Mini-split sizing?

[danb35]

I'm building a 30' x 40' x 14' steel building as a workshop, and I'm planning on a mini-split system to cool (and occasionally heat, but mostly cool) the shop. But I'm having trouble getting consistent information about the BTU requirements--I'm seeing numbers ranging from 16k BTU to 95k. The latter is an outlier, but even discarding that I'm seeing quite a wide range. Hoping for a sanity check on what I'd need--I definitely don't want to undersize the unit, but I don't want to significantly oversize it either (it costs more, and what I'm reading indicates that it reduces comfort levels anyway).

I'm in coastal GA near Savannah. The building is 30' x 40', with the 40' dimension and the roof ridge running N-S. It's a metal building with a 3" fiberglass insulation blanket covering walls and roof/ceiling. Eave height is 14', with a 4:12 roof pitch. It will be on a concrete slab.

There are eight 32" x 52" windows, three each in the east and west walls, and two in the south wall. Two 10' x 10' roll-up doors, and one walk door, in the north wall.

The interior is open, except for an 8' high, 8' wide mezzanine across the south end of the building. Below that, a 24' section is enclosed--both of the windows in the south wall are in that section.

I'll be the only occupant of the building most of the time, and it will be for hobby use--I won't be there full time or anything close to it.

Hopefully I've given enough information about the building--how should I size the unit?

 

[Enjoytheride]

I hade my 30'x40'x10' sized with a 2.5 Ton / 30,000 btu Carrier unit like the attached below but with 3 head units.. I have not pulled the trigger and really on the fence if a mini split is the direction to go in. Looking at around $4500 complete and not sure it will meet my expectations.

https://www.theacoutlet.com/38mgrq3...I34pSNQYf_jSeXqJpm-6Gb8MJYg3h9RcaAtPqEALw_wcB

 

[theoldwizard1]

I'm building a 30' x 40' x 14' steel building as a workshop, and I'm planning on a mini-split system to cool (and occasionally heat, but mostly cool) the shop. But I'm having trouble getting consistent information about the BTU requirements ...

This is ALWAYS a problem !

You have a lot of windows and doors and not a lot of insulation. My only advice is aim high !

 

[justinjoyal]

A proper cooling load calculation is the first thing you gotta get.

www.loadcalc.net

 

[danb35]

www.loadcalc.net

...and that's the one that gave the lowest answer--assuming I entered the details correctly. That's good news/bad news--it accounts for lots of detail that the simpler estimates miss, but in so doing, it makes it possible to get those details wrong.

Edit: but with a closer look at that site, I see some entries I'd missed before, which bring its output into a comparable range with some others. That's making it look like 2-2.5 tons should do the job.

 

[u3b3rg33k]

if your goal is to keep it dry and comfortable, undersizing a system will not hurt you. if you are wrong and it's not big enough, it will by nice and dry inside, and just not as cool as you'd like.

since minisplits have great turndown ratios, find out what yours is, and know that it will short-cycle if the load is below that BTU number. A single head unit usually has a good turndown while a multi-split will have much less turndown (higher minimum).

3" of fiberglass isn't a lot, but it's still insulation. 1200 sqft isn't a big space to cool. 95k is an insane number if you have insulation. not famiiar with GA, but 16k would probably be just fine. upgrade the ceiling insulation with extra money - you pay to insulate once, you pay to heat/cool every time. it'd probably be safe to put in a 2 ton single head unit and call it good.

 

[danb35]

it'd probably be safe to put in a 2 ton single head unit and call it good.

...which is consistent with loadcalc.net, from what I can tell.

There is a further complication: there's a 24' L x 8' W x 8' H room across the south end of the building, which will house (among other things) my server rack. The equipment in that rack burns about 1 kW, and runs 24 x 7 x 365. It doesn't need to be kept cold in there, but at least reasonably cool--definitely not higher than 80F, and 75F would be better.

At a minimum, I'd need a second head in that room, but I'm thinking a second unit at 9k-12k would be the way to go. That would be a higher-SEER unit--running full-time, the ROI looks like about 2 years compared to a lower-SEER model.

