Adding Common Wire w/ Existing Spare Wires


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Old 10-05-14, 12:42 PM
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Adding Common Wire w/ Existing Spare Wires

Hi, everyone. In preparation for adding a more high-end thermostat, I wanted to make sure I had a working common wire in my system. There wasn't one connected, but there was a spare wire in the bundle that was unused, so I connected that to the common terminal on the thermostat, and I connected it on the other end to the C terminal in the control board.

The C terminal already had a wire connected to it, and I'm pretty sure it was one of the two wires that went to the A/C unit. I simply inserted the additional wire and tightened the terminals. The fan and A/C worked fine, but I noticed that when I switched the heat on, it would blow the control board fuse. When I unplugged the wire from the control board, everything worked fine.

Obviously, I didn't wire something correctly, and removing the wire allowed the system to function fine, but now I'm not sure how to add the common wire. I would be happy to post pictures of any wiring setups if it would help. My unit is an American Standard Freedom 90. Any assistance would be appreciated.
 
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Old 10-05-14, 01:03 PM
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American Standard furnaces usually label the common terminal B or B/C.

If the thermostat is not a Trane or American Standard model you would not want to use the B terminal at that end, you would use a C terminal at the thermostat for 24 volt common. Connecting the stat B terminal to the furnace B terminal would blow the fuse unless it is a Trane / American Standard stat..
 
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Old 10-05-14, 01:28 PM
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Thanks for your reply! The thermostat isn't an American Standard or Trane (it's a Lux), so it sounds like there shouldn't be anything connected to the B terminal at the thermostat, and there isn't. My furnace circuit board has C, G, R, W, and Y. The furnace C terminal looks to be wired to the red wire that goes to the A/C unit, so I assumed that would be common, but are you saying that is incorrect?

It seems so weird that the fan and A/C work, but when the heat turns on, it blows the fuse. I checked all the wires, and I don't see any shorts on either the thermostat or the furnace, so I'm not really sure what to do next. Thanks for your help though!
 
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Old 10-05-14, 03:11 PM
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It sounds like the C wire may be shorted to the W wire.
A "common" location for this short is at the ends.
 
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Old 10-05-14, 03:15 PM
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As Houston204 stated previously, American Standard normally uses B as the common terminal. I suggest you double check the connection at the furnace to make sure the C wire is not touching any other wire other than the one on the same terminal going outside to the condenser.

Can you post a photo of your control board terminal wiring?
 

Last edited by firedawgsatx; 10-05-14 at 03:34 PM.
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Old 10-05-14, 04:03 PM
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Sure! Here is the control board. It is C, G, R, W, and Y in case the flash makes it hard to read.Name:  2014-10-05 18.41.24.jpg
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Here is the thermostat.Name:  2014-10-05 18.45.55.jpg
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On the control board, the exposed red wire on the right used to be connected to the C terminal until I found that it was causing issues. I can hook it up again and make sure it isn't touching anything else if you think that wiring should work in theory.
 
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Old 10-05-14, 04:46 PM
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The furnace C terminal looks to be wired to the red wire that goes to the A/C unit, so I assumed that would be common, but are you saying that is incorrect?
Looking at your photos it would appear you have a red wire on C and a black wire on Y going to the a/c condenser.

In the thermostat photo I see a suspect area which I circled in white. It appears the wire going to C may be touching the jumper between the RH or RC terminals. You could shorten the jumper and make sure it is not touching the wire going to C.


Also, you are using the wrong type of wire for thermostat applications at some of the the control board terminals. It appears the C wire has a "wild" strand touching the electronic components below it. You should be using regular thermostat wire of at least 22 gauge instead of stranded wire. At the very least you need to clean up the ends of the wires by twisting the loose strands until it is nice and cylindrical and no "wild" wires sticking out that can contact other terminals, wires or components.

I only see one wire attached to the C terminal in the photo of the furnace control board (I understand you removed one because of the issue). When done, it should have two wires: one going to the thermostat and one going outside to the a/c condenser. The red wire you had attached to the C and removed (circled in white) has way too much copper exposed. Just expose enough wire to go around the terminal screw.

It is hard to tell from the photo: what color wire is coming out of the wall and going to the C terminal. Is the wire with black tape spliced to another or is it the one going to the C terminal?

The green wire at the furnace terminal G needs to have the end trimmed off where it is sticking above the terminal screw. Maybe the red wire you had on the C terminal was contacting that end sticking up since they are both too long. That would definitely cause the fuse to blow.

Can you post another photo of the thermostat wiring on the right side where the C and RH and RC terminals are? Please separate the wires and take the photo at an angle that shows the colors of the wires where they exit the wall.
 
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Last edited by firedawgsatx; 10-05-14 at 06:01 PM.
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Old 10-05-14, 05:57 PM
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First, thanks very much for your detailed response. I know questions like this are probably incredibly basic for someone like you, but I appreciate your time.

As to your questions, I agree about the stranded wire concerns. It appears that the thermostat has the proper wiring, but the red and black wires that run to the condenser are stranded for whatever reason. I'll be sure to clean those up. I also agree about the amount of copper on the red wire. Oddly enough, both ends of the red wire were exposed already (I'm not sure why), but I'll at least put some electrical tapes around the exposed ends.

The wire coming out of the wall and into the common terminal is red. The one with all the electrical tape around it is brown and is also free at the board end, but I put electrical tape around it so it doesn't cause any issues. Basically, that brown wire is unused at both ends, and it is completely taped up so it can't contact the red common. I was going to use the brown wire as a common, but then I found the unused red that was already exposed, so I figured I would use that. Hopefully this photo better shows it.Name:  2014-10-05 20.46.42.jpg
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So far, it sounds like I had it wired correctly, but maybe something is shorting? If so, I'll go ahead and try to clean up the wiring situation and try it again. Thanks so much for your help!
 
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Old 10-05-14, 06:26 PM
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Thanks for the photo with a better angle so it is easier to see the wiring. After viewing the photo the only thing I can see at the thermostat now is that the brown wire that is taped off appears to be touching the end of the red wire going to C. I would separate those two wires so there is no chance to touch.

Stranded wire is for high voltage applications. Solid wire normally 18-22 gauge is used for low voltage applications in hvac. Thermostat installations such as the Nest specifically specify in the guide to not use stranded wiring and other manufacturers state that stranded wiring won't be recognized by their thermostats.

After you clean up the wires and make sure no wires are touching I would try setting it to heat again. If it still blows the fuse I would suggest you use the brown wire instead of the red as the C wire.
 

Last edited by firedawgsatx; 10-05-14 at 07:05 PM.
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Old 10-11-14, 09:04 AM
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Just to give an update, I went ahead and cleaned up the wiring on both sides, but it was still blowing the fuse. After that, I went ahead and used the brown wire on both sides, and so far so good. I didn't bother to get the multimeter out, but I'm guessing the red wire is shorted in the wall or something. Regardless, it looks like this setup will work. Thanks for all the help!
 
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Old 10-11-14, 09:19 AM
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Glad to hear you resolved your issue and thanks for the update. Sometimes a nail or staple is driven into the wall and will hit a wire causing a short.
 
 

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