A fellow employee recently purchased a new (old) residence with substantial remodeling and three HVAC units. He was having terrible problems with high humidity, especially with one of the three.
As the result of my looking around this forum and finding several things mentioned here about causing high humidity (return air from outside, running the fan continuously, etc.) and informing him, he discovered that the return air plenum was comprised of two different diameters of hard pipe with about 3" diameter difference between them! Installer had just wrapped that mistake with insulation; also did not seal the joint between the filter grille and the return air duct. He was also running the fan continuously, mainly to use as "noise masking" at night. Humidity, according to him, has dropped from about 72% down to 52% as the result of these two fixes.
Wouldn't have known about these fixes without this forum. Thanks!!