Common C wire- Old/ Dumb furnace - where to connect for wifi thermostat?

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  #1  
Old 06-10-15, 01:11 PM
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Common C wire- Old/ Dumb furnace - where to connect for wifi thermostat?

Hi, i have an old and dumb(no control board) furnace combination (heating and ac), i want to replace an old thermostat with a new wifi model, luckily the 50 year old wires have an extra unused wire that is yellow.







The middle electrical diagram corresponds to my furnace.






The arrows show the two transformer leads that correspond to the secondary (24v) side.
The green arrow points to the red wire that feeds the condenser.



I tried adding the fire to the R4 terminal and nothing, also added the yellow wire to the Y terminal and have power and fan control works but the ac cooling will not start, it just shows up as -Cooling On- on the lcd control and nothing happens.

I also get power via using the Fan wire - the blue one, but don't want to use that solution since i have the yellow wire that can be used for C -common wire.


Please help with connection, i can't afford to burn anything out.
 
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Old 06-10-15, 02:21 PM
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On the black terminal strip do you have a C connection? It's hard to see the wiring diagram.
 

Last edited by skaggsje; 06-10-15 at 02:23 PM. Reason: adde info
  #3  
Old 06-10-15, 02:31 PM
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That's the problem i'm facing, i do not know how to connect the c wire, the only terminals are the one's that are listed on the diagram:

W
W2
R4
G
Y
FX

In that order, left to right.

Please help with what could be the issue, since when i connect the yellow wire that i'm using as c wire to Y terminal, thermostat works, and fan control work, but it fails to turn on, even though the thermostat LCD shows -Cool On-.


You can right mouse click on the photo and choose view image and then zoom in.
 
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Old 06-10-15, 02:43 PM
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Your Y terminal is to start the AC. I'm not seeing any view image. Follow, per diagram, the wires connected to the SEC side of the transformer. One of those will be the common.
 
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Old 06-10-15, 02:47 PM
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Ok, got a better look at the diagram. FX is your common.
 
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Old 06-10-15, 02:49 PM
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Thanks for the info, one of the pictures i posted has the red arrows pointing to the wires that connect to the SEC side of the transformer, i connected the yellow to the first one on the left, (Designated R4 on the diagram) but no power at the thermostat.

Should i connect to the other wire, the one designated FX, that connects red wire to the condenser? Will that be a safe thing to do, i do not have a voltmeter. thanks for the help.
 
  #7  
Old 06-10-15, 03:10 PM
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I have connected Yellow C wire to the FX, and there is power, and the thermostat works, after a short delay of blinking Cool On, the unit is working, pretty awesome help you guys, thanks a bunch.
 
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