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Unable to turn on air conditioning/compressor

jb76
Community Member

Hi - I recently replaced the thermostat control wire harness with my newly-replaced furnace. My furnace technician confirmed all wiring was correct and I now have power coming to my 3rd. generation Nest Learning Thermostat via the Common wire connection to the thermostat. However, when I tried to set the cool temperature to anything below the current room temperature (71 degrees), only the the furnace blower fan turns on (I confirmed there was air coming out of the ducts).

I then went to the Nest -->Equipment --> Test menu to confirm operation of the Heat, Fan, and Cool operations and both the Heat and Fan tests were successful. When I tried to test the Cool operation, I got a message that the Nest thermostat cannot test the cool/AC unless the outside temperature is 65 degrees or above. Can you confirm this is the case (note: outside temperature was 60 degrees at the time of the testing)?

If this is the case (AC/Compressor) is not allowed to run unless the outside temperature is 65 degrees (18 C) or above, what steps can be taken to ensure that the AC/Compressor will turn on (rather than waiting until it is the summer)? Are there configuration parameters that can be turned off/on to test this function and or a hardware/wiring change that could be used?

Thank you.

J

7 REPLIES 7

zoeuvre
Community Specialist
Community Specialist

H jb76, 

 

Uh-oh! The behavior you described where only the furnace blower fan turns on and the cooling (AC/Compressor) does not engage when you set the cool temperature on your Nest Thermostat below the current room temperature can occur due to several reasons. 

 

Some HVAC systems have an "outdoor temperature lockout" feature that prevents the cooling system from running when the outside temperature is below a certain threshold. This feature protects the compressor from potential damage in colder temperatures. To ensure the cooling system operates properly, check the thermostat settings on your Nest Thermostat and ensure no specific temperature lockout or setback settings are enabled. Verify the wiring between your HVAC system and the Nest Thermostat, ensuring the cooling (Y) wire is properly connected at both ends. Double-check the configuration settings in the Nest app or on the thermostat itself. It's possible that there may be compatibility issues between your HVAC system and the Nest Thermostat, and some HVAC systems may require additional wiring or configuration changes. 

 

The Nest Thermostat may not test cooling operation unless the outside temperature is 65 degrees or above, but it may have internal temperature thresholds for diagnostic tests. The temperature threshold mentioned in the message may be related to the test function, not the actual functionality of the cooling system. With that, you can set the thermostat to cool mode by pressing the thermostat ring to open the Quick View menu, then Turn the ring to the Mode icon and press to select. Turn the ring to choose a mode, then press to activate it, then press again to confirm.

 

Let us know if that helps.

 

Best,

Zoe

jb76
Community Member

Hi - Setting the thermostat to ‘Cool’ mode did not turn on the AC compressor following the steps you provided and instead the furnace fan (unconditioned air) started to come through the vents. It is cool outside - averaging less than 65 degrees so I tried a workaround to ensure my AC works when it is warmer.

To test AC (conditioned air) I turned off power to the furnace/AC, removed the Neet thermostat, and connected the Rh and Yellow (AC control) wires together. I then turned the breaker back on and almost instantly the outside AC compressor turned on and cool air was coming through vents. 

 

This confirms the wiring is correct to the furnace/AC and the AC can be engaged without the Nest thermostat attached to its baseplate. Not sure short of trying to reset the Nest to factory defaults  to remove any “lockout” settings could help but at least this test confirms my AC will work in warmer temperatures.

zoeuvre
Community Specialist
Community Specialist

Hi there, 

 

I'm sorry for the confusion I caused earlier. Based on the information you provided, it seems that the issue lies specifically with the Nest Learning Thermostat's cooling functionality. Since the wiring and HVAC system appear to be functioning correctly when bypassing the Nest thermostat, it suggests that there may be an issue with the thermostat itself.
 

Here are a few additional steps you can try to troubleshoot the cooling problem with your Nest Learning Thermostat. You may skip the steps you already took.

  1. Ensure your HVAC system and wiring are compatible with the Nest Learning Thermostat. What is the make and model of your HVAC system?
  2. Reset the Nest Learning Thermostat to its factory defaults. This can resolve potential configuration issues. Go to Settings > Reset > All Settings.
  3. Ensure your Nest Learning Thermostat has the latest firmware installed to address known issues and enhance performance. Check for updates via the Nest app or thermostat settings menu.

 

Let us know how it goes.

