I built a pretty good house last year, heated and cooled by a 12k btu Mini Split on 1st floor and 9k btu Mini Split on 2nd floor. Toward the end of summer last year our first floor unit was not draining condensate then after partially insulting our basement ceiling with dense pack cellulose, our 2nd floor unit was not draining at the start of this summer. Long story short, after investigating many ideas to the root of the problem, we just recently discovered that the flexible condensate line was pushed around when I dense packed basement ceiling. The corrugated flex, on a lateral run in my basement ceiling, got pushed into a “trap” formation on both 1st and 2nd floor lines and would not allow the units to drain. LESSON LEARNED: always pipe condensate lines from mini splits in hard 3/4″ PVC pipe with generous amount of hangers so that the dense pack cellulose process does not push the drain lines around or break a connection apart. A powerful dense pack machine can really deliver some force and all condensate lines, vertical and horizontal, should be hard piped in my opinion.
We changed to hard pipe in basement ceiling and now the units drain just fine. What a relief!!