Mini-Splits, How many units.

[N_Jay]

I have a 40X42 building that I will be adding a (ore 2) mini-split system for AC (mostly dehumidifying) and heat (Juts to keep reasonable (60 ish).
I am in NC near Charlotte, so not on the coast and not in the mountains.

There ae plenty of calculators to tell me how many BTU I need, but I can't find anything to tell me how big of an areas a single wall unit will cover, so I can decide between 2, 3, 4 ort 5 units.

Yes, less is cheaper, so there is some tradeoff.

 

[justinjoyal]

For that space I would do a single large one if trying to some money and space. If not an issue, go with 2 smaller ones.

You will get better air distribution, plus you can use only one of them when the load is low for more control over %RH.

 

[aluminumwelder]

Well what btu do you need???? R value of walls and have you done manual j calc? Just buy some fans to move air around is simplest

 

[vavet]

Mr Cool has packages with one outdoor unit that connects to multiple indoor units. That might walk the line between good air distribution and lower cost.
I have an 18k btu in my 24x32 and there are certainly cooler spots and warmer spots. For more than twice the space, I think you’d do well to have at least two wall units and maybe some slow moving ceiling fans to keep things moving.

 

[N_Jay]

The calculators show between 24K and 48K, but they don't take a lot into account.
First they seem targeted at the BTUs needed to keep the building in "normal" occupancy temps.
Second they don't (and I don't) have a good way to account for the lack of insulation and air infiltration around the large doors.

Yes, I am looking at the compressor units that handle more than one indoor unit, and given the size of the building and the limited good locations for the compressors, line length starts becoming a cost.
My first "guess" was going with two units each driving two wall units, one maybe having a third close to the bathroom area.

A few things I read talked about one unit being good for a single room "up to 20' x 20' ", since that is about as big as a room would be in a home. That also pointed towards 4 wall units.

 

[PoorUB]

You didn't mention ceiling height, but I would put in two 24K BTU units and call it good. It is a shop, not your living room and if the air disribution is not perfect I doubt you will notice. A ceiling fan or two will help move the air around.

 

[rjacobs]

Im planning a 40x40 building and planning on running 2, 24k BTU Mr. Cool units on opposite walls.

The cost over the 18k btu units isnt much and the 18k btu units are rated for 750 sq. ft.... For my use slightly under sized would probably be ok, but for an additional 200 bucks per unit, the cost isnt much to upgrade. The 24k BTU units are rated for 1000 sq. ft.

 

[N_Jay]

Ceiling is 14 feet at the corners of a 30x40 section and 11 at edge of a 12 x 40 lean-to (Hence 40 x 42 area)
Center of 30X40 section is about 17.
The interior is open, but the lean-to is mostly storage (need to keep reasonably dehumidified) and the bathroom area (will have walls, but open from 8" up)
It is a new steel building with the (so called) 2" insulation.
I am thinking about using a layer of plastic bubble insulation to create a separate air space "attic" and venting that space, but that is a project for another year. (Depending on how well it cools.)

 

[PoorUB]

High side walls, a lot of sun, I certainly would not put in less than 48K BTU.