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I just bought a new WiFi thermostat that requires me to add a C-Wire at the furnace. I have read the questions related to my issue but would like a suggestion on my particular (incredibly OLD) furnace. I have run 18/5 wire down to the furnace and just need to know where the C wire would go in my particular case.

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marked as duplicate by BMitch Nov 8 '13 at 20:52

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What model furnace? –  Niall C. Jul 23 '13 at 3:20
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Find the transformer. It should have two wires on the primary side, and two wires on the secondary side. One of the wires on the secondary is R, the other is C. Don't mess with the primary side, it's at line voltage. –  Tester101 Jul 23 '13 at 13:17
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1 Answer

It looks like the transformer is the thing on the bottom, with the red and yellow wires attached. You'll need a multi-meter to verify this as you go - I wouldn't make any assumptions here, especially since it's older. Mis-wiring could cause something to break, and there's a chance finding a replacement will be very difficult.

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Use a multimeter (on the "VAC" setting), and you should be able to measure 24V AC across the red and yellow wires whether the furnace is on or not.

Next, you need to figure out which power wire (red or yellow) is already going to the thermostat. Turn the furnace power off (you should now measure 0V AC on the red/yellow wires) AND the thermostat itself off, and use the resistance meter (Ω) on your multimeter to figure out which one of the red or yellow wires goes to the thermostat. You should measure 0Ω (or <1Ω anyway) between one of the wires of the thermostat and either red or yellow. Test both thermostat wires against both. The others will be infinite (not connected). If it's red, then the yellow is your "C" wire, or vice-versa.

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Thank you very much for your comment. VERY informative. So, below is a closer view. You can see the words Thermostat and 24V above the wires. The yellow wire is connected to the wire on the far left labeled thermostat. The red wire just runs straight to the transformer. I am on my way to buy a multimeter, but this seems correct to me? imgur.com/hjT3xoH So I would connect the c wire along with the red wire at the transformer right? –  Anthony Jul 23 '13 at 19:16
    
The picture is hard to see (small), but my guess is the wire on the top left is the thermostat, and bottom left is the fan? It also looks like the two middle terminals are connected? If so, seems like yellow is the C wire.. Anyway, your multimeter will confirm this as I said in my answer. –  gregmac Jul 24 '13 at 4:38
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