which "common" wire for furnace

Hi, We're replacing our 1970's mercury thermostat with a modern programmable. The existing wire is 2 conductor, and has 24vAC (I confirmed with voltmeter ). We replaced the wire with a 7conductor-18AWG "thermostat" wire, and wil l properly dispose of the mercury t-stat.
I know we need to connect to the two original wire terminals on the furnace (with 24vAC difference), but I understand these thermostats need _3_wires to work. This is for heat only, no AC (but maybe in future). I know one i s designated the "common wire" but it's confusing because if you short the 24vAC, the furnace comes on (of course, like with the old mercury tstat). So not sure where the 3rd wire is going to go, and instruction booklet is a bit confusing. Has anyone been through this process and can offer guidanc e? Thanks
Theodore.
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On Mon, 19 Oct 2015 21:48:27 -0700 (PDT), snipped-for-privacy@yahoo.com wrote:

The third wire goes to an SD card embedded in the thermostat. It keeps track of how long you use your furnace every day. If you have a late model furnace, or whenever you eventually get one, the furnace takes the data and transmits it over the AC wires to the USECA, the US Energy Conservation Agency. There your energy use is reviewed and any applicable fines are levied.

Yes, but it was 32 years ago. Programmables were just starting to come in, and mine is mostly mechanical. There are two 48-position switches for the times, and 7 dip switches to say whether there should be one setback or two each day of the week. Honeywell.

Still, I'm not sure you need more than 2 wires. (oops, see below) Most of what I know relates to my 36-year old furnace, which has provisions for AC, and it is clear that if you have no AC, you only need two wires.
Red and white in the case of this Carrier furnace.
But you're not saying the furnace needs 3 wires. But that the thermostat does. I guess it needs a ground, or a return to keep power to it when the themostats switch itself is open.
My thermostat instructions are files away, but I think it's the G screw at the furnace, which you should have conenected to the green wire, that should be the third connection. It's not the color of the wires that matter so much*** but where they are connected on the furnace.
***Proper colors are used so the next guy won't be confused.

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On Tuesday, October 20, 2015 at 12:48:31 AM UTC-4, snipped-for-privacy@yahoo.com wrote :

The existing wire is 2 conductor, and has 24vAC (I confirmed with voltmet er). We replaced the wire with a 7conductor-18AWG "thermostat" wire, and w ill properly dispose of the mercury t-stat.

ce (with 24vAC difference), but I understand these thermostats need _3_wire s to work. This is for heat only, no AC (but maybe in future). I know one is designated the "common wire" but it's confusing because if you short th e 24vAC, the furnace comes on (of course, like with the old mercury tstat). So not sure where the 3rd wire is going to go, and instruction booklet is a bit confusing. Has anyone been through this process and can offer guida nce?

If the new thermostat is battery operated, it should work with just two wires on the R and W terminals. The common wire is used for thermostat s that need power or optionally can use power. Usually though with any furnace you have a third wire that goes to the fan so that you can activate the fan manually if you want to run it without heat, that would go to the G terminal. If yours doesn't have that or you don't care about it, then two wires should do. Wires are color coded, R - red, W -wht, G - green.
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Problem that I have with the instruction is that it refers to a number of labeled existing wires on the furnace, none of which exist in my case.
This is a photo of the furnace wire terminal.
http://i899.photobucket.com/albums/ac194/millinghill/image_zps1eax1m8u.jpeg The two I've cut have 24vAC across them, and used to go to the mercury thermostat. There is no other control wire for the fan or anything else in the exisitng condition.
The new thermostat is a Honeywell wifi programmable thermostat model RTH6500WF. It specifically indicates that it needs a Common wire to function.
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On Tuesday, October 20, 2015 at 10:34:19 AM UTC-4, snipped-for-privacy@yahoo.com wrote:

Then in addition to connecting the existing two wires to the R and W terminals you need one new wire to one terminal of that transformer. If you have a meter, measure between each of the low voltage terminals and the R wire at the furnace. At one terminal you should have zero, the other 24V. You want to connect the new common wire to the terminal that shows 24V. Alternatively, if you can follow the R wire back, it should be connected to one terminal of the transformer. The new common wire goes to the other one.
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Trader_4, sorry, but I want to make sure I understand what you're saying. I have 2 terminals available, per the photo. I know they have 24vAC differ ence. I can confirm which of these terminals is +24vAC vs. neutral by usin g a voltmeter against ground (i.e. the frame of the furnace). Your recomme ndation is that the wires going to the R and W terminal on the thermostat s hould go to the +24vAC and neutral respectively; and the wire going to the Common terminal on the thermostat should connect to the +24vAC?
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On Tuesday, October 20, 2015 at 11:45:16 AM UTC-4, snipped-for-privacy@yahoo.com wrote :

I have 2 terminals available, per the photo. I know they have 24vAC diff erence. I can confirm which of these terminals is +24vAC vs. neutral by us ing a voltmeter against ground (i.e. the frame of the furnace).
Whoow there Pilgrim. There is no neutral on the 24V side and not sure the transformer output has any relation to the eqpt ground, but it might.
Your recommendation is that the wires going to the R and W terminal on the thermostat should go to the +24vAC and neutral respectively;
The R and W terminals on the new thermostat should be connected to the two wires you had with the old thermostat. One of those, the R, if you follow it back, should be connected to one terminal of the transformer. The common gets connected to the other terminal. Look at the transformer as a battery. One terminal call it A, goes to the thermostat. From there, if the thermostat closes, it goes back to the heat terminal on the furnace, through the furnace control closing the relay or whatever there, back to the other terminal B on the transformer. You want the common connectio n to go to that terminal B. The thermostat then has wires going to both A and B, which it uses for power.
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understood. Many thanks for the words of caution, and the extra description.
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