but I guess I did a good job because the thermostat is still on the wall :D

Glad to hear it's up and working!

Do you go though the set up menu on the t-stat? If so, any questions on it, or want me to review it for you?

Hi Jay,
I did change the number of heat cycles per one of your previous posts from 5 to 3 (0240). Does it matter if my furnace is about 25 years old?

I didn't change the compressor cycle rate setting (0220) because I wasn't sure what it should be. I just left it on default of 3. Does that sound right?

Thanks again, I really appreciate it. I really like this t-stat, especially that you can program each day separately, and that I can see it in the dark.

3cph is fine with your furance. If you find that the temp swings too far above set point, then you can change it to 4.

Ok, thanks Jay. So far, it's been working great.

Great! Glad to hear all is working well for ya.

Thanks for the update.