Heat pump fan runs when aux heat is on

[JAD-16]

Gen 3 thermostat is set up for Dual Fuel with heat pump as the primary and gas radiator as the aux heat.  The aux heat turns on when temp falls below temp set, but the heat pump fan also runs blowing cold air (compressor is not running which is correct).   Everything else works as it should.  The installer is stating they will need to add a relay for the fan.  Is this correct or is it a config/programming issue?

[luciaaa]

Hi, @JAD-16!

Thank you so much for posting in the community. I apologize for the heat pump fan running when the auxiliary heat is on. I’m happy to help you further.

To effectively troubleshoot, I’d like to check the configuration and installation. To do this, please provide the following information:

• A photo of your old wiring configuration: This will help me visualize the wire connections.
• If possible, a photo of your control board: It’s usually located near the furnace. This will help me compare the installation with the thermostat.
• A photo of the current wiring configuration on your Nest Thermostat.
• A photo of the thermostat settings on the Nest app: Please avoid sharing the serial number information.
• On the Nest thermostat, go to Settings > Equipment > Continue > Continue > Pro Setup. Select the dual fuel system option and take a photo of the wiring information. Share this photo. This will help verify if any wire is not recognized. After taking the photo, select done to return to the settings menu.

Also, please let me know when you installed the Nest Learning Thermostat 3rd gen. 

Please keep me updated.

Best regards,

Ana

[JAD-16]

Here are pictures of the thermostat wiring and config as well as the equipment settings in the app.  The thermostat was already in place to run what is now the alternate heat.  The Heat pump that is now the  primary heat is new.  It is sealed and I cannot get to the control board. The thermostat was installed over 4 years ago. 

[JAD-16]

Sorry - forgot one picture.

[luciaaa]

Hi, @JAD-16!

Thank you so much for your response and for sharing all this detailed information, it helps me to identify the best next troubleshooting steps. I’m happy to assist you further.

Since the heat pump is new, please let me know when it was installed. Also, make sure the orientation for the O/B wire is set to "O”.

To provide you with the best assistance, please answer the following questions: 

• Is the thermostat displaying any error messages?
• Are there intervals or specific patterns to the strange behavior?
• When was the HVAC system last checked for maintenance?
• Does the issue happen only on heating, cooling, or both?
• Is this the first time you're using the heat since the season changed?
• Have you experienced very low temperatures recently?

In the meantime, try these troubleshooting steps: 

• Check Thermostat Energy History.  to see if any features are causing the fan to run while the heat auxiliary is turned on.
• Check Google Home app routines to ensure none are interfering with the thermostat. Check this article for instructions: Change Home & Away Routines settings.
• Ensure wires are fully inserted: The wires may not be making a proper connection with the terminals. Make sure at least ¼ to ½ inch of the copper wire is exposed, straightened, and fully inserted into each of the required connectors. If necessary, strip and reinsert the wires.
• Factory Reset:
• Perform a factory reset and set up the thermostat again.
• During the setup process, specify whether the fan should turn on in heating or cooling mode to ensure it functions correctly. This will help us determine if the inconvenience is related to the set up process or with the heat pump installation.

Please let me know how it goes.

Best regards,

Ana 

[JAD-16]

Below are the answers to your questions:

• Is the thermostat displaying any error messages? No
• Are there intervals or specific patterns to the strange behavior? No
• When was the HVAC system last checked for maintenance?  Last week
• Does the issue happen only on heating, cooling, or both? Heating - there is no dual fuel for cooling.
• Is this the first time you're using the heat since the season changed? No but the first time the temp has dropped below 32
• Have you experienced very low temperatures recently? Yes - below 32 as mentioned above.

Please reread my original post for the situation specifics and my question.  Thanks

[luciaaa]

Hi, @JAD-16!

Thank you so much for your prompt response. I really appreciate you answering the questions I asked.

After carefully reviewing the wiring installation, Nest app configuration, and all the information you provided, it seems everything is fine. As the technician suggested, you may need to install a relay for the fan. A relay is an electrically operated switch that controls when a fan turns on and off. It receives a signal from a control unit and activates the fan motor without overloading the control circuit. This is commonly used in applications like radiator fans that are activated by a temperature sensor when the engine overheats.

Please feel free to reply if you have any questions.

Best regards, 

Ana