I installed nest thermostat without turning off power to furnace. Blower kicks on but not furnace. Is there anything I can do?

  • 1
    Call a service person and see what you shorted out/did not connect right. Just because it is low voltage and won't hurt you too bad, does not mean a short will not burn out a control board.
    – crip659
    Jan 18 at 20:10
  • 2
    Can you post photos of the wiring at both ends and of the wiring diagram of your furnace please? Jan 19 at 3:16
  • "Blower kicks on" ... when you do what? And what did the Nest say at first start up and going through the setup? If it didn't even make it that far, then yikes... Put the old Tstat back... does it work?
    – Mazura
    Jan 19 at 3:17

2 Answers 2


Hopefully there are fuses on the control board for the furnace, and hopefully one of them is blown. If you're comfortable opening the case of your unit (after turning off all the power), look around for some removeable fuses and test them. If you don't find any that are blown, or can't find them at all, then its probably best to call in a pro.

Best case - $2 fuse, worst case - $$$ on a control board.

  • Pretty sure with a blown fuse it will do absolutely nothing.
    – Mazura
    Jan 19 at 3:17

A typical furnace attached to an air conditioner can turn on the blower independently of the furnace. This allows for the blower to be on when used with the air conditioner, and it also allows for "fan on all the time". So this may just be wires in the wrong places, but could be something much more serious.

Pictures and other details about the furnace and the thermostat would help. Keep in mind that while there are some fairly standard color codes in use for thermostat wiring, they are not always followed. If a couple of wires were connected previously without following the usual color coding and then you connected them to the Nest based on the usual color coding, things won't work quite right.

  • Guessing you're already aware of this, but in case you and other readers are not: Nest setup should walk you through entering what colors your wires are and what terminals they're connected to on the old thermostat, which tells it 1) whether it's compatible with your system and 2) where to put each wire on the new thermostat. Setup guide also tells you to label each wire (and box provides labels) so you're not dependent on wire color. Jan 18 at 21:36
  • @FredricShope I know that. I also know sometimes people look at colors and disconnect from the old without looking at labels on the old thermostat do if it was wired non standard with the old one it won't work so easily with the new. Jan 18 at 23:05

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