Nest A/C woes

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  #1  
Old 05-18-19, 04:58 PM
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Nest A/C woes

I have split level HVAC systems, upstairs and downstairs unit. Downstairs consists of a single HVAC unit that is outside, and the upstairs unit has a furnace in the attic, and a unit outside as well.

The heat works fine with my nest tstats, however, the air works with neither. While set to "Cool", the air will blow but its essentially only the fan. In my opinion, the compressor is not kicking on. The HVAC units are new, as are the nests, so the units do not need charging. I confirmed this by hooking up two old thermostats that I had laying around, the AC worked fine with these.

I've attached pictures of the wiring at one of the tstats (they're both wired the same), as well as the the "upstairs" unit outside and in the furnace. The C "Common" wire was not used in the installation, could this be the reason why I am not getting air? There is an intermittent issue where, while the AC is on, the compressor will attempt to kick on repetitively, with no success.

I called the installer back, twice, to look at the wiring and he saw no issues with the wiring or setup, and blamed the issues on the nests. What steps can I take to make AC work with my nests? I am comfortable doing any modifications to the wiring myself, but would greatly appreciate anyone's thoughts or opinion. Surely I am not the only one who has had this specific issue.

Downstairs Unit
- Byant 577ENWB42090AATP (outside unit, installed last year)
- Nest learning tstat Gen 3

Upstairs Units
- Lennox ML180UH (furnace in attic, installed last year)
- Rheem RAAC-024JAS (outside unit, original, 1991)
- Nest learning tstat Gen 3

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Last edited by PJmax; 05-18-19 at 09:53 PM. Reason: cropped/resized/labeled pics
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  #2  
Old 05-18-19, 09:58 PM
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Make sure you program the nest for standard gas furnace with A/C.
I don't see any issues in the pictures but I highly recommend using a C connection.

You have the available wire. At the nest...... connect the blue wire to C.
At the furnace.... connect the blue wire to the C terminal along with white from the condenser.

As a pro installer..... I will not install without a C wire. Power stealing thermostats are historically a problem regardless of the manufacturers claims.

Also..... when creating a call for cooling.... the compressor startup is delayed five minutes to stop short cycling.
 
  #3  
Old 05-19-19, 09:46 AM
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At the furnace, I connected the blue C wire from the thermostat to the C terminal, with the white wire.

At the tstat, I connected the blue wire to the C terminal.

Then, I received the error in the image attached. I tried to switch the red wire from RC to RH, and received a similar error message. I double checked that power was back on.

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Last edited by PJmax; 05-19-19 at 11:16 AM. Reason: resized pictures
  #4  
Old 05-19-19, 10:03 AM
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You used a meter to check for 24vAC at the R and C wires at the nest ?
 
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Old 05-19-19, 11:06 AM
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Correct, there is no voltage between R and C.
 
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Old 05-19-19, 11:12 AM
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You probably touched the C wire to R. Check for a blown fuse on the furnace board.
It should be a 3A-5A plastic fuse.

Remember..... the system will be dead with blower door open.
There is a safety switch on the door.
 
  #7  
Old 05-19-19, 11:14 AM
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The 3amp fuse on the motherboard of the furnace checks out to be good, has continuity.
 
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Old 05-19-19, 12:30 PM
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Hi, are you getting 24VAC across R & C at the control board?
Geo
 
  #9  
Old 05-19-19, 06:10 PM
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I had left the blower door off! I now have 24vAC between R and C. The nest seems to be accepting the common wire for the first time. I've had the air on for a few hours, and it is cooling and cycling on and off normally.

Now if I can repeat the process for the downstairs unit...

This will be my project for tomorrow.
 
  #10  
Old 05-19-19, 08:50 PM
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Good job so far.
 
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