Random heating while on cooling

[PhillipCrosby]

I have my Nest set to cooling mode only as its the summer. Also have a fan only schedule set.  However the heating (NG furnace) comes on randomly.   I have had to pull the white heating wire out of the thermostat to get this to stop from happening.  I have checked all the wiring making sure nothing is faulty or shorted.   I never had this issue with previous thermostat.

[gmelanymelissa]

Hi @PhillipCrosby,

Thanks for posting here in the community. I'm sorry to hear your Nest thermostat randomly switches to heating mode when it's set to cooling. I'm happy to help!

To better understand the situation, I'd like to ask a few questions:

• What kind of Nest Thermostat do you have?
• After removing the white wiring, Is the heater still running?
• Is this a new installation
• Do you have a heat pump system or a conventional system?
• When was the last HVAC maintenance?
• Could you share a picture of the wiring configuration?

Keep me posted. 

Best regards,

Melany

[PhillipCrosby]

To clarify, its set to cooling mode only and doesn't change to heating mode, just turns the heat on randomly. I'll feel hot air out the vents and see a flame lit in the furnace. 

• What kind of Nest Thermostat do you have?
  • Gen3 Learning
• After removing the white wiring, Is the heater still running?
  • No
• Is this a new installation
  • Older HVAC system, added Nest fall of 2023
• Do you have a heat pump system or a conventional system?
  • Conventional forced hot air natural gas, with AC unit
• When was the last HVAC maintenance?
  • 2020
• Could you share a picture of the wiring configuration?
  • Not on hand, but has typical 5 wires connected as instructed (24VAC, common, heat, cool, fan)

[gmelanymelissa]

Hi @PhillipCrosby,

Thanks for the follow-up information. I appreciate all the information you've provided. Let's try to fix this!

To isolate the inconvenience with the thermostat base and the HVAC system, we need to check the following:

• Verify if the thermostat screen turns blue when you want cooling (or orange for heating) but the HVAC system doesn't turn on.
• Performed a wire swap between W1 and Y1. Once you swap these wires test the system for heating but expect cooling and check if the cooling has the same behavior. Test cooling but expect heating and check again if the inconvenience still persists. Important: Before touching any wires, turn off the power to your HVAC system at the breaker box.
• For alternative solutions, you can install the previous thermostat and test the system. This method is useful to determine if there’s a maintenance problem with the system. For reference, you can check out this article: Troubleshoot strange heating or cooling behavior.

Let me know how it goes.

Best regards,

Melany

[PhillipCrosby]

The AC does turn on when set to cooling with a blue screen. Orange screen with heating demand and furnace. 

If wires are swapped, cooling demand turns on furnace, I have not noticed AC turning on with this situation.  The problem is extremely random and infrequent, so could happen at another unpredictable time. Seeing as it is the middle of summer I won't be keeping it in this configuration. Heating demand also turns on AC, again have not felt heat in this configuration either. 

Previous thermostat never exhibited this behavior. Will try again. 

[ovidioj]

Hello @PhillipCrosby,

Thanks for replying. 
I appreciate your detailed response. It's clear that we're dealing with an intermittent and unpredictable issue.
Given the complexity of the problem, I recommend contacting a professional HVAC technician to inspect your system. They have the expertise to identify potential issues with the wiring, thermostat, or HVAC unit itself. You can contact them on your own, or you can contact one through this link: Find Nest Pro.

While you wait for the technician, you can try the following:

• Monitor the system closely. Keep a log of when unexpected heating or cooling occurs. This information might be helpful for the technician.
• Check for drafts: Ensure there are no drafts that might trigger the thermostat to call for heating or cooling.
• Verify the thermostat settings. Double-check all settings, including schedules and modes, to ensure they are correct.

I understand this isn't ideal, but it's important to have a professional diagnose the root cause of the problem. As soon as the technician gives you an answer to what's causing this random heating while cooling, please let me know. I am here to help you further in any case where you need it. Thank you. 

Regards, 
Jorge