Heating Randomly Won’t Turn off

[Pompeyjas0420]

Hi there, after reviewing many posts and trying everything I can think of and find this is my Hail Mary before reinstalling the old generic thermostat. Occasionally/randomly the heat will keep running past the desired temp. I was keeping the temp at 70 and it would run and reach this temp and then keep running altho the app and thermostat wouldn't show that it was still running. “Heat pump” would be shown on the screen but it wouldn’t be orange as though the heat is actually on. There are a few times the temps inside have reached 75-77 deg and the only way to stop the furnace/heat pump is to flip the breaker and it shuts off and then doesn’t happen for a couple of days. I’ve noticed it was happening more when the temp was set around 70-72 and happening every couple days (less often) around 68. I have turned off all schedules, auto, and learning capability’s but at a loss as it is still happening. Any thoughts, comments or suggestions are really appreciated. 
- Jason 

[Pompeyjas0420]

@Patrick_Caezza

Maybe you have some thoughts? 

[Dolo]

I’m struggling with the same issue. I can turn the system off and it turns itself back on to heat to 90

[moralesvicente]

Hello folks, 

Thank you so much for posting in this community. I’m sorry to know the thermostat isn’t working as you expected regarding the heating. Don’t worry; it’s a pleasure to help. 

To provide you with accurate troubleshooting steps, I would appreciate your assistance in answering the following questions:

• What type of Nest thermostat do you have? For your reference, you can use this article: How to tell which Nest thermostat you have. 
• When did the issue start?
• What other strange symptoms have you experienced since the problem started?
• Are there any error messages? If so, what are they exactly?
• When was the last maintenance check for your HVAC system?
• Have there been any recent changes or upgrades to your HVAC system?
• Do you have a common ground wire or C wire?
• Have you experienced any power outages within the last few days or weeks?

If you have any more information that could be helpful, please let me know.

Best regards,

Daniel

[Pompeyjas0420]

• What type of Nest thermostat do you have? nest learning thermostat gen 3 

• When did the issue start? Once the heating started coming on, never had an issue with cooling 

• What other strange symptoms have you experienced since the problem started? Randomly when this happens sometimes it shows heat is on, sometimes says heat pump and sometimes it shows nothing is on but the heat is actually on and running but it’s like the thermostat doesn’t realize it or know. 
  
• Are there any error messages? If so, what are they exactly? No errors or anything have ever popped up. 
  
  
• When was the last maintenance check for your HVAC system? July 2024 

• Have there been any recent changes or upgrades to your HVAC system? No it was installed in 2022

• Do you have a common ground wire or C wire? C wire 

• Have you experienced any power outages within the last few days or weeks? No I have not 

[Pompeyjas0420]

 

The old thermostat wiring before switching to nest learning gen 3. 

[Pompeyjas0420]

@CoolingWizard  any suggestions or thoughts please l?…I’ve seen that you’re quite a helpful chap

[CoolingWizard]

@Pompeyjas0420 ,

When you check the power using the display , what is being reported?

Choose SETTINGS->TECHNICAL INFO->Power:

BATTERY

VOC

VIN

LIN

AC Cooling Wizard

[Pompeyjas0420]

Thank you so much for the reply, here is the information collected from the unit.

BATTERY - 3.931V

VOC - 31.76V

VIN - 31.76V 

LIN - 200 ma(c)

[CoolingWizard]

@Pompeyjas0420 ,

In your original post, you stated “occasionally “ this happens. Can let me know the frequency of occurrence?

Clearly you have ample power coming from the air handler and this leaves hardware or software. 
Hardware can be a defective temperature thermistor or a digital relay that is failing to open. 
This could be software although not very likely.  
This case should be escalated. We need diagnosis of the ambient temperature sensors (thermistors) and obtaining internal log files of the software system signaling the relay to open. 

