Help with understanding my current furnace wiring

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Old 11-06-19, 08:54 AM
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Help with understanding my current furnace wiring

I am trying to upgrade my thermostat and noticed the setup of my current setup is a little unusual. I tried asking questions at Home Depot and Lowes but they were not sure either; hopefully someone here may be able to help.

Picture 1 is my current thermostat - as you can see there is an adapter that splits a single black wire into the Y and W wire. No one seems to know what this adapter is, or if it is simply a splitter.

I then went down to my furnace to see the origin of the black wire. In picture 2, you can see there is some sort of adapter that is connected between the furnace and the wires that go to the thermostat. What is the purpose of this adapter? Picture 3 shows the wire that goes to the thermostat. You can see there are only 4 wires, and that there is a jumper between the Y and W and the black wire is connected to the Y.

The reason I need to sort this out is that with my new thermostat, the Y and W are on the opposite side. I will not be able to use the existing Y/W splitter setup to connect both Y and W to their respective terminal hole (the wire is not long enough). Ideally I can get 2 separate wires for Y and W? Assuming I have an extra unused wire, can I just remove the jumper between Y and W (in Picture 3) and attach a new wire to Y?

I apologize if this sounds confusing but this is the first time I'm messing with a furnace/thermostat.

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Last edited by PJmax; 11-06-19 at 08:13 PM. Reason: cropped/resized pictures
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  #2  
Old 11-06-19, 11:39 AM
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Hi, interesting, any model numbers , mfg on that control in your hand? also furnace.
Geo
 
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Old 11-06-19, 08:18 PM
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Welcome to the forums.

That's called an "Add-a-C" module. It converts four wires to five wire operation. It's done to get a C wire at the thermostat. The white wire hanging around at the thermostat is the C wire. Hopefully it's taped off as if it hits something there will be a short.

That white wire could be connected to that stat and then it will run without batteries. Typically a C wire is needed when an upgrade is made to a wifi thermostat.

So for the future.....
W = W
Y = Y
Red = R
Green = G
White = C
 
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Old 11-07-19, 08:52 AM
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What thermostat are you wanting to install?
 
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Old 11-07-19, 12:45 PM
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If your thermostat doesn't need the common and can run off batteries, you should remove the device on both ends and use the black wire for Y.

The white can be used for W.

The wires need to be moved on both ends.

Those devices can fail and it's not worth running the risk just to not have to change thermostat batteries.

It's worth it if your stat can't run without a common or you need an extra wire for something else.
 
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Old 11-11-19, 09:10 AM
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Thank you for all the replies.... my question is why do I need this "Add-a-C" module? My new thermostat does request a "C" wire... however, if you look at the control panel directly on my furnace (see new picture attached), it appears I have room for 5 wires including the "C". The furnace is only about 9 years old. My question - do I really need this "Add a C module", or can I connect the thermostat directly to the furnace using the 5 wires?
 
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Old 11-11-19, 09:13 AM
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Yes, if you have enough conductors in the thermostat bundle.
What thermostat are you installing?
 
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Old 11-11-19, 10:24 AM
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I am looking to replace the thermostat with a Honeywell T9
 
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Old 11-11-19, 07:09 PM
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I hadn't noticed the extra wire in the cable. I would recommend the following wiring color change.
There is currently a white and black wire coming from the A/C condenser.
They must remain on C and Y as they currently are.

Red - R
Black - C
White - W
Green - G
Orange - Y

Remove the add a C modules at both ends.
 
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