Running Nest common wire from Weather King air handler.

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Old 08-09-19, 11:01 AM
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Running Nest common wire from Weather King air handler.

May someone please confirm location from where I can get the common wire for the Nest thermostat (it recently started shutting off due to insufficient power). The thermostat controls two separate heating and cooling units. I only have two wires going from the heating boiler in the basement but should be able to relatively easy run an additional wire from the air handler in an attic. I'm attaching model info and schematics from the handler and current Nest wiring diagram. Thanks, Chris

Larger diagram image on Dropbox here.

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Last edited by PJmax; 08-09-19 at 06:00 PM. Reason: resized pictures/labeled one
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Old 08-09-19, 01:15 PM
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Hi, from this drawing you will use the Brown wire as C on the Tstat, R from the AH will go to RH on the Tstat, I would run a new 18/5 from the AH.
Geo
 
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Old 08-09-19, 03:21 PM
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Geo - I think you have a typo. Normally the R from air handler goes to Rc on Nest.
 
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Old 08-09-19, 03:38 PM
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Thank you! That's what I thought.
 
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Old 08-09-19, 05:45 PM
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In your case you can use the brown wire off the transformer or any metal screw on the air handler. On that unit... the common is tied to ground/chassis.
 
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Old 08-10-19, 05:59 AM
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Astuff, you’re right.
Geo
 
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Old 08-11-19, 10:26 AM
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Thanks again! Got it to work! Chris
 
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Old 12-07-19, 09:21 AM
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Hello all! I have the same AH wiring and used the tip provided. I had a spare wire in the bundle and used it as C connected to the Br. I installed a new Sensi thermostat and it powered up and seems to be fine. HOWEVER, when activating for heat or AC, AC comes on for both and heat never comes on?!? Have I mis-wired or can I only use the C from the AH for AC control? Thanks!
 
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Old 12-07-19, 09:43 AM
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Update: I was incorrect that the heat does not come on. When I ask for heat, I get heat and AH running, no compressor. When I ask cool, I get AH and Compressor. Appreciate any help with this mystery!
 
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Old 12-07-19, 07:57 PM
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Welcome to the forums.

Is the mystery solved or do you still have a problem ?
 
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Old 12-08-19, 07:13 AM
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Thank you! The mystery is not solved. No matter what I ask for, hot or cold, the air handler comes on. I assume this is because I am obtaining my C wire from that unit. Not sure how I can resolve any other way short of a separate C wire from a different source.
 
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Old 12-08-19, 02:53 PM
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Configure the thermostat for gas heat ( equipment controls fan in heat mode) if you don’t want to run the indoor fan in heat mode.
 
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Old 12-08-19, 03:40 PM
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I will give it a try. Thanks!
 
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Old 12-08-19, 04:54 PM
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I'm not so sure about gas heat. The member is mentioning the condenser is not running in heat mode. That would indicate a heat pump.

We're going to need clarification of what type of system you have.
 
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Old 12-09-19, 11:32 AM
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I have baseboard hot water heat from a gas furnace. I will try the suggestion when I get home tonight or possibly tomorrow. Thanks again.
 
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