Getting a common wire from air-handling unit to thermostat
#1
06-11-20, 11:16 AM
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?
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
06-11-20, 11:25 AM
Group Moderator
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.
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.
#3
06-11-20, 11:55 AM
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 :-)
#4
06-11-20, 11:58 AM
Group Moderator
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
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
#5
06-11-20, 12:20 PM
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?
The question is which terminal is the common wire. Is there any way to test that like measuring voltage or resistance across terminals?
#7
06-11-20, 12:31 PM
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?
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?
#8
06-11-20, 12:39 PM
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
https://www.acdirect.com/media/catal...spd_series.pdf
#9
06-11-20, 04:02 PM
Group Moderator
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Info off the furnace
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wiring at the main board inside the furnace
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The main board itself
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This crazy setup from the power company. I don't know if this has anything to do with it
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A brown cable goes from the board and is tied into this
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[i]A hot day out side temp 43 C[/i]
[i]A hot day out side temp 43 C[/i]
[i]A rainy evening out side temp 30 C[/i]
