Gas and Oil Home Heating Furnaces - how do I Know if I have a "common wire'?

I bought a thermostat that needs a common wire on terminal C. I followed advice on other post and tried the standard blue, but it didn't work, How do I know if the common wire is another wire? Thanks for the help.

I am guessing that your thermostat is not working right.

If your stat has a clock or is a programmable stat, it MAY need the common wire hooked up. (Some don't.) The common wire is not needed to control the heating, cooling or fan operation. The stats that require a common wire will function just fine on battery backup until the batteries die.

If something is not working correctly, tell us what is or is not happening.

The thermostat I am trying to install is a Chronoterm III digital programmable by Honeywell that says needs a common wire. My current stat is a mechanical that aparently doesn't need/have? one. I tried conecting the blue as common but didn't work. The fan works fine. When I conected it, it shows the sing "-AC" and the instructions say that that happens when there is no 24V between R and C. Right now in my old thermostat I have these wires:Orange to O, Green to G , Red to R, and Yellow to W and Y. THere is a jumper from 1 to 2. The terminals on the Chronoterm III are O,W,G,C,R,B, Y, so I conected O,G,R,W (with jumper to Y) and the blue to common and wouldn't work. I disconected the blue and didn't work either. My understanding is that batteries in this model are for back-up only to prevent program loss.Looking at the installation diagram it seems that the common wire carries the 24 V, right? now that my original is a mechanic maybe makes sense that I don't have a common wire. This is all for a single stage heat pump. I hope this is enough information.Thanks for the help.

Chances are that you hooked up the blue wire at the stat but NOt at the air handler or furnace which is the source of control power. So do that and you should be in business.

If not, let me know.

HI, thanks for the info, I climbed to my attic to look at my air handler ( at least 130F up there) to realize that I am not sure what to do. I see a cable that goes in and one that goes out, the one that goes an "interruption" with wires joined by conectors (this might be confusing-it is what I saw-like if they run out of wire maybe) then that cable goes to the air handler. The other one goes to the stat. Where do I conect the blue wire, I think has to be in the one that goes in, but I am not sure . Is there a way to identify where exactly conect the wire? Will I see more wires and a "free"conection for the "C"? I realized up there that I wasn't ready to start so I deciced to write again. Thank you for your quick responses, and honest desire to help.
My heat pump is an Intercity 1997, the AIr Handler look just the same design of label with specifications etc, so I am assuming it is the same. THANKS AGAIN

The cable from the furnace to the stat needs to be carefully identified. Apparently the cable is spliced (perhaps to send a blue and a yellow wire to the outdoor unit.

The blue from the stat can connect to the furnace terminal C or to the blue wire IF it was spliced to go to the outdoor unit.

OK,here I am learning a lot about this stuff. I climbed to the atticc, found the trasformer at the Air handler. It has 2 cables, one that comes from "outside"(I am asumming too far to tell) but one that definitely goes to the Stat, 100% verified. The "interruption" that I mentioned before it is on the cable that goes INTO the airhandler not the one that goes to the stat just like they run out of wire, but interestingly they conected, red,orange,yellow and brown. When this cable arrive to the trasformer it is conected to the plate and it joins the Outgoing(to the stat) on Red,yellow and Orange only( on R,Y and O). From the plate to the stat goes a single wire from G (green) and a Brown wire that I found conected to 'C" at the trasformer's plate so I thought I HAVE A COMMON WIRE! so I went and conected the stat using the brown (coming from the trasformer on C) and the same, didn't work ,still showing the -AC on the display. Because of a mistake in delivering my order that guy I bought it from sent me 2 stats, so I tried both with same results. SO to make the story short, is that brown wire that I have coming from the plate of the tranformer my common wire, my common sense told me so, do I still need to conect something there? seems like it would be the same, just diferent color of cable, anyway, I got excited thinking I have it now, but aparently not, I appreciate your help.THANKS

The brown wire connected to C on the transformer is the common wire. (This wire is needed by the outdoor unit and is already in use for that purpose.)

Connecting the stat to this same terminal should do it. (But it didn't.) Since you see a splice in the cable to the outdoor unit it is not much of a stretch to think that there may be a splice in the stat cable somewhere. Look (but it may be hidden).

Sometimes a wire in a cable will simply be defective.

TEST: You could carefully note the existing wiring on paper. Double check it. Disconnect all wires from the stat on both end of the cable. Then reconnect Red to R and use the Green wire (which was used to run the fan and is a proven good wire) to connect from C on the transformer to C on the Stat.

If that TEST works and you don't find a splice to repair, replace the cable and it should work fine.

HI, finally got it to work, I just hooked up the blue wire to the conector in C from the trasformer to my stat. The brown wire as you say goes to the compressor probably for the fan to work, so I conected the blue wire the to my stat and works beatiful, thank you so much for your help, it has been fun to learn all this stuff.Thanks again Lynn.

In your original post you said, "I followed advice on other post and tried the standard blue, but it didn't work."

You ended up with blue working. So was something else happening or is it a mystery to you?

the blue wire was not conected anywhere, I conected it from the transformer to the stat and it worked, I picked blue as "standard" for the common wire.
Thanks for following up.