Replaced a traditional thermostat with a basic nest this past spring and it was fine during cooling season. When heating, the unit goes into fan mode, and the burner doesn't kick in. It's odd. Also, the option for fan only is gone?
I did go from an older thermostat with a jumper from Rh to Rc, but instructions say I done need it?
You shouldn't need the jumper if you just have it for heating (Rh). With cooling (Rc), you would run a separate line to it and then the AC compressor would route to (Y).
I am having the same issue with the furnace - it worked fine for years, then out of nowhere, the furnace turns on, but the fan doesn't. Every single thread on here about it has been closed with no resolution...
So after a lot of troubleshooting, testing, and frustration here is what I found.
1. The first issue is that whoever set up the furnace wires used cat5e instead of the correct gauge furnace wiring.
2. They did not setup the C line supplying 24V DC, which the Nest needs in order to stay charged. The rapid cycling we saw prior was due to these power issues. I replaced the wiring with the correct gauge and that issue was solved.
3. I then tested the furnace ignitor and found that even though it still had continuity to the board, when power was applied it did not light the gas. This led me to check the board and the gas valve. I was able to eliminate the gas valve because I could smell the gas released during initial start up - the reason the furnace shuts down is because the flame detector was doing it's job and shutting down when no flame was detected.
4. I then pulled off the control board and tested the primary resistors, as well as the power relays to make sure the NC (normally closed) and NO (normally open) switches were correct. Further checking the link between the coils showed continuity as indicated by the relay wiring diagrams.
5. I replaced the ignitor, reassembled the heating manifold and when I fired it up, nothing. BUT now the control board was showing an error 3 lights. This was the Rollout; Aux Limit switch. It's the one between two purple lines by the ignitor. I examined the switch and reset (pushed it in) and the next test fired up perfectly.
All told, I think the Nest was not the issue, it appeared to be the furnace ignitor and the Rollout;Aux Switch.
I hope that helps everyone. I'll keep and eye on the thread if anyone has questions.
I am having the exact same issue.
my old thermostat turns on the heater and the burner and the fan both start normally.
when I install the nest the fan will start. The nest says heating but the burner won’t start.
The connections to the old thermostat are R,W,G,Y. With a jumper between Rc and Rh.
i made theses same connections to the nest and even added a C wire with no change.