Nest thermostat turns on heat and AC at same time

[TXJIUJITSU]

Today I upgraded my existing nest thermostats to third generation nest thermostats, which replaced first and second generation nest thermostats that functioned flawlessly for years. One of the thermostats would run continuously and never quite cool off the zone it covers. Just on a whim I opened the HVAC closet for that zone and I discovered the furnace was running at the same time as the air conditioning. I got on the phone with customer support and created a workaround with the wiring that solved the issue, however, the thermostat is no longer being charged, so I have lost network connection etc.

The house was so hot I did not want to disconnect the “solution “wiring and just keep things cool until I could work on it again. I was also thrilled that my AC was not running continuously while I was gone. I would like to figure out the wiring issue and I’m not the kind of person who calls a technician, but rather does everything myself. I figured this was the best place to go for help. 

So my second floor new thermostat functions perfectly, wired it up, and it worked right off the bat zero issues. Functions perfectly with the app, etc. Previously a first generation thermostat ran that hvac unit perfectly as well. HVAC unit is two years old. Downstairs unit which has the wiring issue is 23 years old and I have never ever once had a problem with it, and previously it was controlled by a second generation thermostat which worked flawlessly for years.

My best guess is I screwed up the initial setup on thermostat. @CoolingWizard i saw a thread where you helped troubleshoot wiring and wanted to reach out regarding this issue. Any thoughts? While speaking to a Nest customer service representative about the thermostat we went through hardware testing, and I was told there is no issue with the base plate or the thermostat itself.

Thanks in advance! 

[TXJIUJITSU]

Closer inspection revealed a broken white wire hence no charging…..fixed that and now we are back online and connected to network. Still uncertain as to finalizing  workaround wiring though…..  

[CoolingWizard]

@TXJIUJITSU , I am sorry you are having problems. By chance did you save any pictures of the old wiring versus the new wiring? I am currently driving on vacation p; driving to Washington.   

AC Cooling Wizard 

[TXJIUJITSU]

Unfortunately no, but I can show you the new…….I will also show the upstairs wiring as I believe I copied it. at a minimum you can see the work around wiring that google instructed me to use. Not sure why I never used the blue downstairs…….

UpstairsDownstairs workaround

[CoolingWizard]

@TXJIUJITSU ,

you assigned Green Wire to the Y1 and Y1 calls for Air Conditioning.  You put Yellow Wire in G and G call for the indoor blower fan.  Typically Y1 is Yellow, and G is green.  However, we need to verify by going to the indoor furnace/air handler and see which terminal yellow is attached to and which green is attached to. We also need to ensure that Blue is attached to the common side of the transformer. 

AC Cooling Wizard

[TXJIUJITSU]

Ok so do I look inside service panel on hvac unit / furnace etc? We have this……….prior to chatting with Google nest thermostat customer service I did have yellow green wires assigned as you mention. And furnace was coming on with the air conditioning. Blue power wire was not installed and I think that could be the problem. I will move Y an G back, and add the Blue common, and see what we get…..

[TXJIUJITSU]

Unit is a Rheem Criterion II. I’ve returned to y and g wires to the base as you said and at least the furnace does not come on at the same time as the air conditioning but nothing is charging the internal battery to thermostat. See photos below, it appears blue is, indeed, not connected to circuit board. Moving red from Rh to Rc seems to do nothing. Please do pardon my earlier attempts at wiring photos as I now am oriented to the air handler and furnace panel covers as below. Things are running smoothly but low battery warning present and no Wi-Fi connection. 3.638v currently but it fluctuates a bit. I do have 27v across C->R. 

[CoolingWizard]

@TXJIUJITSU , the white wire on the Furnace Control is going outside. The red wire attached to Y is also going outside.  So, all you need to do is connect the blue wire of the other cable to the C along with the white wire that goes outside. Keep the blue wire at the thermostat connected to C. 

AC Cooling Wizard

[TXJIUJITSU]

@CoolingWizardcurrent set up that works is below, seems to take a while to cool but it’s really hot in TX right now…..I will add the blue wire at the board C and at the display C next…..see these connections without C. Adding C now…..

[TXJIUJITSU]

New C wire added……working still…….

[CoolingWizard]

@TXJIUJITSU , to get an idea of your system cooling performance, the two copper pipes feeding the indoor unit, the small one should be very hot and the larger one in foam should be cool.  Using the blue wire attached to C and both ends will improve the charging of the internal battery  

AC Cooling Wizard

[Sdotnance1]

@CoolingWizard I have seen where you helped a few others on here to convert their 3 wire Honeywell VisionPro with EIM to a nest and while I have a similar setup to another thread is not exactly the same.  I have a heat pump with an auxiliary electric heat strip.  And I can provide you detailed photos of the wiring going to the EIM.  All I would need would be a chart detailing the connections that need to be made to bypass the EIM and connections to the nest learning thermostat. Any help would be greatly appreciated.
https://www.googlenestcommunity.com/t5/Nest-Thermostats/VisionPRO-IAQ-to-Nest-Learning-Thermostat/td...