A/C for a 70 x 96 - Multiple Mini-Splits?

[Lonnies Performance]

I'm in the process of trying to figure out the best way to cool my 70 x 96 steel building. It has R19 insulation in walls & ceiling with 9 insulated Clopay R18.4 garage doors. It has (5) 9x8 & (2) 12 x 12 doors on front wall & (2) 12x12 on the right side wall.

Since the front is essentially all doors & I want to put pallet racks along the rear wall for storage... the two 70ft side walls are a natural place for the A/C units.

My ceiling height is 15'-3" (edge) to 18'-3" (center) so I have a lot of volume to cool. About 105,840 cu. ft. based outside dimensions.

Any thoughts on using 2 mini-splits on each side wall?
I could mount them between 8' & 12' off the floor to the steel existing structure. I have an electrical sub panel on each side wall so wiring would be easy (about 10'-15' for each unit).

Here is my reasoning... single zone systems have a higher Seer rating than multi-zone units & the cost in most cases is cheaper with single zone units.

Also saving the cost of duct work sounds attractive.

Four 30,000 BTU units (18-20 Seer) give me about 10 tons of cooling & range from $1,800-2,200 ea. whereas a 10 ton conventional split or two 5 ton units get pretty expensive once I do all the duct work.

I'm not looking for 70 degree comfort, but would like to keep the humidity out, maybe keeping it a 80 when unoccupied (or controlling to humidity) & 75 or so when I'm in it (mostly nights & weekends).

I also would typically work on 1 side so I don't need perfect distribution & could just bump the temp down where I'm at.

I have in floor radiant heat (2" under floor insulation), so I don't really need heat capabilities.

Any thoughts or recommendations?

 

[justinjoyal]

If primary goal is proper cooling (meaning temperature AND humdity control, for overall comfort) then you need a cooling load calculation before you can decide what size unit(s) you will choose.

I am not a fan of mini-splits in very large spaces.

What kinda work/activities will be done in the garage?

 

[Lonnies Performance]

I've seen many cooling load calculations & the big variable is the actual insulation quality.

Problem with the insulation is it is compressed between the siding & purlins/girts so its really only full thickness in the middle of span. The actual installed value is a big guess in my opinion.

I've heard anywhere from 6-10 tons depending on what they have been trying to sell me.

The building is primarily car storage with some hobby car fabrication/restoration nights & weekends.

 

[PurdueSD]

I have 4 tons in SW Indiana servicing my 2900 sqft shop. It has kept up in its first winter with below average temps. Your shop height is is about 2' higher overall and that will add a bunch more volume. Im just guessing but i think your about 2-4 tons short. I'd definatly install some ceiling fans no matter what.

One thing nobody told me was the lowest set point for heat on minis is 62 degrees. Its a bit of a bummer, 62 is about 5 degrees higher than id prefer to keep my shop. I did my research and missed it.

 

[theoldwizard1]

Doubling the insulation in the ceiling will make a BIG difference !

 

[kj_mustang]

Any thoughts on using 2 mini-splits on each side wall?
I could mount them between 8' & 12' off the floor to the steel existing structure.

Have fun cleaning the filters on all those units every month with them up that high.

 

[Lonnies Performance]

Have fun cleaning the filters on all those units every month with them up that high.

LOL... Lately, since building the garage, I've been getting used to working on a lift.

I never needed a 17' ladder before, but I now have one along with a fork lift & a safety harness.

 

[justinjoyal]

It’s gonna be difficult to come up with a representative cooling load if you don’t know the R-value you’re working with.

Too much tonnage will cool the air but will not remove enough humidity due to short cycling. Not so much a problem if living in a very dry climate. But short cycling is never good for long-term reliability.

 

[Mr onetwo]

As stated before....you need a very accurate study of heating and cooling loads.First, most important step.Look into an "air rotation" heating system.Put one in an aircraft repair hanger once...it worked fantastically well.You could not even tell that it was running.They can be oil or gas direct fired, steam or hot water coil based with separate boiler and can have a DX coil or chiller setup installed for space cooling.http://www.thermocycler.com/Thermo_Rotationr_/thermo_rotationr_.html

 

[Rob7181]

I have 4 tons in SW Indiana servicing my 2900 sqft shop. It has kept up in its first winter with below average temps. Your shop height is is about 2' higher overall and that will add a bunch more volume. Im just guessing but i think your about 2-4 tons short. I'd definatly install some ceiling fans no matter what.

One thing nobody told me was the lowest set point for heat on minis is 62 degrees. Its a bit of a bummer, 62 is about 5 degrees higher than id prefer to keep my shop. I did my research and missed it.

Are your units Mitsubishi? With the add on Kumo Cloud, you can lower the set point to 50 degrees. I just started a thread, as no one told me this until I already added them. It was a nice surprise!

 

[Lonnies Performance]

I have 4 tons in SW Indiana servicing my 2900 sqft shop. It has kept up in its first winter with below average temps. Your shop height is is about 2' higher overall and that will add a bunch more volume. Im just guessing but i think your about 2-4 tons short. I'd definatly install some ceiling fans no matter what.

A typical Sq.Ft. rough calc states about 500-600 Sq.Ft. per ton for heat which is on target for what you have.

Here is the interesting part....

The locals want to sell me about 10 tons based on a heat pump calculation.
The Load calc worked out to about 4 tons actual, which seemed light to me.

My calcs show 29,000 BTU for cooling & 10,000 BTU for latent heat (dehumidification) assuming a tight seal on doors & R19 insulation. Thats 3.25 tons...

I have my doubts that the insulation really calculates to R19 as it is compressed under the purlins. Even with a generous reduction to R11 insulation, the load still only goes up by a ton.

