Nest Thermostat is sending 18 volts from the O terminal when switching from heat to AC.

[keithf01]

Is 18 volts enought for the air handler to recognize. It used to work fine but on a hot day in the winter I tried to run the AC but it only blew heat. I called my HVAC company and they told me the thermostat was not sending a signal. After doing some research I tested it myself and see it is sending 18 volts but not 20 or 24. So my question is what voltages should it be sendig for the reverse signal, is 18 enough or is the unit shot?

[keithf01]

Looks to be the Nest thermostat went bad. I installed a cheap thermostat for testing and was getting 28 volts in AC mode. Wired it back up at the air handler, put it in AC mode and started getting cool air. These Nests are a bit expensive to be breaking down for this. I've seen multiple YouTube reels on this issue. Not sure if I want to put out the money for another Nest Thermostat or not after going through this.

[Jake]

Hey there,

I wanted to follow up and see if I could be of any help. Please let me know if you are still having any trouble from here, as I would be happy to take a closer look and assist you further. 

Best regards,
Jake

[keithf01 · Accepted solution]

Thanks for replying. After further troubleshooting,  purchased a cheap manual thermostat  to verify the O voltage and talking with another HVAC person who said it has to be a minimum of 20 volts, I was able to determine the Nest was defective as only sending 18 volts.  The manual thermostat sent 24 volts and the AC kicked on. Later purchased a Honeywell wifi thermostat that also worked.

[Jenelyn_O]

Hi there,

Thanks for the update and for being a step ahead of us. May I know when you purchased your Nest Thermostat? Please be advised that the Nest Learning Thermostat has a two year warranty, and the Nest Thermostat E and Nest Thermostat 2020 have one year warranty. If your thermostat is still in its warranty range, please let me know so we can check for any options. 

Best,

Jenelyn

[Jenelyn_O]

Hi keithf01,

I wanted to follow up if you still need help. Please let me know if you’re still having any concerns or questions from here, as I would be happy to take a closer look and assist you further.

Thanks,

Jenelyn

[Jenelyn_O]

Hello there,

We haven't heard from you in a while so we'll be locking this thread if there is no update within 24 hours. If you need assistance or want to discuss topics, feel free to start a new thread.

Regards,

Jenelyn