 

Thanks,

Zoe

jb76
Community Member

Hi - My HVaC is a Carrier SB820 forced air furnace and Carrier AC. I have not been able to configure my Equipment to setup ‘Cooling’ in the Settings menu due to the limitation that Nest will not try to turn on the AC if the outside temp is 60 F or lower (which it has been as it is winter in North America). I also checked in the Nest app if there was a firmware update (Settings menu) and there was no update- my 3rd gen Nest firmware is version is 6.2-27 (latest?).

Also, can you confirm if under Settings of the Nest app. when opening ‘Equipment’ I should see both ‘Heat Source/Type’ and Cooling Source? Currently, I only see ‘Heat Source’ which I suspect is because the Nest refuses to confirm/control the AC (Y1) unless the outside temperature is above 60 degrees F. I also tried to configure the AC in Settings —>Equipment but was unable to set/test the AC equipment. I can set the Cool mode to a lower temp. but all that turns on is the furnace blower (unconditioned air) and Nest thermostat screen turns blue. I also reset all settings in the Nest (under Settings) and I am still unable to select the AC equipment during setup.

Thanks.

 

J

zoeuvre
Community Specialist
Community Specialist

Hello jb76

 

Based on the information you provided, it seems that the limitation you're experiencing with configuring cooling settings on your Nest thermostat is due to the outside temperature being 60°F or lower. This limitation is in place to prevent the AC from turning on when it's not appropriate, such as during the winter season.

 

Regarding the settings in the Nest app, under "Equipment," you should typically see options for both "Heat Source/Type" and "Cooling Source." However, if you're unable to see the "Cooling Source" option or configure the AC equipment, it could be due to the current temperature limitation or a compatibility issue with your specific HVAC system. Follow these: 

 

  1. Check your thermostat's wiring.
    • Turn off the power to your HVAC system at the breaker and remove your thermostat display. Your HVAC system might have multiple breakers so make sure to turn them all off before moving on.
  2. Check the air filter.
    • It’s important for your system to have good airflow to run properly. A clogged air filter in the summer can limit how much air flows over your cooling coils. If this persists your cooling coils can freeze solid and cause your system to shut off.
  3. Check your drip pan or drain tubes.
    • Air conditioners (AC) and heat pumps work to cool the air in your home. Like a glass of ice water, water droplets condense on the cold coils of your AC or heat pump and drip off. Usually this water drips off the coils into a drip pan and flows down a tube outside your home or down a drain. If your drip pan or drain tube becomes clogged and water starts to back up, your AC or heat pump may shut itself off to prevent an overflow of water. When this happens your system stops sending power to your Nest Thermostat.
  4. Check the HVAC fuse. 

 

Let us know how it goes.

 

Regards,

Zoe

jb76
Community Member

Hi - Sorry for the late reply - I went through the last 4 steps you provided and did not find any issues - no issues with the HVAC fuse, drip pan, air filter (replaced 1 month ago), nor the Nest wiring. I ran a test again today to see if the AC Compressor would come on - still below 60 F here and observed the same behavior as before - AC Compressor would NOT turn on and only the blower would come on. I also confirmed the Rh and Y1 wiring "workaround" still works (AC compressor turns on and am getting cold air through vents). I also did another complete reset of the Nest Thermostat and am still not seeing the Cooling system details in the Nest App. (under Settings) - only my Heat system details. I also tried to 'Test' function the Cooling system in the Nest system menu - but I got a warning that Nest will not test the cooling system if the outside temperature is below 60 degrees F (currently 58 F).

Any other ideas to try or is my Nest thermostat malfunctioning/"bad"?

J

jb76
Community Member

Just an update on this and possibly a warning for other Nest Thermostat owners: my Nest thermostat stopped controlling my AC not because of any fault with my HVAC equipment but the Nest Thermostat baseplate malfunctioning. This is the not the first time this issue has occurred with other Nest owners and there is an aftermarket for working Nest baseplates for owners that would otherwise (like me) be forced (warranty is 1 year folks)  to purchase a new Nest Thermostat (baseplate is not replaceable).

 

Multiple hours of troubleshooting and visits from HVAC pros.($$$) to tell me there is no problem with my HVAC....Google Support's response - "sorry, but would you like to do additional troubleshooting?"

 

For reference about this being a more common issue than Google Nest Support acknowledges or is aware of (so they could tell their customers to stop spending money for HVAC techs. to come out) - here:

https://www.googlenestcommunity.com/t5/Nest-Thermostats/Failures-in-Nest-3rd-Base-Plates/m-p/92178

 

J