No ac

[Scubasquad6]

Had new furnace and central air installed today and was told by installer that something was wrong with nest learning thermostat as it would not turn on ac. He installed non smart thermostat and everything works fine. Nest worked fine with old unit and new unit is compatible. I would really like to have the smart thermostat back up. Any ideas?

[CoolingWizard]

OK, this happens when the HVAC system type changes.  Your nest smart thermostat can indeed control your system unless it uses a proprietary communications protocol. Let’s work together to figure this out.  Step one is to snap the thermostat off the backplate and take a picture of the wires being used and what terminals they are connected to.  
From there we can proceed.  What was the type of your old system?

- AC Cooling with Electric Heat

- AC Cooling with Gas Heat

- Heat Pump

And is your system a split system, ie., outdoor system and indoor air handler or,

a packaged unit where all is outside and ducts connect it to indoors. 

Ken, the AC Cooling Wizard 

[Scubasquad6]

Old system was the same as new system, ac cooling and gas heat. Forced air furnace in the basement and central air unit outside. 

[CoolingWizard]

Then there is no reason the Nest will not work unless the HVAC tech intentionally changed the wires at the  air handler.  Do this, snap the new thermostat off the back plate, note which color wire are going to which terminal screws on the backplate. Sometimes techs do this to sell you a thermostat and make more money. If the new thermostat can initiate cooling, the Nest will indeed do it too. 

The terminals in use should be Rh/Rc, Y, W, G and C.  

Ken, The AC Cooling Wizard  

[Scubasquad6]

Yellow to y

green to g

blue to c

gray to w

red to r

[Scubasquad6]

[CoolingWizard]

This is a normal, setup.  So, the wire terminals are as follows.  R (red wire) is the incoming AC power, the C (blue wire) is the COMMON wire to the R.  The W (gray) is for calling for heat. The G (green) is for the Blower Fan, the Y (yellow) is for calling for cooling.  That blue thing at the bottom connects Rc and Rh together. 

Your nest will be set up as AC cooling and Gas Heating. Connect the colored wires as you have noted them to the same lettered terminals on the nest back plate. Connect the Red wire to Rc, the Green Wire to G, the Yellow Wire to Y1, the White Wire to W1, the blue wire to C.  

Ken, The AC Cooling Wizard

[Scubasquad6]

I will try that setup tomorrow evening and get back to you. Thank you for all your time and help!

[CoolingWizard]

[Scubasquad6]

Sorry for the delay. Hooked up according to your picture and still won’t turn ac on. Thermostat says it’s on and gives a time to reach set temp but unit doesn’t turn on. Is it possible thermostat is broken? Seems like that would be a strange coincidence but I guess possible. Thanks again for all the help

[Ryan_G]

Hey folks,

I just wanted to follow up and see if you're still in need of help. Please let us know as we would be happy to answer any questions you may have. 

Best regards,
Ryan
 

[CoolingWizard]

Hmm, this is indeed very puzzling.  I cannot imaging why it is not connecting the Rc to the Y1 to initiate Cooling.  This next statement is a crude way to check the system.
1. turn the power off,

2. then you remove the the Rc (red) wire, and the Y1 (yellow) wire,

3. twist them together,

4. Turn the power back on and see if the AC system turns on and starts cooling.  

Ken, The AC Cooling Wizard

[Ryan_G]

Hey Scubasquad6,

Thanks for being here in the Google Nest Community.

I'd like to jump in here and check if you've seen CoolingWizard's response with suggestions. In addition, before touching any wire, make sure to turn off the HVAC fuse to be safe.

Let us know how it goes by replying to this thread.

Regards,

Ryan

[Jeran]

Hey Scubasquad6,

Did you have any more questions or need any additional help? If not, I'll go ahead and lock up this thread in 24 hours.

Just checking up,
Jeran