Nest Learning 3rd Gen won’t energize G (Fan → On) in heat-only system — old thermostats worked fine

[sai99]

I’m running into a frustrating issue with the Nest Learning Thermostat (3rd Gen) that I’ve confirmed is not wiring or furnace related. Hoping someone from Google/Nest can clarify, because this seems like a firmware limitation.

My setup:

• Furnace: Carrier 58MCB (heat-only, no A/C)

• Thermostat wiring: R, W, G, C

• No cooling equipment connected

What I want:

• The ability to run the blower fan independently of heating (Fan → On or Fan Schedule), for air circulation, filtration, and comfort.

• This worked perfectly with:

  • My old mechanical thermostat

  • The newer Nest Thermostat (2020 basic model)

Problem with Nest Learning 3rd Gen:

• Heating works fine — W1 energizes blower as expected.

• Fan → On or Fan Schedule never energizes G.

• Multimeter confirms:

  • R–C ~28V, stable power

  • R–G = ~28V and blower runs when shorted manually

  • G–C = always 0V, even when Fan → On is selected in the app

• In Pro setup, Nest detects “Heating + Fan” correctly, but fan test fails (no blower).

Tried already:

• Full reset and reinstall

• Pro setup mode (Heating + Fan)

• Dummy Y1 wire trick → did not work

• Shorting R–G during setup → blower runs, but Nest never drives G

• Fan schedule in the app → no effect

Other observations:

• Y1 energizes 24V if Nest is put into Cool mode (even though I don’t have A/C)

• • terminal energizes 24V when configured as humidifier

• But G never energizes, even when Fan → On is commanded

Conclusion:

• Furnace wiring and blower relay are fine (R–G short works every time)

• Power supply is fine (28V across R–C)

• The issue is that the Nest Learning Gen-3 never energizes G in a heat-only system, despite detecting it during setup

Why this matters:

• This is a basic thermostat function that every mechanical thermostat supports

• Even Google’s cheaper Nest Thermostat (2020) supports it properly

• Ecobee and Honeywell also handle it correctly

• For a premium thermostat, this should be a must-have feature

Question for Google/Nest:

• Will this be addressed in a firmware update so the Nest Learning Gen-3 can honor G (fan-only) in heat-only systems?

• If not, can Google officially acknowledge this limitation so customers don’t waste time troubleshooting perfectly good wiring and furnaces?

This has been reported by many users on forums and Reddit since the Gen-3 launched. Please let us know if there’s a fix or at least transparency on why this was disabled.

[PeterP1]

I’ve seen the same behavior with the 3rd Gen Nest Learning Thermostat in heat-only setups. Everything checks out with wiring and power, but the G terminal simply never energizes when Fan → On is selected. Older thermostats and even the newer Nest model handle this correctly, so it seems like a limitation or bug specific to the Learning Gen-3.

From what other users have shared, this isn’t something that can be fixed with wiring tricks, and it may be a firmware limitation by design. The most helpful step right now is to submit feedback through the Nest app and directly in the Home app so the engineering team can see how widespread this issue is.