Getting a common wire from air-handling unit to thermostat


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Old 06-11-20, 11:16 AM
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Question Getting a common wire from air-handling unit to thermostat

I'm trying to replace the existing old-school thermostat with a Nest, but there are only two wires between the air-handling unit (SpacePak ESP-3642D) and the thermostat (green and red, no common wire), and the Nest wouldn't power up.

First off, is it reasonable to believe the problem is the common wire, as my google searches indicate?


If so, I have some 18-gauge wire that I can easily get between the air-handling unit and the thermostat, but when I opened it up to see where to connect the common wire on the AC, I found no indication or labeling on the terminals other than numbers (see pictures).

Can anyone help me figure out where to connect the common wire on the air-handling unit?




 
  #2  
Old 06-11-20, 11:25 AM
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Welcome to the forums.

I think there is a three wire cable between that unit and the stat.
Look closely at the stat end.... my guess is you'll see a white wire cut off or folded back.

I'm trying to find the wiring code for your unit.
 

Last edited by PJmax; 06-11-20 at 11:50 AM.
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Old 06-11-20, 11:55 AM
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Thanks for responding. The cable to the thermostat only carries 2 wires, the red and green you see connected to terminals 6 and 7 of the air-handling unit. I also checked SpacePak's website for documentation, but couldn't find anything relevant. I wrote to their technical support 3 weeks ago and still haven't received any answer. These forums are really my only hope :-)
 
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Old 06-11-20, 11:58 AM
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Trying to come up with actual wiring.
That company has little to none helpful online information.
I'm still checking but ultimately you will need three wires from the stat to the unit.

1 - y ??
2 - x (blind)
3 - x (blind)
4 - x (blind)
5 - Common ? bridge
6 - green - blower
7 - red - 24vAC
 
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Old 06-11-20, 12:20 PM
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You're right - on closer inspection I do have 3 wires going to the thermostat. Don't know how I missed the white one at first (which I believe should be yellow, per your comment and the label on the thermostats). This photo shows how they're connected at the thermostat, and the white wire goes to terminal 2 of the air-handling unit.
The question is which terminal is the common wire. Is there any way to test that like measuring voltage or resistance across terminals?


 
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Old 06-11-20, 12:21 PM
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5 appears to be the C. Verify by following its blue wire along with the yellow wire on 7 (R) - should go to the transformer with the green label underneath.
 
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Old 06-11-20, 12:31 PM
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The blue wire does go into the transformer with the green label (photo below).
Thanks a lot!
So I should be able to run a single wire from terminal #5 to the thermostat's C terminal? If we're wrong about #5 being Common, what's the worst that can happen?


 
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Old 06-11-20, 12:39 PM
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I also found this manual has the wiring schematic in Figure 2.13 (page 12), though it doesn't say anything about a Common wire or terminals L3-L5. However, someone with more knowledge about wiring may be able to read the schematic better - it shows where the blue wire in R5 goes (transformer).

https://www.acdirect.com/media/catal...spd_series.pdf
 
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Old 06-11-20, 04:02 PM
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That site has issues with viruses.
Go with the terminal 5 as the C connection.
 
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Old 06-11-20, 06:32 PM
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Here is the drawing from the document. 5 is wired to the transformer so acts as a C connection.


 
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Old 06-12-20, 07:53 AM
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Great, thanks a lot guys! I'll use L5 for the C wire then.
 
 

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