Heat and AC Runaway

[HenriB]

Good Day,

We have a new home we moved into 5 months ago. Pretty basic setup with FHA gas furnace in attic and AC outside, no heat pump.  The system is controlled by a Nest thermostat (specs below). We are in SW Utah so for the first 2.5 months the system was in cool mode and AC worked fine. Sometime around early November I switched the Nest to heat mode and the heat worked great until last week.

One day last week we noticed the house cooling down and I could hear the furnace running. When I checked the display it was set to 71 (calling for heat) but the temp was 67 and getting cooler. I then noticed a strange noise outside and when I went out I noticed my AC was running, sounded like it was straining in max mode (outdoor temp was 34).  The air coming out our vents was cool.

After calling HVAC service company I pulled the disconnect at AC and set Nest to mode ‘off‘ but the furnace keep running and no heat was being produced.  After waiting 5 min I killed breaker for furnace and after waiting a few more minutes turned it back on. I set Nest back to heat mode and the furnace came back producing heat normally this time. After the house was back up to temp I put the AC power disconnected back in place. All seemed good, the system ran normally fine for 5 days and the AC stayed off.

This morning the exact same thing happened again and I followed the same procedure as above, the only difference is that I haven’t put the AC power disconnect back in place yet. Thank goodness we were home both times this happened. The only other thing I noticed is that once I reset everything and the heat comes back on, the air has a bit of a musty smell (for a few minutes).

My HVAC company has not come out yet but I don’t want to waste time / money when the tech I spoke to was stumped. All we can figure is that the Nest is causing this by calling for heat and cooling at the same time but why after disconnecting power to the AC was the furnace running without heat being produced? It’s like the Nest says ‘everyone give me all you got’ and ignores any input, even being set to mode ‘off’.

Anyone ever see this or have ideas?

Equipment Y1, G, Rc, W1, C

SW ver 6.2-27

model display 3.7 backplate 5.26

[Anonymous]

I believe at this point, I'd recommend to have HVAC tech look into your system since you mentioned you put the thermostat into OFF mode and the furnace keeps running but no heat was produced as you mentioned.

Btw, Do you have a voltmeter?

[HenriB]

I do have a volt meter. 

[HenriB]

Had an HVAC tech take a look yesterday and he couldn’t find any issue so he’s suspecting an issue with the Nest thermostat. I’ll be monitoring the system and when it happens again I’ll pull the Nest off the wall, if everything stops, that will confirm the theory.

[aatienza]

Hey HenriB,

I’m sorry to hear you’re having trouble with this. Keep monitoring your system and let us know how it goes.

I appreciate the help, eveluna.

Thanks,

Archie

[aatienza]

Hey there,

I just wanted to follow up to see if you still need our help. Please let us know as we would be happy to answer any questions you may have. 
 

Thanks,

Archie

[aatienza]

Hello there,

Just checking in to make sure that you've seen our responses. Please let me know if you have other questions or concerns as I will be locking this in 24 hours if I won't hear back from you again. Feel free to start a new thread and we'll be happy to help.
 

Thanks,

Archie

[HenriB]

I’m still monitoring the situation. So far this week there have been no problems so I have not been able to test pulling the thermostat during an incident.

[Jenelyn_O]

Hi folks,

@HenriB, I saw your post and wanted to chime in. I just want to confirm if everything is working fine. Please let me know if you have any questions or concerns, as I'll be willing  to assist you from here. 

@aatienza, thanks for your help.

Best,

Jenelyn

[EmersonB]

Hello everyone,

@HenriB, I'm dropping by to ensure that everything is covered here. How's it going with your Nest Thermostat? In case you have an update, you know where to find us.

I appreciate the help, Jenelyn and Archie.
 
Thanks,
Emerson

[Markjosephp]

Hi HenriB,

It has been a few days since your last reply, and we're going to lock the thread within 24 hours. If you have any questions, please feel free to create a new post.

Regards,

Mark

[HenriB]

Sorry for the delay. The incidents were normally several days apart and then I was away for a week. The last time it occurred I pulled the AC disconnect and after 10 minutes the furnace / circulation fan was still running. I pulled the Nest off the wall and the furnace shut off but the circulation fan ran 20 minutes until I reset the breaker. The HVAC tech arrived during that incident and said he was getting strange voltage readings on the wall plate low voltage wires. He felt it may be caused by damage to the wire or an issue with the system control board. We had the low voltage wire replaced yesterday so now we wait and see. In the end, this does not appear to be an issue with the Nest device so this thread can be closed. Thanks!

[Markjosephp]

Hello HenriB,

That's a relief! I appreciate you posting and updating us on your efforts to address your concern and find a fix. It looks like we can consider this one complete, so I will lock the thread within 24 hours unless I can help out with anything else.

Regards,

Mark