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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.

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  • 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?
    – ThreePhaseEel
    Aug 7, 2020 at 11:47
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    it's possible for a mis-wired thermostat to damage the transformer.
    – Jasen
    Aug 7, 2020 at 12:41

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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.

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