AC Compressor running all the time

[TQGregar]

I had a new HVAC system installed in Feb.  Prior to that the Nest was running the heating and AC for a few years or more on the old unit and another 6 months on the new unit.

Came home to find that the AC compressor unit running all day with no fan.  Shut off HVAC unit, compressor still ran.  Flipping breaker only way to shut off breaker.  

HVAC blamed the Nest, saying they had seen this problem before.  Replaced with new thermostat (Honeywell) and problem went away.  What happened to cause this with the Nest?

[humbertogarcia]

Hi TQGregar,

Thank you for posting in the community! I understand you had a stressful experience finding your HVAC compressor running all day without a fan. It was smart to immediately flip the breaker, as that was the only way to shut the unit off. I also understand that you have since switched your thermostat to a different brand. I'll be glad to investigate to respond to your question.

Based on your description, the cause of the problem was almost certainly an HVAC system hardware failure, not an issue with the Nest Thermostat.

Please consider that the Nest Thermostat's job is to send a low-voltage signal to your HVAC system's control board to tell it to turn on or off. When you shut off the HVAC unit from the Nest Thermostat, the "OFF" signal was correctly sent. However, if the compressor continued to run, this is an indicator that a component on your HVAC system's control board was stuck in the "ON" position. This can be a critical hardware failure within the HVAC unit itself, and the steps you took were appropriate.

To confirm our statement above and rule out any Nest-related factors, please provide the following details:

• What was your Nest Thermostat's display showing after you had turned the system to "OFF"? Was it displaying any error codes or unusual messages?

• Confirm if your thermostat was a Nest Learning Thermostat 2nd gen?

I look forward to your answer. Let's work together to find an explanation for the issue.

Regards,

Humberto

[TQGregar]

Could a software update have caused this?

[TQGregar]

I confirmed that the nest was Gen 3.  There were zero errors anywhere from the thermostat when this error was occurring.  I have not had any issues since a low end thermostat was installed.  

[humbertogarcia]

Hi TQGregar,

Thank you for the update. It's good to hear that your AC compressor issue was resolved, but I'm sorry you had to replace your Nest Learning Thermostat 3rd gen.

The problem could have been caused by several factors. To find the root of the problem, we can perform a few steps with your thermostat installed, if you're willing.

If you have any other questions or issues, don't hesitate to contact me. I'll be glad to assist you.

Regards,

Humberto