Nest Thermostat stops heating prematurely

[Worzie]

Back in Dec 2023 our Google Nest Thermostat model G4CVY on the Nest Renew energy saving program was keeping us in the cold too often.  We had to override the system to get the temps back up to the point I was fed-up.  Unfortunately Dec 26th the controller board on the furnace stopped working and we had it replaced a couple days later.  With the new system, we continued getting left in the cold by Renew so, I found how to cancel the subscription hoping all our troubles woud go away.  Instead, the Nest continuously would fall behind 5-10 degrees.  The Nest said it was heating but no signal was being sent to the furnace yet the Nest keeps racking up virtual hours of running the furnace.  Whatever the Nest does to realize something is wrong and finally send another signal is beyond me.  I've tried bumping the temp below the current then back up and it usually works then but, not always.

I have 4 wires and using the Nest Power Connector for the missing C wire.  I have gas forced air and an AC condensor so, R, G, W, B with the blue going to Y.

I have tested a few ways:
Test 1: I removed Nest Power Connector and put G (blower) to C at both ends. Temp fell too much again and eventually said it was heating when it was not.
Test 2: I reinstalled original analog Honeywell mercury thermostat with G back on G.  All systems well, heat stayed on for entire time to reach set temp.  Unfortunately too good as a slight nudge launched it up 5 degrees too hot!

Test 3: I logged account off the Nest and factory reset all data.  Installed the Nest Power Connector again.  This time, leaving most defaults except the schedule and desired temps.  After a few hours the heating when not started again.
Test 4: thinking perhaps the issue has been there all along and the AI of the Renew program had been mastering it all along... I resubscribed to Nest Renew program again.  Nope!  

To live with this now expired warranty we'd have to set the temp a few degrees higher and have it keep resetting when it reaches some minimum threashold and resend heat signal to get up to temp over a few tries and 30-45 minutes of trying.  I don't wish to go back to the analog yet as it behaves like it has a sticky spot that launches the mercury on for too long.

[Worzie · Accepted solution]

Well, I feel the need to post an update from the weekend.  I have historically had to do this for an old furnace as well as with a couple other friends' furnace even!  The issue turns out to have been a dirty flame sensor.  After using a fine grain sandpaper on the probe, not leaving any oils from fingerprints, the system is working wonderfully now.

The fortunate/unfortunate is, I ended up buying the 3rd gen Nest Thermostat thinking all was well until, the next morning when a 5 degree deficit was too much to overcome in the early morning.

Typically the flame prob issue had no blower going in  the past experiences but, this was going for about 10 minutes.  Each try I could hear the gas flame up and a moment just before the blower kicked on I could hear the flame shut down blowing ambient air.  I'm not sure why I didn't pay closer attention before this.

[Lance_L]

Hey Worzie,

Thanks for posting and sharing your experience with the Community. We’re delighted that this has been resolved despite the initial challenges. If you have any further questions or need assistance, don't hesitate to reach out to us at the Nest Help Center.

Regards,

Lance