Prevent mold in mini-split heads

Do your best to keep it reasonably clean.

That's about all you can do with anything that spends it's entire life at 100% RH.

I've never seen a minisplit that collects water? The ceiling cassettes do but as far as I know they're uncommon.

All of the ones I've installed had a hose that runs directly outside and drains freely by gravity.

I've never seen a window unit with more than a very tiny area to drain and they always clog to the point I started drilling holes in all of mine to let it just drain outside and keep the pan dry rather than the stupid slinger setup. Once I did that they almost never clogged again. A GE Carrycool has something like a little 1/4" slot behind the compressor.

I also had two that had a plug you could remove which was nice but any that didn't have it I just drilled a 1/2" hole in.

Standard minisplits have a small tray under the evap that's pitched towards the hose and it runs outside via gravity. Much better than any window unit I've worked on and they do not collect water unless you neglect it and it clogs. Same thing as a standard split really.

These systems need to run near 100% from time to time. Very poor dehumidification at lower speeds.

I feel like the mold issue is overblown in standard residential applications. It's a good idea to get in and make sure everything is staying clean periodically, best to clean the coil as well (once a year) Maintain proper airflow by staying up on the main air filter (vacuum or wash clean every two weeks give or take, if it is filthy when you clean it you waited too long) most also have a thicker filter that should be cleaned every couple of months. They do make UV lights that go inside mini split heads which I have mixed feelings about, they will keep mold at bay and should keep the indoor coil cleaner. I've only seen mold become an issue in my area when people are using these in big marijuana greenhouse operations (these are legal here and big money for our industry) to counter the massive lights being used. There is a ton of moisture in those spaces, they closely resemble a pool room, and mold growth is a big concern in that application.

Several things have been bothering me on this topic

First, why would a minisplit, or any inverter system not dehumidify well at lower speeds? Wouldn't it be better, if anything? They usually run EEVs, so I'd expect the evaporator to be good and cold especially at lower output levels. I

Next, cooling too fast to dehumidify. I have ZERO experience with water chilled systems etc, I've only worked with small (tiny, relatively speaking) systems with the evaporator directly cooling the air. How can it cool the air without dehumidifying? Even if it does it rapidly, it still had to pull the air through the evaporator which was at a fairly low dew point. If the system moves more air faster, it's still moving X amount of air through the evaporator, typically 350-400 cfm per ton. Honestly, I don't understand and I have not personally seen an oversized system not dehumidify well. I don't get it? Can someone please explain?

To add to @pecmsg 's comment, I have a feeling he's talking about keeping the blower wheel clean.

They seem to crud up if they run slow most of the time, running it fast blows the stuff out.

I think the mini splits dehumidify fine. The only time I have seen humidity problems cause by the equipment is an on-off unit that is too large. It will cool the space and shut down but doesn't get enough run time to dehumidify especially at lighter loads.

2 things here.

On systems that modulate you have to match the airflow to the modulating refrigerant flow to keep the coil full of refrigerant and cold for good dehumidification.

on single stage systems it is an issue but i'm not sure why. my system is oversized because i had plans to finish the attic and cool it from the same system 25 years ago. it gets clammy when it is not super hot and very damp out. Maybe it has to do with the off time between cycles and infiltration.

I suspect with the kitchen you will have to clean the blower and evaporator regularly with degreaser to keep it from becoming felt covered.

The amount of dust and crap in the air that is unseen is unreal. My GF cleans and vacuums all the time, and we no longer have cats.

I have two HEPA air cleaners one in the LR and one in the bedroom due to allergies.

The LR unit has a "change filter light" on it that blinks when 90 days are up.

The amount of stuff it filters out in 90 days is unreal. Looks like a lint filled dryer lint screen. I can imagine that sticking to a window unit or mini split coil or filter.

I have noticed using the air cleaner that our through the wall AC units the filters stay pretty clean

we see projectors that for the most part get little in their filters. in the office and classroom spaces there is very little dust. where we have projectors near surgical units where there are lots of linens we see lots of dust. things that intake high off the floor like a projector or window ace or most RTUs with ceiling returns get little dust in their filters. forced air returns near the floor tend to pick up a lot of dust.

I've never seen a mini split that is not mold box after about two years. Even when it drains perfectly and the hole to the outside is sealed perfectly. Sizing doesn't seem to make any difference, the more it runs the more filthy it gets. The UV lights make the plastic brittle. The only thing you can do is deep clean them with the bib kit and power sprayer. Maybe this doesn't happen in less humid areas, but it's a real problem in the northeast. I wouldn't want one in my home. Although some manufacturers have models that are easier to clean than others.