How to install a C wire for wifi thermostat

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Old 05-05-18, 05:40 PM
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asi
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How to install a C wire for wifi thermostat

I have old HVAC system. It is all electric both for AC and heat furnace. My compressor is from mid 70s and furnace is from 1991.

My furnace circuit board is only 4-6" from thermostat.

1. Ist picture is overall furnace and thermostat is in the hallway, opposite to furnace.Name:  IMG_20180505_173152537.jpg
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2. Pic 2. It seems like there are 3 cirucuit boards, 2 in the back and one on Left side.Name:  IMG_20180505_173354485_LL.jpg
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3. pic 3 is, back wall circuit board (2 connection boxes) Name:  IMG_20180505_173420536.jpg
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4. Pic 4 is side wall circuit board Name:  IMG_20180505_173409996.jpg
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I have not seen any set up like that in any youtube videos.


Now It seems like there is a big brown wire is coming from thermostat and I has 5 wires. 4 of them (red, green, yellow and white) goes back to circuit board and 5th one blue is just hanging there.Name:  IMG_20180505_173803675_LL.jpg
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But on the thermostat side I can only see 4 wire (red, white, yellow and green). perhaps who ever installed this unit cut 5h Blue wire which may be hidden inside big brown covering on thermostat side.Name:  IMG_20180505_152154240_LL.jpg
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Any suggestion how to run 5th (C wire/blue wire)? I think best option is buy a single 5th wire and run along big brown wire from thermostat. But only thing is where it will go once inside.


Most wires either go to top back circuit board or lower one. Then some goes from back board to side left circuit board. There are no name like R, W, G, Y on the circuit board inside.
 

Last edited by asi; 05-05-18 at 06:40 PM.
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Old 05-06-18, 02:15 AM
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Pic 3 shows the transformer on top and fan relay on bottom. Blue wire is your common.
 
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Old 05-06-18, 08:07 AM
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Hi, remove the Tstat and see if there is any slack in the cable,if the blue wire is present at the furnace it must be in the cable at the stat.
Also, the cable going out to the condenser will have the common wire you can connect the blue wire to, it will be the opposite wire from the one that the Yellow wire from the Stat is connected to.
Geo
 
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Old 05-06-18, 09:49 AM
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I think I have it labeled correctly. Blue is C.

Name:  cc.jpg
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Old 05-06-18, 09:56 AM
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Thanks PJmax. Blue one is just hanging out in front. Pay attention to label G, it is just left of it. Blue is not attached to anything.

I checked thermostat side. There is a blue wire buried inside big brown covering.
 
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Old 05-06-18, 10:03 AM
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I labeled the wirenuts with their function. That loose blue thermostat wire would connect to the blue wire junction. You should end up with two blue wires and one white wire in that splice.
 
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