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Hi Dan, I'm Brian and I'm here to help!
This is a pretty common problem but finding it can be tough sometimes. Can you give me a little more info please? Anything that's changed with your ac or heat system recently, new t-stat, service performed, etc... What is the brand and model numbers of your ac/heating equipment?
Great, thanks for the info Dan. That helps a lot. It sounds like since the fuse blew when you turned on your ac that it is probably ac related. The most common cause for this is the thermostat wire outside between where it exits the wall and enters the ac unit has become compromised and shorted together so I would look carefully at that first. The symptoms also lend itself to that since this happened when you haven't even been running your ac! The next likely cause(s) are in the ac unit itself and could be failed compressor contactor, a rubbed wire shorting through the insulation, a failed time delay (if equipped) and a few other less common issues. We can do some testing without any meters by disconnecting low voltage wires to see if the fuse still blows. This is just process of elimination. The simplest way to find it is to find the short to ground with an ohmmeter. So check the wire outside and turn power off the the ac unit and remove the control panel and look to see if something doesn't look right and let me know what you find. We can go on from there.
Very good then Michelle, I'll look for a post back later. Thanks much, -Brian