I recently had Mitsubishi Minisplits installed throughout my 1950's cape cod in eastern PA. While the 2 units on the first floor control the humidity and keep it at a comfortable level, the 2nd floor is a different story. The 3 head units upstairs are hooked to a multi-zone Mitsu OD unit and all 3, particularly the 2 small bedrooms (~100sqft, 6000BTU units) have high RH (sometimes 70%+ @ 72F) after they hit their temp setting. The installer is confident they are sized properly (he showed me the manualJ calcs which were based on his pretty thorough measurements of the house) and also, given the fact the Mitsu's are variable speed, they can ramp up and down as needed. They've replaced the main board on the outdoor unit (all under warranty) and installed wireless thermostats, but still high RH. I recently had the crawl space above air sealed and additional insulation blown, so I'm pretty confident we're not getting high humidity air in.
Even with keeping the fan speed on low (to slow amt of air over the coils) the problem still remains. However the RH isnt always high - if we lower the room temps and the units call for cooling, the RH drops down to the 50-55% range. The DRYING mode on teh units isn't helpful because it basically drops the room temp down as it periodically cools.
Is this just a inherit flaw of the Mitsu minisplits? It's been pretty frustrating that we can't get a constant, reasonable RH level. As I said - the same system is installed on my first floor is working great with comfortable RH. The installer is somewhat stumped - saying b/c we have hardwood floors they suck up and give off alot of humidity - as well as the extreme heat we've had. But I feel like that's an excuse - there should be a better way to fix this problem?