Nest thermostat not providing 24 volts to Y wire

[ereilly220]

I am getting 25v from the transformer to the base plate, however when the thermostat calls for AC im only getting 18-20v to the Y wire which is not enough to turn on the condenser. I had contacted google about it last year and they told me it was the base plate so I swapped that out with no luck. If I hook up my old honeywell thermostat the ac works fine and is getting 25v to the Y wire. Anyone thats had this issue ever figure it out? Google now saying its out of warranty so its troubleshoot or throw it in the trash. 

[CoolingWizard]

@ereilly220 , you are having  voltage sharing problem in all likelihood.  Do you have a C wire attached to your thermostat? If not, you are having a power sharing problem.  This can be corrected by adding a 5-watt, 220 Ohm resistor between Y and C in the air handler.  

AC Cooling Wizard

[ereilly220]

So you would just add the resistor between the 2? What is causing this to happen? 

[CoolingWizard]

The Nest uses the Y wire to charge the battery when there is no C wire. On some systems this creates a power sharing bridge that is not compatible. The resistor drains the power bridge to allow the full current is available for the compressor contractor.  The resistor is added between the Y terminal and the the Common terminal.  Your other choice is to add a C wire or a Nest Power Adapter. 

AC Cooling Wizard

[ereilly220]

Interesting, I will order one now and let you know if that fixes it. Thanks for the help here I haven't seen anyone mention that and I have been researching this forever. 

[ereilly220]

wait I think I misunderstood this, I have added the c wire, my ac unit doesn't have a board on it, the C wires coming off 1 side of the transformer. That was added because the nest wouldn't charge correctly without it. 

[CoolingWizard]

Well, in that case this is not power sharing issue since you have a C wire. 

AC Cooling Wizard 

[zoeuvre]

Hi folks,

@ereilly220, thanks for reaching out to us, and I'm sorry to hear about the situation. I want to check if you managed to see the response above. Please let us know if you still have questions or concerns, as we'll be willing to assist you further. 
 

I appreciate your help, @CoolingWizard. 

Best,

Zoe

[ereilly220]

Of course I have questions this isn’t resolved 

[ereilly220]

If anyone is having this issue and cant figure it out, do yourself a favor and go return it to whatever store you got it asap. Otherwise google will string you along till its too late and will not replace it for you without paying for a nest pro installer to come install it first. 

[CoolingWizard]

@ereilly220 ,

when you measured the the voltage and current output from transformer itself, when the thermostat calls for cooling, what were the actual readings? Does your system have a humidifier or dehumidifier installed on it? You cannot measure the output of the backplate since the display must be attached to activate the cooling call.

Your HVAC system requires a 40VA class 2 transformer and if it was replaced with a smaller or inferior model, or if the total current draw exceeds 40VA, then the problem is not the Nest Thermostat. 

AC Cooling Wizard

[ereilly220]

No humidifier or dehumidifier on this unit. I took the reading from the transformer which is reading 26.7v, at the plate its the same. When I hook the thermostat back up and call for ac, the reading at the inside unit going to the condenser is where I see it jumping all around from 18v-27v, if you go outside to the unit its the same reading and you can hear it kick on for a split second here and there. 

fyi I appreciate your help so far, my frustration is with google support as Ive contacted them multiple times and they refuse to replace the thermostat

[CoolingWizard]

You should run a test with the wires.  Go to the wires, and connect R, Y and G together.  Then go to air handler, turn power one and make voltage measurements again.  

AC Cooling Wizard

[ereilly220]

I can test that tomorrow but what would that show? The unit works great with regular honeywell thermostat

[CoolingWizard]

Trying to help you in determining if this is a wiring problem, a nest backplate problem, or a power supply problem.  By connecting the R,G,Y wires together, we eliminate the backplate. If the problem goes away, it is the backplate.  If the problem continues, it is likely in the wiring somewhere.  

AC Cooling Wizard

[ereilly220]

The thing is the ac works perfectly normal with the honeywell which is what ive been using for the ac, last summer when I contacted google they told me it was the backplate and I had to pay for a new one then return the old one for a refund. That didnt work same issues occured. I have 2 units in my house they are the exact same indoor and outdoor units, the upstairs unit works perfectly fine with the nest E. Downstairs gives me an issue with the nest learning.

Also, when the honeywell thermostat is calling for ac, there is no voltage fluctuation on the Y wire to the outdoor unit.

[CoolingWizard]

You mention no fluctuation on Y for heat. You have a heat pump type of a system? 

AC Cooling Wizard

[ereilly220]

Sorry that was a mistake I meant to say calling for ac, i will edit that message so its clear for anyone searching.