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Install new thermostat but not sure if I have wire C


kindoof's Avatar
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02-03-13, 03:05 PM   #1 (permalink)  
Install new thermostat but not sure if I have wire C

Hello,
I moved to a new house and would like to install a thermostat with WiFi, I found 4 wires only (W,Y,G,R) behind the current thermostat plus an extra hidden unused wire, but I found 5 in the furnace box (C,W,Y,G,R) the colors don't matches between them (furnace and Therm.) which confuses me.

In addition I noticed an transformer which have 4 wires connecting to it. My theory that the old owner had an advanced Tthermostat but he took it with him then the bank had to put a cheap one to run the heat. Can some one confirm the extra hidden wire is the ''C" and this is related to the transformer?

Thanks in advance...

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Astuff's Avatar
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02-03-13, 06:02 PM   #2 (permalink)  
The C (common) wire is supposed to be blue and normally attached to chassis ground at the furnace. Get a meter and check voltage between the R and C. If 24 volts at the thermostat you should should be good.

The red/white wire cable are on C/Y going to the air conditioner compressor. The other cable should have all 5 - maybe yellow was cut off.

standard colors
C - blue common ground (not normally used)
R - red 24V AC power from furnace's transformer
W - white heat
Y - yellow air conditioner
G - green fan


If the wires don't match at each end they may be spliced somewhere in the middle. Did you find out where the transformer's wires go? It might be for some other accessory. The yellow/green and red/black look like they might be twisted together/shared.


Last edited by Astuff; 02-03-13 at 06:31 PM.
 
kindoof's Avatar
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02-03-13, 07:01 PM   #3 (permalink)  
Thanks for your reply. I just ordered a meter online, will check the voltage when I get it, but I would like to learn why there are 6 wires on the furnace side but 4 active wires + 1 non active wire "red" on the thermostat side. On the other hand there are 4 wires on the transformer.

Here is another picture for the furnace where it shows two cables one with 4 wires and one with 2. as what it shown the one has red and one has blue but both are sharing the Y.


I cannot tell if the transformer for something else it's going to the wall which is very close to the thermostat location.


C: Cable 1 white
G: Cable 2 green
Y: Cable 1 red + cable 2 blue
W: Cable 2 white
R: Cable 2 red

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Astuff's Avatar
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02-03-13, 07:52 PM   #4 (permalink)  
That model transformer just plugs into a normal 120V wall outlet. It looks like a grey cable. Do you have a grey or brown at the thermostat?

Cable 1 should be going outside to the air conditioner compressor as it needs the Y and C. The Y goes to both as it needs to be fed from the thermostat.

How much slack is in cable 2? You should be able to peel back some of the covering to expose the other wire.

 
kindoof's Avatar
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02-03-13, 09:04 PM   #5 (permalink)  
I just chipped the dry paste and found the source of the non active wire. it's coming from additional white cable that has 3 wires (red, blue and white) in addition I noticed what's written on the Transformer's cable and it says "...Alarm.." so it seems it might belong to the security alarm systems which I don't use, maybe I should detach it to check if the it connecting the alarm.

What caught my eyes that the black cable on the thermostat side has matching colors and connection wires with the brown one on the furnace side.
W: white Y: blue G:green R: red

the white cable has 3 wires (red, blue and white) but on the furnace side is brown and has 2 only (white and red)

Are those non active 3 wires are related to the furnace? how I can check that? is one of them is the C wire ?

Thank you again for helping :-)

Here are the pictures.

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Astuff's Avatar
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02-05-13, 05:24 AM   #6 (permalink)  
The black thermostat wire must be spliced somewhere in the middle. Try following the bigger brown cable from the furnace. You should find the splice. Hopefully you will find the white cable there as well.

Of course the best solution would be to run a new cable (brown 5 conductor) all the way from the thermostat to the furnace. That way next time you won't have to re-think the whole solution.

 
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02-05-13, 02:56 PM   #7 (permalink)  
If you are going to run new wire then run 18-8 for future upgrades and incase you have a wire short or open.

 
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