air handler for air conditioner has worked fine since I moved into the house but this year both times the air conditioner has turned on it has not restarted. Both times so far the problem has been that the fuse on the inside of the air handler has been blown. It seems to run fine while it is running. The fuse doesn't blow while its running and shut it off prematurely. It just blows after the air conditioning is shut off by the thermostat.

  • root cause ended up being bad thermostat. – nonzero Jul 13 '15 at 23:39

Are you 100% certain that it blows when the thermostat opens the call for cooling circuit to shut it down? Or, does it run for a good while, shut down, and then you notice that the fuse is blown? If I were you, before going down that rabbit hole I would put one more fuse in it, run it for 5 minutes or less, shut it down from the tstat yourself, and see if the fuse is blown.

If you do find that the fuse blows as soon as the tstat shuts it down, one reason it would blow after the call for cooling is gone is perhaps there is a blower keepalive timer (common in newer HVAC systems to push the last of the cold air out of the ducts) and that is somehow shorted out, causing it to blow the fuse when that circuit kicks on to run the blower after the tstat tells it to stop.

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  • I just tried your suggestion and shut off the a/c after 5 minutes checked the fuse and it was fine. Does that suggest that it is the call for the cooling circuit to shut it down that is causing the circuit to blow? – nonzero May 10 '15 at 20:39
  • If you can run it and shut it down from the thermostat normally with a short runtime, it sounds more like there is something inside the unit overheating and causing the current to spike, blowing the fuse to protect it from fire or damage. Which fuse is it that blows? A low current one (5A or less) for control circuitry or a larger one (15-20A) for the blower motor power? – Jeff Meden May 11 '15 at 15:11
  • further developments...As of yesterday I can't start the a/c at all w/o instantly blowing the fuse. The fuse is a 5A. – nonzero May 12 '15 at 12:10
  • Instantly blowing the fuse = short circuit going to ground somewhere. If you have a multimeter and a circuit diagram of the unit, you can probably find it and fix it with continuity mode and some patience. If not, I would recommend consulting a pro. – Jeff Meden May 12 '15 at 14:04

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