Control window AC unit with Nest thermostat?

Old 06-09-16, 08:19 PM
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Control window AC unit with Nest thermostat?

Hi everyone. I currently have central heating in my condo and no air conditioning. I have a basic thermostat with a Rh and a W wire connected to terminals. Rc and Y terminals are free, as well as G, C, O, and B. I was going to buy a basic 115V window AC without a built in thermostat and wanted the thermostat to control the window AC. From what I've read (I have no knowledge of electrical engineering, HVACs, or anything of the sort prior to investigating this), it seems like this is possible by using a relay switch. Would I just run a wire from Y to a relay? How do I set this up?! Which relay should I buy? If I switched my thermostat to a Nest would it complicate matters? Please use layman terms if possible! Any help is much appreciated
Old 06-09-16, 09:15 PM
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Technically possible but a challenging for someone not familiar. You would need a transformer as well to supply 24 volts AC connecting to the Nest's Rc, Y, and C. Then you would need to mount all of this in a box with a cord and receptacle. - with diagram at

Cheaper version with better relay
Old 06-09-16, 10:17 PM
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Welcome to the forums.

If you were real handy and mechanically inclined you could go into the A/C and get compressor control and fan. That would be beyond the scope of most DIY work, a little dangerous and would void the A/C warranty.

I'd recommend buying an A/C with a mechanical thermostat. You'd leave the thermostat turned all the way up and your remote thermostat would control the 120vac to the entire unit. It could also be done with a 240vac A/C.

You could use a RC840T-120 combo relay from Honeywell to power the thermostat and switch the A/C. This setup would allow for the A/C and your existing heating to both work from a nest.

The link is to the relay at amazon. You can shop around for a better price if you want.

You would mount the relay in a 4" square electrical box, use a 4" box receptacle plate and a short HD power cord. Fairly easy to build. Pretty much under $50. Not the prettiest looking but effective.

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Old 06-10-16, 09:27 AM
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Thank you all for your input. A few questions in response.

1. Why would your recommend a window AC with an internal thermostat? Wouldn't an AC with just a dial and an on/off switch left in the on position do the same thing?

2. Do I need a transformer or not?

3. I attached an image of my set up. If somebody is willing, could you connect the wires of how to rig this thing and repost the image? Do I need to build the second receptacle as PJmax mentioned or could I just use the outlet right below it?

Any input is MUCH appreciated!

PS. Ignore the 1st image I posted. I accidently left off the Y terminal on the Nest.

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Last edited by eeisen88; 06-10-16 at 09:56 AM.

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