A year ago, I had a brand new smart thermostat that caused the A/C fuse to keep blowing. I replaced the thermostat with a duplicate, swapped the fuse out and everything has been fine. Now a year later, my AC fuse again blew. I checked the thermostat on another ac in the house and it blew that fuse too. I replaced both fuses. Now the AC with the problem will not supply any power out of the 24v side of the transformer, as tested with a multimeter. I've ordered a new transformer and a replacement thermostat for the third time.

Is it coincidence it's the same unit and I've now had 2 bad thermostats? It is possible the transformer has been bad this whole time and finally gave out, taking the thermostat with it? This AC is not even 2 years old but the company that installed it is pretty poor and won't stand behind their install.

  • Please edit your post to include make & model of both the thermostat and the AC unit.
    – FreeMan
    Aug 7, 2020 at 10:53
  • Yes, we'll need to know the make and model of the various things involved here. Also, when you say "A/C fuse", I take it you're referring to the (often automotive-type) fuse on the low voltage side of the transformer, typically found in the indoor unit/air handler? Aug 7, 2020 at 11:47
  • 1
    it's possible for a mis-wired thermostat to damage the transformer.
    – Jasen
    Aug 7, 2020 at 12:41

1 Answer 1


Pretty much can guarantee either you've wired up your thermostat wrong, or that you chose a thermostat whose current draw on the connection to the A/C unit exceeds the A/C port's rated capability.
Consider checking the manuals.
Then see if you can power the thermostat off a different source, such as a local DC supply, rather than hanging it off the A/C block.

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.