 

[u3b3rg33k]

1kW is 3400BTU - that multi split you linked only does one mode at a time (heating OR cooling), and the cooling range of the multi-split is 5810-21940BTU/hr, so it's too big to drop in a small closet if you have no other cooling load. with a small closet like that you can short cycle something to death very easily.

 

[danb35]

The multi unit was someone else's link, not mine; I'm looking at something like this for the smaller room: https://www.highseer.com/ultra-high...ductless-mini-split-heat-pump-wys009gm22.html

The loadcalc.net site indicates a cooling load of 2800 BTU, based only on the exterior walls of that room, and not accounting for the equipment. If I'm reading the datasheet correctly for the unit I linked above, its cooling range is 3785-11941.

 

[u3b3rg33k]

i's hard to argue with that price, assuming it works like it claims.

 

[eddieK]

I'm building a 30' x 40' x 14' steel building as a workshop, and I'm planning on a mini-split system to cool (and occasionally heat, but mostly cool) the shop. But I'm having trouble getting consistent information about the BTU requirements--I'm seeing numbers ranging from 16k BTU to 95k. The latter is an outlier, but even discarding that I'm seeing quite a wide range. Hoping for a sanity check on what I'd need--I definitely don't want to undersize the unit, but I don't want to significantly oversize it either (it costs more, and what I'm reading indicates that it reduces comfort levels anyway).

I'm in coastal GA near Savannah. The building is 30' x 40', with the 40' dimension and the roof ridge running N-S. It's a metal building with a 3" fiberglass insulation blanket covering walls and roof/ceiling. Eave height is 14', with a 4:12 roof pitch. It will be on a concrete slab.

There are eight 32" x 52" windows, three each in the east and west walls, and two in the south wall. Two 10' x 10' roll-up doors, and one walk door, in the north wall.

The interior is open, except for an 8' high, 8' wide mezzanine across the south end of the building. Below that, a 24' section is enclosed--both of the windows in the south wall are in that section.

I'll be the only occupant of the building most of the time, and it will be for hobby use--I won't be there full time or anything close to it.

Hopefully I've given enough information about the building--how should I size the unit?

Okay - 1,200 sf with 14' ceiling, roughly 17,000 cf.

Off the top of my head I would recommend a high seer 22 + (preferrable 30) SEER single zone 30 or 36k that senses motion/activity and 9,000 btu single unit (high seer as well)for the server room . I'm assuming high temps in summer, 90's plus and high humidity.

When you use a multi - zone unit you lose SEER rating from 22 plus down to 16. When unoccupied keep the temp at a high setting, say 80 degrees, this will knock down the humidity and help you achieve a reasonable temp in short order.

Brands I recommend are Gree - (A/C Pro) - Mitsubishi (kinda pricey) - Fujitsu - (mid range cost)

 

[theoldwizard1]

if your goal is to keep it dry and comfortable, undersizing a system will not hurt you. if you are wrong and it's not big enough, it will by nice and dry inside, and just not as cool as you'd like.

Oversizing with mini-splits should not be an issue. Just turn one or more off !

 

[theoldwizard1]

The multi unit was someone else's link, not mine; I'm looking at something like this for the smaller room: https://www.highseer.com/ultra-high...ductless-mini-split-heat-pump-wys009gm22.html

The price on that is very impressive, but like any appliance, if it goe ****-up, can you get it repaired ?

 

[u3b3rg33k]

Oversizing with mini-splits should not be an issue. Just turn one or more off !

it's usually not with single zone gear - but it is for multi-split systems. That's what I thought he was looking when I said it. i'm sure it would be for a 95k system, but i doubt OP is going to buy a system that big!

 

[danb35]

i'm sure it would be for a 95k system, but i doubt OP is going to buy a system that big!

No, if my needs actually called for 8 tons I'd be looking at changing the problem. Fortunately, it doesn't sound like that's the case at all.

 

[finn]

The price on that is very impressive, but like any appliance, if it goe ****-up, can you get it repaired ?

For less than $700, does it matter?

Assuming the unit can be verified to be halfway reliable.