Is the insulation new ? My bet is the house is tighter than it used to be but I need more info.

What does the J calc say your cooling load is, and what tonnage of your mini split system? If you were in my area, it would sound like your system is oversized for cooling and doesn't run long enough to reduce humidity. Could be a climate situation though - What's your general area/climate?

The J calc says 3 tons, but we had to go over because we needed 14btus for my first floor and Fujitsu only sells 12 and 18. I checked with the engineer first and he said if I went with the 18 it would be ok because everything is variable speed.

I’m in Philadelphia, so 80’s/90’s and humid during the summer.

So we ended up with a 4ton outdoor halcyon compressor.

Patricia, yes the insulation is new since we had the old ductless units.

4 ton overall for a 3 ton calc, that seems a bit high and could be contributing. I wouldn't expect 18k btu vs a load calc of 14k to be that oversized as to cause humidity that high (only 1/3 ton), but I'm not a pro. Just the engineer kid of an HVAC tech that's learned a lot about sizing and ventilation in our new home design. Some questions that may help others, how old is the house? To Patricia's point, have you done any renovation or insulation recently that would have made the house "tighter"? Do you have an ERV or HRV for ventilation?

Thanks so much for the help!
Ya the engineer told me that since things are variable, oversizing isn’t a problem. But at this point I don’t know who to believe or what is up or down.

And it is true that I’ve noticed the unit that supplies my bedroom does turn on for only 5 minutes at a time and then off again. I’m confused about how that is variable speed.

The Manual J said:
3rd Floor: 8k
2nd Floor: 14k
1st Floor: 14k
Total: 36k

What we did:
3rd Floor: 12k
2nd Floor: (2) x 7k units
1st Floor: 18k
Total: 44k using a 4ton outdoor unit with one head unused

What’s an ERV or HRV?
House is 100 years old brick and some wood framed in a few places.
We spray foamed the roofs.
2 party walls shared with neighbors. Exterior walls are plaster directly on brick or wood framed with fiberglass insulation.

Thank you all!

Humid air is less dense/more buoyant than dry air. Accordingly, the relative humidity will tend to be highest on the upper-most level of your home. Unfortunately, your 3rd floor mini split is 50% oversized. Oversizing results in shorter cycle times and reduced dehumidification. Your best option is to reduce the fan speed of the mini split cassette to the lowest possible setting in order to extend the run time and maximize dehumidification.

The house is a lot tighter than it was before too, with all the new insulation. Things can be very different. Before a house that old would have had a lot of air leaks and air exchange and now, it doesnt, which could explain all the extra humidity.

Air leakage in the floors, walls, and 3rd floor ceiling would allow hot, humid air to enter the home, increasing the relative humidity. The attic, however, can be a different case. If the OP has a sealed, unvented attic it could indeed elevate the relative humidity compared with a conventional, vented attic.

@Erin Smith is the spray foam insulation installed on the floor of the attic (in the 3rd floor ceiling joists) or under the roof deck? If it's under the roof deck is there a vapor diffusion port or other type of vent installed? Is any conditioned air introduced into the attic? How does the attic relative humidity compare with the 3rd floor?

That was my thinking that the spaces are now more airtight so nay humidity tha enters can't escape .

Thank you all so much for the help!

Our 3rd floor (top floor) doesn’t have an attic. It never had a true attic to begin with, but it did have airspace above an old plaster ceiling and below the roof deck. We tore down the plaster ceiling, spray foamed the underside of the roof deck between the rafters, put up a new, taller ceiling that followed the roofline. Unfortunately there is no vent. Perhaps I could add one now/after the fact?

Regarding the 3rd floor cassette being 50% oversized - it seems to be running all the time as it is despite the oversizing. Maybe because my townhouse stairwell naturally funnels hot air to the top floor?

My (2) 2nd floor units are definitely short cycling. There are (3) small bathroom/laundry registers that have not been roughed-in because those rooms are waiting to be renovated shortly and the installer is waiting until the ceilings are framed out so he can place the registers. I’m thinking that too much air is coming out of the bedroom registers right now because of this and satisfying the thermostat too quickly. I’m also wondering if the tstats should be moved from the bedrooms into the hallway near the returns.

I’m particularly confused by the idea of oversizing. I thought these units were supposed to be variable speed and adjust to meet the demand of the spaces? When I called the engineer and told him he called for 14k on my first floor and I was considering the nearest larger size of 18k, and told him the installer was worried about short cycling, he said “not to worry, you can’t oversize them because they’re variable speed”. I don’t hear the cassettes varying speed, they just click on and off.

I’m so frustrated by the idea that these things are so finicky. It just doesn’t make any sense to me. If I wanted my room temp to be 77 and it’s only 85 degrees out, of course the units don’t need to run all the time. Shouldn’t they be designed to automatically spit out a tiny amount of cold air over a long period of time?

I really appreciate all the time you all are taking to give me advice here.

If the inside air handler is the Fujitsu ADUH18LUAS1, this may be the Operation Manual. It mentions a "Dry Mode" which is designed to help reduce humidity. Also, there's Economy Operation which may cause the unit to run longer, better dehumidifying the room.

"I’m also wondering if the tstats should be moved from the bedrooms into the hallway near the returns."

The Best and Worst Places to Install a Thermostat...

wdccruise - thank you! I will check the product numbers on the units this evening.

@Erin Smith,

If the rafters are filled to full depth with spray foam and the underside of the rafters is covered with drywall there is no vent required because there is no attic air volume in which moisture can accumulate. You will, of course, have higher relative humidity on the 3rd floor due to moist air being more buoyant, so the mini split heat pump for the 3rd floor will need to deal with that.

You should be able to vary the fan speed of the indoor cassettes and/or utilize a dry mode or similar dehumidification mode. It's the speed of the outdoor unit that ramps up/down to adjust to the required cooling capacity.

Mini splits are variable speed units and have good turndown capability, but you can oversize them. If the units have a 4:1 turndown ratio, a 12K Btu/hr unit would bottom out at 3K Btu/hr; an 8K Btu/hr unit would bottom out at 2K/hr

We turned all 4 cassettes on ‘dry mode’ for a full 24 hours, and I went around and measured everything. My husband read about it and said dry mode runs them on low speed continuously, and it seems like that’s what they did, never turned off. I’d say dry mode lowered the RH a few percentage points here and there, but we were still between 57-62% generally.

Could my return sizes need to be increased? I have (1) linear 6x36, (3) 14x14.

Could the locations of the returns be making this much of a difference? They upper level returns are all in hallways and not in the bedrooms.

Are these units just less effective than ductless splits?

My very nice neighbor down the street is borrowing my portable thermometer/hygrometer today and testing her house be because it’s almost identical to mine.

Many forced-air HVAC systems are oversized with respect to required cooling load and most ductwork is undersized with respect to required return air flow.

You should have a return in each bedroom for comfort. In the interim, you may find some improvement by leaving the bedroom doors open.