I had a really bad situation where a semi-clogged sink drain and an AC drain were feeding into each other and really creating problems for each other. After having to cut out some plumbing and blasting it with the outside hose, things are working again. There are also some smells that can't be "unsmelled". For the next round, hard water calcium build up in the AC drain line was causing blocks. After that was broken free, things started working again. For those who can handle the semi-dangerous chemistry and don't have metal drain pipes, it's much better to regularly flush the line with dilute muriatic acid instead of bleach or even drain opener chemicals. Acids dissolve the scale build up. I did that for a week to make sure the drain line was really clear. In the future, I'll be using acid instead of bleach in the bi-yearly flush. As a safety warning, NEVER directly swap from an acid to bleach (alkaline) or drain openers without a heavy water flush. Direct mixing acids and alkalines create a lot of heat and toxic fumes that can melt your lungs.
