How to wire the AC & Thermostat into Furnace

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Old 09-21-19, 02:26 PM
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How to wire the AC & Thermostat into Furnace

I was going to replace my 5 wire (with no C wire) with an 8 wire. However I couldn't budge the existing. When I opened the Furnace panel I found that the installers spliced the 2 wires from the AC from the Thermostat and ran short wire to the Furnace. I was going to try Venstar Add-a-wire next ,but I don't know what to do about this wiring job
 
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Old 09-21-19, 05:15 PM
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Welcome to the forums.

If the thermostat wiring was in the wall from when the house was built..... it will be stapled in place.

If the wiring goes down.... you should be able to enlarge the hole behind the stat....... go to the basement and locate the thermostat wiring there. You can use a snake to fish between those holes. You could also try pushing the new thermostat wire up from the basement. It would have to be kept very straight to go up high enough to be seen at the stat hole.

Otherwise..... we'll need pictures of what you have for wiring to be able to help.
How-to-insert-pictures.
 
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Old 09-21-19, 05:31 PM
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Thermostat wiring issues

Use a Honeywell Redlink wireless thermostat. Then the only wiring will be to the condensing unit (2) and then wire the transmitter to the furnace.
 
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Old 09-26-19, 07:57 PM
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Exclamation Venstar Add-A-Wire, but there are 2 "C" Terminals

I gave up on trying to run the wire through the house. Today I installed the Venstar Add-A-Wire (VAAW). I carefully followed the instructions. But there is 1 complication. There are 2 wires coming from the AC condensor. The hot wire goes to one of the (2) labeled C terminals. I have included a screenshot of my furnace manual showing the terminals and it doesn't even show that there are 2 C terminals. The VAAW diagram and instructions show to have their brown wire and Green wire go to the C Terminal. So I'm at a loss if I add these 2 wires to the same C terminal that the AC hot wire goes too? or the other C terminal. So frustrating. I have it all wired up but I'm afraid now to turn the system on. *one more thing, One of the photos shows the AC wiring + Thermostat wire + another wire going to the Furnace panel. I figured out why they did this: The installers didn't have enough length when using the original wire, so they cut off a short run and splinted it and ran the rest to the furnace panel.

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Last edited by PJmax; 09-27-19 at 09:43 AM. Reason: resized pictures
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Old 09-26-19, 09:30 PM
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Update: I opted to put the two Add-a-wire wires on the open C terminal and left the AC hotwire on the other C terminal. I have a Nest 2nd Gen Thermostat that I've now connected and turned the system back on. The fan is running and the Nest seems to recognize everything. But when I went back to the basement, the furnace was making sort of angry buzzing noise that I've not heard before. So, from the Nest App, I shut the system down for the night. I don't know where to go from here.
 
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Old 09-26-19, 10:09 PM
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There should only be one wire on C from the add a wire.

You can not split the C wire and may have done the circuit board, add a wire and thermostat damage.

The C wire is a straight path to the transformer.

You use one of the other wires.



The nest is known to be a really bad product and I recommend returning it. There are far superior products made by ecobee and honeywell.

The add a wire isn't great, you should really get some fish-tape and pull an additional wire if you can, leaving the existing one in place.

I recommend hooking W2 to the thermostat so it can stage that furnace properly based on what the house needs. Right now it uses a fixed timer - like 10 minutes on low and if stat isn't satisfied, it switches to high until the cycle is complete which defeats the purpose of having a 2-stage furnace. You can't get extended heating cycles on low fire, nor switching from high to low with the timer. Dip switch setting needs to be changed to disable the staging timer.
 
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Old 09-27-19, 09:53 AM
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The VAAW diagram and instructions show to have their brown wire and Green wire go to the C Terminal.
Only brown goes to C. Green should go to G.

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Old 09-30-19, 07:56 AM
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So, why are there 2 terminals labled C? Which one could be the real C wire terminal? Is the blue wire from the compressor attached to the true C terminal? I wish I could run new wire, but the the only path goes above the furnace along the joists to the wall then does a 90 turn up the wall in an area that is completely inaccessible (as far as I can tell anyway).
 
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Old 09-30-19, 12:29 PM
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Your control board has 2 C connections for your use. They are connected together internally.
 
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