Do mini splits dehumidify less than ducted systems?

Motorapido

I know, I know, so many variables. But suppose you had an absolutely perfectly specified and installed traditional ducted system with compressor outside and air handler inside, measured against a perfectly specified and installed mini split system. Which will dehumidify more effectively?
For instance, in my house, I have a traditional ducted system downstairs that takes care of only the downstairs, and a more recently added mini split system to cool the upstairs. I always find a pretty big difference in humidity, with the upstairs split system not dehumidifying as well as the downstairs ducted traditional system. And at my vacation house, I just had a Mitsubishi mini split system installed. I love the cold air and the silent operation. But as an example, today's outdoor relative humidity is 77 percent and indoors it is 64 percent (high temperature today was 88) -- and the system has been running for 48 hours, and the house is very well air-leak sealed. I would have expected to see an indoor relative humidity of around 55 percent.
Can one make the general statement that traditional systems dehumidify more effectively than mini split systems, or is this just my individual experience?

GroundUp

I feel like the dehumidification factor should be very similar if not the same, in theory. However I have a ducted system in my house and a ductless in my shops (one 2x6 with fiberglass and one solid 5" high density foam, like zero air exchange), and the humidity in my shops are always high regardless of setpoint unless I also run a dehumidifier.

EBEBRATT-Ed

Could the mini split be oversized? Humidifacation control gets worse with oversizing. A typical ducted system will give a 20 deg air split, less in high humidity more in low humidity.

Don't know about ductless.

Jamie Hall

One thing to remember with any AC unit and dehumidification: you should be looking at dewpoint, not relative humidity. The best a unit can do is drop the dewpoint to the coil temperature. Thus if the unit can modulate, if it is running on a light load the coil temperature will be warmer (over the whole coil) and the dewpoint -- and humidity -- will stay higher. Not sure whether that's a factor here.

Hot_water_fan

The comparison is ductless vs. ducted - “minisplits” can be either. It’s extremely confusing terminology the industry uses. If the sensible heat factor is the same, they should remove the same amount of humidity? At least to my ears. 

yesimon

Use the "dry" mode on your mini-split. Normal "cool" mode cranks up the fan speed to increase SEER but don't dehumidify as much.

bburd

Right. Lower evaporator fan speed equals more dehumidification.

TAG

There is this idea that you can't oversize a mini. Since they modulate you can't oversize them ... right? This seems to be very common with the multi-head systems as many installers don't look at the actual outputs of the heads.

Since so many split systems are way oversized my initial feeling was mini's did a better of dehumidifying a room. Having installed a two speed compressor over 15 years ago and now a few VS compressors I know the split systems work just as well.

One note -- having a beach house I can attest to how fast and how much humidity can be pumped tot he second floor when the system is off and it can take a while to remove it.

Jamie Hall

bburd said:

Right. Lower evaporator fan speed equals more dehumidification.

That's really the key to it -- not so much that the fan speed is lower, but because that means the coil temperature is lower and hence the exiting dewpoint. Up to a point, of course -- you do need to have enough air flow to cool the space, too!

ChrisJ

Jamie Hall said:

bburd said:

Right. Lower evaporator fan speed equals more dehumidification.

That's really the key to it -- not so much that the fan speed is lower, but because that means the coil temperature is lower and hence the exiting dewpoint. Up to a point, of course -- you do need to have enough air flow to cool the space, too!

I often run the fan on our minisplit in the office on a manual slower setting rather than auto.
Personally, I'd take a warmer dry room over a cooler damp room.

But, by paying attention and adjusting it I can usually keep it nice and dry and cool at the same time. Usually.

And I oversized this single head minisplit. It's 12K cooling in 300sqft office.

ratio

I was told by an Old Guy to leave the fan on auto so that the mini split computer can use the fan speed to help with humidity control, but what @Jamie Hall said about modulating makes a lot of sense to me.

ChrisJ

ratio said:

I was told by an Old Guy to leave the fan on auto so that the mini split computer can use the fan speed to help with humidity control, but what @Jamie Hall said about modulating makes a lot of sense to me.

Doesn't work great on ours.
It ramps up and down too much. So, I do it manually.

TAG

The hypers don't ramp down as much ..... if you look at the specifications for the Mitsubishi GL12 it can ramp down to 1500btu's. 1/8 The others are much higher 3600BTU. They typically are 28% ... but that GL12 will go lower than than the 9k. It's a great unit if you don't need the hyper.

These are singles .... the multi often have issues as each head is limited to the min from the outdoor unit .... gets really bad when people have some of the heads off. Can't really do that with them in most cases.

Matching that minimum is the key as the unit will run very long and remove the humidity

jumper

EBEBRATT-Ed said:

Could the mini split be oversized? Humidifacation control gets worse with oversizing. A typical ducted system will give a 20 deg air split, less in high humidity more in low humidity.

Don't know about ductless.

When sizing A/C err on too little and suck it up on rare occasion when A/C can't keep up? In addition to better humidity control temperature swing will be less.