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After installing a Google Nest, my furnace heats until it reaches the correct temperature, but if it sits, the controller light turns off and I need to turn the breaker off and then back on in order for the furnace to kick in. The controller light turns off, but does not flash as far as I have seen. I have switched back to the original thermostat, some basic Honeywell one, and it works fine. There is a C wire connected, though I had to add that and I'm not sure if it's totally legit. Currently the C-wire is connected at the T terminal.

My furnace is a Lennox G14 series (I'm not sure which model exactly).

I am fine using the old thermostat, but I am bummed because I was looking forward to automating this stuff. Thanks for any help!

Wire diagram

Thermostat wires connected to furnace

Controller with light on

Correllion115
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  • can you show us the problem maker (Nest) wiring – asinine Nov 11 '22 at 18:54
  • where did you get the C wire from ? – asinine Nov 11 '22 at 19:10
  • @Ruskes I never took a picture of the Nest wiring, but all the wires went to their respective letters (i.e. Red => R), and the blue wire went to C. I pulled the new wire with the blue included awhile ago, and the original thermostat is using it now. – Correllion115 Nov 11 '22 at 21:04
  • I am interested in helping you, but need more information. What was connected on Nest ? R, W, Y, G, C ?? What would shut down the furnace ? The Limit switch ! / why ? – asinine Nov 11 '22 at 21:35
  • @Ruskes R, W, Y, G, C were all connected. The furnace would reach the correct temp, turn off, and then most times not come on again, with the inside temp being 5-10 degrees below the setting. That's when I would have to cycle the breaker, which would turn the furnace on immediately. – Correllion115 Nov 11 '22 at 21:54
  • OK, right now you do not need Y and G, disconnect them and leave only heating W, . Your home brew C worries me, and it is connected to T on the furnace which is grounded on the furnace . – asinine Nov 11 '22 at 23:19
  • @Ruskes What would be the reason to disconnect the Y and G? I guess I'm not totally sure of the implications of having it attached to T, since there is still 24V between that and R. – Correllion115 Nov 12 '22 at 00:20
  • Do not know, just eliminating possible sources of problems, at this time you do not need Y (cooling) and G the fan in heat exchanger. The furnace has its own fan. As said some major problem that kills the Heating, so you have to reset it got get it working. I assume none of the LED on the furnace control board indicate anything. The T on the control board is 24 Volt common, but it is unusual to me for it to be grounded. – asinine Nov 12 '22 at 00:34
  • One more question, as the heating reaches the set point, and the fan stops working, are you sure the door interlock is not activated due to the pressure change ? that would cut the power to the heating. – asinine Nov 12 '22 at 00:40
  • @Ruskes Ah I see. From what I can tell there is just the one green light on that controller, unless I am looking in the wrong place. I'm not sure about the pressure change though, how would I check that? Would the two thermostats affect that differently? When I had the nest installed, I left the fan control to the furnace. – Correllion115 Nov 12 '22 at 01:57
  • "it is connected to T on the furnace which is grounded on the furnace" - then it's the same as C. And according to this it should work : [Can I use the T terminal in my furnace as the C for a Wifi Thermostat?](https://diy.stackexchange.com/questions/56049/can-i-use-the-t-terminal-in-my-furnace-as-the-c-for-a-wifi-thermostat) "Lennox terminal block inside the furnace uses T label on the terminal block for common" – Mazura Nov 12 '22 at 02:06
  • Next time don't power cycle the unit, instead pull the thermostat off and then clip it back in. If it starts up then it isn't the furnace in lockout, it's the nest. **Factory reset and redo the setup.** Maybe it doesn't like chassis ground? **Hook blue straight to the transformer** opposite red. – Mazura Nov 12 '22 at 02:06
  • Deleted answer from that link: "I'm dealing with the same scenario. **I added the blue C wire to the T terminal and it seems to be working, except that I saw the thermostat (Ecobee4) reboot once when the fan shut off**. It started fine, kept it's settings and programming, but I haven't been able to catch it spontaneously again since installing the thermostat a few days ago (I would rather see it happen on it's own than force the fan to stop)." - sounds like they don't like chassis ground. – Mazura Nov 12 '22 at 02:16
  • Quick check on Nest. When the furnace wont start, use a pice of wire and **bridge** the R and W. That is to send a signal to the furnace to start the heat. It does not explains why it shuts down in first place. – asinine Nov 12 '22 at 02:49
  • @Mazura I realize it's been a few months, but I think I did what you said to no avail. I connected the common to the common on the furnace circuit. I am leaning towards the Nest sending a signal to turn off that overloads the control board, forcing it to need to be reset. – Correllion115 May 25 '23 at 03:03
  • "Nest Thermostat Gen3 won't work with York furnace/thermostat that has A-wire. **Called Johnson Controls** & was told there is an adapter that makes my thermostat compatible with most 3rd party thermostats. Anyone know about this or how to obtain adapter?" https://www.googlenestcommunity.com/t5/Nest-Thermostats/Nest-Thermostat-Gen3-won-t-work-with-York-furnace-thermostat-that-has-A/m-p/94966 - No idea what an A wire is. If the old tstat makes it work, then it's not the furnace's door switch or anything like that. Call Johnson and mention a nest, then call nest and mention johnson. – Mazura May 26 '23 at 00:35

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The solution that seems to be working so far is adding a 1kOhm 5W resistor between W and T, that being the call for heat voltage. After contacting Johnson Controls, I was redirected to an engineer at Baso (which now owns this particular model of Johnson Control) told me that older controllers sometimes expect more of a load when heat is called for, and newer controllers have these built in.

Correllion115
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