AC Cooling Wizard 

[Pompeyjas0420]

Thank you for the information and following up with me. It was happening every 2-3 days When I had the thermostat set at 70 or higher, it’s only happening once or twice a week now that I have the temperature set lower between 65 and 68. I haven’t noticed anything common when The heat decides to not shut off and keep running, like sometimes it’s during the day and sometimes it’s at night. Let me know if there’s any other information I can pull from the unit to share with you. In another comment, I shared the pictures of the prior thermostat and the new wiring to the nest if that is helpful. Thanks again! Jason

[Pompeyjas0420]

It’s happened again just today, thermostat on 68 and not showing that the system is on but it was on and blowing full heat and the internal temp was registering as 72 in the house. It’s like it fails to realize that the temp is reached and to communicate to the heater to turn off. But the crazy thing is that it happens very intermittently and not consistent. I took a picture of what the screen looked like today but I can’t add it to this reply. In order to stop the heat from blowing I had to flip the breakers to the furnace/heat pump. 

[moralesvicente]

Hello Pompeyjas0420, 

I appreciate you keeping me in the loop. To continue providing you with further information, could you please attach a picture of the wiring configuration in the Nest Learning Thermostat 3rd Gen? 

Keep me posted so I can provide you with further assistance.

Regards, 

Daniel

[Pompeyjas0420]

Hi there, thank you for the reply. Here is a photo of the nest wiring. Thank you for your assistance. @moralesvicente 

 

[CoolingWizard]

@Pompeyjas0420 ,

You have a heat pump and your old thermostat had the brown wire attached to EMERGENCY HEAT. You need to move the brown wire from W1 and place in the programable * terminal. Then need to configure that as Emergency Heat. I am not saying this will fix your runaway heat problem but will correct the software thinking you have might have  a duel fuel system. 

AC Cooling Wizard

[Pompeyjas0420]

Thank you for that suggestion!I have completed it. 

Does this look correct? I did as you instructed, and then updated the settings in the software. Anything else you can think of that I can try? I will let you know if this happens again. 

[CoolingWizard]

@Pompeyjas0420 ,

The W2 wire should be configured as Auxiliary Heat and not alternate heat. You will need to go to SETTINGS: EQUIPMENT: PRO hit Continue twice, then you will see a picture of the backplate with colored wedges. Any yellow wedge needs attention.  Rotate and select W2 wedge and select. Then change the setting to Auxiliary Heat. 

AC Cooling Wizard

[Pompeyjas0420]

I have made the changes that you have suggested. It did not stop the heating situation where it doesn’t turn off as it happened again yesterday. I took some pictures of the display screens, but I’m not sure how to send them to you because it won’t let me attach them to this reply. Can I email them to you or what do you suggest? Thank you so much 

[Pompeyjas0420]

 

[Pompeyjas0420]

 

[Pompeyjas0420]

Hi there, please advise on how I need to move forward,  do I need to call or email someone? I’m still experiencing this issue and woke up to 80+ deg this morning because it just kept running. I’m very close to taking off the nest and putting back on the regular thermostat. 

[CoolingWizard]

@Pompeyjas0420 

everything appears to be configured properly now you have all green wedges so the thermostats happy with the selections. And the display shows that they’re probably set up as emergency heat and auxiliary heat. Now the next thing we need to do we need to figure out if we’ve got a bad temperature sensor and by the way, the thermostat has three of them in there and it averages the temperature between the three to be very accurate. We need to see if one of those are two of those are acting up and that’s why it keeps going. The other choice is we could have a bad electronic relay that is sticking in that leaves it on too long. At this point, I’m gonna escalate your case so we can get some high-level support to look in and possibly they may need to get some information from the display that I don’t know how to get such as what the temperature sensors are reporting in ohms and what that relay is in and what that does. A software thing to relay is in. 

AC Cooling Wizard 

[Pompeyjas0420]

Okay sounds good, thank you for all of the help so far. What do I need to do from here? Do I need to reach out to somebody or what happens? It didn’t shut off again last night, got up at 4 AM to a house blazing hot in the mid 70s. Please let me know and I really appreciate the time you’ve spent with me

[CoolingWizard]

@Pompeyjas0420 

We need to verify the call for heat.  To do this we need to set the desired temperature then when it overruns by 1°, we need to go to the air handler and check the Y1 to C on the control board.  If the Nest Thermostat is still calling for heat, you see 24VAC on your multimeter. If the system continues to run and the Voltage is near zero, then the problem is not the thermostat. If you see 24VAC +/- 5VAC then the thermostat is the failing component. 

AC Cooling Wizard