A/C vinegar
I've had many issues in the past with one of my bathroom sinks getting clogged over time. Specifically just one bathroom drain line gets clogged with gray sludge which I initially thought was just normal drain stuff, however it seemed weird that I only seem to get it in large amounts on this one drain. I realized that my A/C condensation drain line goes to this sink because there is an extra pipe coming out of the wall. My HVAC guy recommended periodically putting vinegar into my A/C condensation drain line to reduce clogging in the lines. However, on one of my A/C units (in my attic) the exhaust vent is kind of blocking access to easily pour vinegar into the pipe. It's hard to tell from the pic, but it's nearly impossible to pour something into the line without spilling everywhere. So I used some leftover PVC from previous HVAC installs, and now it's much easier to get vinegar into the pipes without making a mess.