If you live near the ocean, wouldn't the water that flowed through the dripline contain a little salt from the sea air which, over 15 years, could build up in the line? Maybe the solution is to simply flush the line every few years.

That drain line should be maintained yearly. Each season before it is used run water and bleach through the lines. Probably would not kill you to run some compressed air through there to just to be sure. I do this every year and have never had an issue with a clogged drain. An ounce of prevention is worth a pound of cure. 15 years and no attention will yield strange things.

Thank you both. The salt idea makes the most sense. The outside of the drip line was also caked in white chunky material too, so a buildup of salt seems to be the only reasonable idea. We have been negligent about flushing the lines, and will do so yearly from now on. Would vinegar be another reasonable flushing agent? Not sure which is better at removing the salt residue.

Our AC guy told me to pour bleach (about a quarter cup) once a month to keep the line clean. We're on the Gulf Side

on the bay. I think it's mostly mold that plugs it.

Why would you think air conditioner condensate is distilled water? Distilled water is made by boiling source water, collecting the vapor (steam) and cooling/condensing it back to liquid. Air conditioner cooling coils are cold, not hot, so there's no steam/water vapor to condense. The condensate is moisture that collects on the COLD coils (like on the outside of an iced beverage glass on a hot day) and drips off. Even if the system is a heat pump, the indoor coil doesn't get anywhere near to boiling temperature.

We initially thought it was calcium build up, but of course, it's distilled water - there shouldn't be any calcium in it.

The condensed water is clean no matter what temperature it condense at and probably very clean. Condensing is a layman engineering term as the liquid considered in balance # molecules leaving equal to number of entering the liquid. With condensation more molecules enter the liquid and than leaves.

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The water in your condensate drain in no way as clean as distilled. First there is a debris and dust as the water drips over dusty coil and washes rusted drain pan; than there could be bacteria if pipes stay wet or drain pan, than evaporator coil coatings, at 15 years also contains rust and mechanical .., it could contain some minerals in your air.

Can it clog up drain line? Of course after 15 years most definitely.

I called plumber because I thought my hot water heater was going out because of water on floor. My hot water heater and furnace are next to each other. Nope, the water line from air conditioning was clogged and spilling on to floor. Had to pay the plumber for service call for him to point this out. It was 20+ years and never had a problem and didn't know it was something I should check on. My mom said she clears out theirs once a year with air compressor. The tube on in hers is much smaller diameter than mine so probably clogs easier. Now that I know about it I check it every year...... that reminds me, I should do it again, it's been a while.

This line probably clogged with some gunk is what I called it. I think about a gallon of water came out once I cleared the line.

Thank you all for your great knowledge and information. I don't know why I thought it should be as clean as distilled water. My "salty air" is in fact, air with salty water molecules, and it makes perfect sense that those salty water droplets are what condense in the coil and down the line. Lesson learned, and luckily, we were able to break up the clog ourselves and didn't loose much of value in the flooded closet, so it wasn't an expensive lesson.

Now, every few months we're going to completely flush out the system, followed by a blast or two of compressed air.

Thank you all again for the insight!

I use condensate pumps - one for my Hybrid Hot Water Heater and one for my A/C & Furnace. The water in those pump reservoirs get quite nasty. Years ago I had a stopage similar to the OP's and cleared it with a couple of shots of compressed air.

In any event, many years ago I started adding a tablet of DiversiTech PROTREAT-161 Drain Pam Treatment to each pump reservoir on a monthly basis... They are now clean as a whistle and cloggages are non-existent.

Condensate Treatment

EDIT/PS: When I have my A/C Serviced each year the contractor offers to "Treat/Service" my condensate pump... he adds two of those very same tablets and charges like $25.

BTW, there are many brands of treatment that are available- the one I use is posted simply as an example. YMMV.