A/C running continuously -- even when heating

[Clough42]

It's heating season, and only gas heat should be running, but every couple of weeks, my A/C switches on and runs continuously.  The heat keeps cycling (on most of the time) to keep the temperature up, but the A/C compressor and fan are always running.

Checking with a meter, cooling is being commanded.  The only way to stop it is to disconnect the yellow wire at the thermostat.

This is on a Payne gas heat and A/C system, with a common wire.  Connections are Y1, G, Rc, W1, C.  The thermostat detects all wires and everything functions normally for a couple of weeks, until it happens again.

Is this a known issue?  Do I have a bad thermostat?

[Houptee]

When you tested the Y was getting power were you testing outside at the AC or inside at the furnace?

Does the Y wire go to the furnace Y terminal screw? Is there another Y in the same screw that goes to the outdoor unit? I ask this because it could be something wrong with the furnace control board if the Y wires connect to the board.

[Clough42]

I was testing at the furnace control board.  The last tech who came out to diagnose it wedged the safety switch so I could open the panel and test without resetting the system.

The wires from the thermostat connect at the control board, and then a parallel pair goes to the condenser compressor outside.

To determine what was actually closing the Y circuit, this is what I did:

1. Pulled the display unit off the thermostat base.  This caused no change.  The Y circuit was still closed, commanding cooling.
2. Pulled the Y wire out of the connector on the Nest thermostat base.  The Y circuit then showed open at the furnace, the compressor stopped, and after the 120 second delay, the blower shut down.

Based on this, I think it's the thermostat itself that is calling for cooling, but I would be happy to discover that I'm missing something important.

[Houptee]

Hmm interesting.

Maybe pull the nest and remove the red wire and the y wire.

Put meter in continuity test mode and use the small holes in the nest tabs and see if you have continuity between R and Y tabs with no wires in the tabs. It should read open. If it's showing continuity then something shorted in the base.

[Clough42]

That would be interesting to try, though as soon as I pull the wire, it re-detects equipment and resets and works fine again for a couple of weeks.

[Markjosephp]

Hello folks,

Thanks for helping here, @Houptee.

@Clough42, I understand that it's been an intermittent issue, as we couldn't specify what might be the cause if it's currently working. I wanted to ensure that everything was covered here. Are you getting any wiring reports or message codes on your thermostat? Also, please gather the power readouts on your thermostat's "Technical Info" settings.

Regards,

Mark

[Markjosephp]

Hi Clough42,

I wanted to see if you still needed help. Please let us know if you have any other questions or concerns.

Thanks,

Mark

[Clough42]

I still don't have a solution to my problem.  Your last message indicated that you can't do anything to help if it is intermittent and not currently happening.  But if I wait for it to happen, I'm not willing to let it run while discussing in the forums.  I need to know what troubleshooting steps I can take in the few minutes before I need to shut it down.

I have not seen any errors or messages from the thermostat.  When we entered heating season for the first time with the Nest, it warned me that the furnace was short-cycling, and we had that fixed by a professional (adjusted the gas pressure regulator and increased the air flow).  Since then, there have been no messages or issues, except the A/C turning on and staying on.

Here's the technical info I see in the app:

• Model: Display-3.7
• Backplate model: Backplate-5.26
• Software 6.2-27
• Updated July 1, 2022
• Battery: 3.93V

For wiring, I have:

• Yellow - cool - Y1 (currently disconnected to prevent the issue from happening)
• Green - fan - G
• Red - power - Rc
• White - heat - W1
• Blue - 24V common - C

My conclusion at this point is that the yellow circuit is being closed at the thermostat backplate.  When the A/C comes on and stays on, I can see +24V on the yellow wire at the control board at the furnace.  If I pull the thermostat off the backplate, the A/C keeps running.  If I pull the yellow wire out of the thermostat backplate, the +24V power on the yellow wire disappears (measured at the furnace control board), the compressor shuts off immediately, and the blower shuts off 120 seconds later, as configured by the jumper at the furnace control board.

If it were the furnace control board asserting the call for cooling, I would expect that to continue when I disconnect the yellow wire from the thermostat backplate.

Are there additional troubleshooting steps I can take?  If I reconnect the yellow wire and wait for it to happen again, what can I check at that time to help determine the cause?

I see these options:

• File a warranty claim and replace the Nest Learning Thermostat
• Give up on Nest and buy a different brand of thermostat
• Replace the control board in the furnace, even though there's no evidence it's bad

Is there another path I'm not seeing?

[Markjosephp]

Hey there,

I'm sorry for the late reply, and thanks for letting me know about this. We'd like to take a look into this for you.

Please fill out this form with the needed information and let me know once you're done.

Regards,

Mark

[Clough42]

Done.

[Markjosephp]

Hi Clough42,

Thanks for completing the form; we have received it. Soon, a member of our upper team will contact you via email. So please continue the conversation there, as this thread will be locked within 24 hours.

Regards,

Mark