Nest Thermostat w/Power Connector keeps generating error messages M20/N260

[ExBexTexMex]

Help.

I have a Nest Thermostat (the basic model) with a Power Connector wired in at the furnace (only 2 wires in the wall).

I've had it hooked up about a week, and it has operated the furnace fine.  However, I keep getting errors, seemingly at random, about the power supply.

1.  M20 Error: "Not receiving consistent power."  This has happened at least 2 times.  And each time, I go into the settings menu, check for power, and find it is receiving around 35v.  After exiting back out of the menus, the error message is gone, without having actually done anything to fix it.

2. N260 Error: "A C wire or Nest Power Connector is recommended for your thermostat."  I've seen this one at least 3 times, despite having a Power Connector hooked up!  I then go into the settings menu, down to the "power status" and find that the thermostat is NOT detecting the Power Connector.  I run the power test, and within seconds, it detects the Power Connector and everything is fine.  I exit out of the menus and the error message is gone.  And it works fine, usually for days, until the error pops up again.  

3. Disconnected from Wifi Error: This shows up at least once a day, usually multiple times.  Every time though, I go into the menu and it shows that it has a connection.  It seems to reconnect automatically.  I haven't done anything with this error since it seems to resolve itself.

The battery is charging and always says it is "good".  When first installed, it was around 3.14.  Now, it consistently shows around 3.01-3.05.  After the N260 errors, it seems to drop to 2.98ish, but goes back over 3 once the Power Connector is detected again and it has time to charge. 

I have an old gas forced air furnace, and based on other posts here, I have the Power Connector wired this way on the gas valve:

W --> TH

R -->  TH-TR

C --> TR

Again, every time I go into the menus, it shows around 35v of power.  I don't know why the Nest seems to randomly lose detection of the Power Connector.  It is always able to detect it again after I manually go into the menu and run the test.  

I'm frustrated.  I like the Nest otherwise, but if it's not reliable, all the fancy stuff doesn't mean anything.  Any suggestions?  Thank you!

[ExBexTexMex]

It made it 5 days without a power error.  But I woke up this morning to another N260 error. 

I entered the settings menu, and it showed that the Power Connector was detected, with 35v at the thermostat.   I exited the menus, and the error was gone.  

[Markjosephp]

Hello ExBexTexMex,

I appreciate you posting and updating us on your efforts to address your concern and find a fix. I understand that it can be confusing, but I'd love to know more about this.

Could you please check your thermostat's "Wiring info" and "Power Info" in its Equipment settings? Also, update its software and ensure it's on version 1.1-11.

Keep us posted on how it goes.

Thanks,

Mark

[ExBexTexMex]

Hi Mark, thanks for the reply.

Wiring Info:

   Connected to W(P)-R

Power Info: 

   Vin: 34.808 V

   lin: 385(W)

   PS: W-385, WP-16

Regarding the software, the Nest required a software update at the time of installation for the Power Connector to function at all.  (I think it was error E297.)  I just checked, and it's running version 1.1-11. 

Thanks. 

[Markjosephp]

Hey there,

Thanks for letting me know the status of your thermostat's power and wiring information. It appears that it's in normal condition, as it was showing the power readouts it needed. But I wanted to ensure that everything was covered here. How's it going with your Nest Thermostat? Are you still experiencing any power issues? I'd be glad to hear from you, and I'd like to make sure everything is working.

Regards,

Mark

[ExBexTexMex]

I've gotten two more M20 errors this week.  But I did more research, and I think I know what's going on now... 

These old gas air furnaces have a temperature sender that has two functions: turn the fan on/off and provide a high-temp safety cutoff if the furnace gets too hot.  

I suspect that safety circuit is the issue.  It functions by cutting power to the low-voltage transformer, which turns off all low-voltage power in the system, thereby turning off the gas valve to stop gas from flowing.  Obviously, that will also cut low-voltage power going to the Nest via the Power Connector, since it feeds off the low-voltage gas valve connections.  Once the furnace cools, power is restored to the transformer.

This is likely happening on colder days, when the Nest is trying to increase the temps by a bigger margin, like from Sleep to Comfort settings in the morning.  The furnace runs continuously for too long and gets too hot, tripping the safety circuit for a few minutes until things cool down a bit.  By the time I see the error and check the settings for power, the system is already cooled and the safety circuit has already turned the low-voltage power back on. 

There is a way to wire that temperature sender so that it will cut power to the gas valve AFTER the transformer, so that the transformer never loses power and so that a separate common can run from the transformer to the Nest Power Connector for continuous power in all conditions.  That would be the solution.

I haven't done that yet.  As a temporary workaround, I changed the scheduling in the Nest to only increase the temp by 1 degree per hour, so that the furnace won't run for as long.  Need a few more days to see if that is working.

This whole thing has already been such a hassle.  (None of this explains why I still get multiple wifi errors every day.)

[Markjosephp]

Hi ExBexTexMex,

I'm sorry for the trouble this may have caused you, and thanks for keeping us in the loop on what you've done to have your concern isolated. I'm just checking in — how's it going? Still need our help?

Thanks,

Mark

[Markjosephp]

Hey there,

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

Thanks,

Mark

[EdwardT]

Hi ExBexTexMex,

I'm just checking in to make sure that you've seen our response. Please let us know if you have any other questions or concerns as I will be locking this in 24 hours.

Thanks,

Edward