A few years ago I was living in a home where the air handler was up in the attic. I neglected to ever maintain the air handler's drain pipe so it eventually clogged up and started leaking water down through the bathroom's exhaust fan.
That unfortunate experience taught me to check, clean, and flush out the pipe at least once a year. I took pictures of the process this time and created a quick guide.
Here's the gallery page -
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A wet/dry vacuum makes the procedure easier, but just flushing out the pipe with water is usually enough to keep the pipe clear.
Cheers, Paul Michaels Ft. Lauderdale, FL