Here is what I'm thinking.... If I have multiple units, nothing says I have to run all of them. In June maybe 1-2 units are enough, August may require 3-4 units. It limits the oversizing concerns & still lets me bring the temp down more rapidly if I decide to change the temp in a more rapid fashion.
I would have more flexibility with respect to sizing & enjoy the benefits of a higher SEER rating of multiple smaller units. Am I missing something here?

The biggest concern I have is airflow on these units.
I will typically work on 1 side of the shop, so 2 units here will likely keep me cool. If, I'm in the middle of the shop, I would be 50' away from my cooling source(s). I'm not sure of the temperature variation that could exist, or if it even matters. Obviously as stated above, ceiling fans would be a good idea.

 

[justinjoyal]

Mini-splits don’t offer the best air distribution and filtration.

You have a large space to deal with. I would go with a forced air split system. Less maintenance too. You can get built-in electrical strips as backup heat as well if needed.

One outdoor unit, one indoor unit, one thermostat. Much simpler.

 

[theoldwizard1]

I'm in the process of trying to figure out the best way to cool my 70 x 96 steel building. It has R19 insulation in walls & ceiling with 9 insulated Clopay R18.4 garage doors. It has (5) 9x8 & (2) 12 x 12 doors on front wall & (2) 12x12 on the right side wall.

You need to DOUBLE the insulation in your ceiling if you want decent performance of any A/C system

Four 30,000 BTU units (18-20 Seer) give me about 10 tons of cooling & range from $1,800-2,200 ea. ...

First, single zone min-splits are more efficient than multi-zone. Second, larger units have lower SEER ratings (at least the specs I am looking at). In order to get 20 SEER you may have to drop down to about 20,000 BTU each. You really need to read and understand the specs.

With a building that large you are going to need addition fans to move the air around. You would probably be happier with 4 air handlers (2 per compressor) on each of the longer walls, even if you only get 17-18 SEER. (Back to add insulation !)

It the costs work out, I would go for it

 

[theoldwizard1]

One outdoor unit, one indoor unit, one thermostat. Much simpler.

And a HUGE amount of ductwork !

 

[theoldwizard1]

Here is what I'm thinking.... If I have multiple units, nothing says I have to run all of them. In June maybe 1-2 units are enough, August may require 3-4 units.

A friends of a friend built a super insulated house with 2 mini-split. Most of the time the big one is turned off ! The little unit is rated at about 30 SEER, while the big one is about 20 SEER. He has a large ceiling fan in the main room (cathedral ceiling) that runs 24 hours/day.

Now that saves a lot of money !!

 

[justinjoyal]

And a HUGE amount of ductwork !

A simple lenght of supply duct and a main return near the air handler.

Very simple and cost effective.

 

[theoldwizard1]

It’s gonna be difficult to come up with a representative cooling load if you don’t know the R-value you’re working with.

Too much tonnage will cool the air but will not remove enough humidity due to short cycling. Not so much a problem if living in a very dry climate. But short cycling is never good for long-term reliability.

This is a big bonus of multiple mini-splits !! Turn some of them off in "shoulder" months !!! Saves a lot of money also.

 

[justinjoyal]

This is a big bonus of multiple mini-splits !! Turn some of them off in "shoulder" months !!! Saves a lot of money also.

And then you get poor air distribution/cold spots.

A single split system will require less current than the mini-splits to produce all of the needed heat. It will run less while maintaining constant temperatures throughout the shop. Properly sized, it will work great in cooling at design specs while maintaining a comfortable workspace even in shoulder months (even more so using a 2-stage or modulating heatpump.)

I don’t understand why someone would spend more on multiple mini-splits only to have more maintenance to do, consume more power and require multiple wiring circuits, have lesser filtration, poorer air distribution, etc.

 

[Lonnies Performance]

For insulation... more would be nice, but 6700 sq. ft. of insulation plus the ceiling to hold it up is pretty expensive. Remember, there is no attic, only the roof purlins. If I conditioned the space 24/7 I would be more inclined to spring for more insulation.

For wiring I would straddle my sub panels with a unit on either side.... only about 15' wire for each one. I can't do anything on the long walls... 1 side is all doors, the other side will be pallet racks to the ceiling. Four 30A circuits is not a big deal.

I need no heat... only A/C. I have radiant floor heat.

For cost, most mini-splits are around $1000/ton, not much difference from a conventinal split... mini's probably a little cheaper.

For a conventional split system, I would only consider it if I could wall mount it high up off the floor. Running a nearly 100' of supply duct down the center of the building does not appeal to me either.

I do have concerns about the filtration of mini-splits, but I have one in my house garage & it's not that bad to take care of.

I appreciate the comments, as I'm trying to reason all this out for myself & need some good perspectives.

 

[Lonnies Performance]

Update...

Well, I bit the bullet & installed 4 30,000BTU Daikin units.
The line sets ended up being about 15' each.

I planned on 24,000 units, but for some reason, the 24's output typically 21-22,000 BTU, which was below my comfort zone. I'd rather have some excess than not enough.

I mounted them 8' off floor as I'm not concerned how cool the ceiling is, only where I can stand. If you get on a ladder, it's definitely a lot hotter about 12' up.

With these units, I can feel the air movement near the center of the building & the temp at the middle of the building matches the wall temps. I do leave the fans set on high speed.

The weekend I was installing them, we got a 92-93 deg weekend with high humidity to test them out.

I'm happy with my choice as I can selectively run between 1 & 4 units. 2 units maintain temps very well (72 inside) until it gets around 90 outside, then inside temps start to climb to the mid 70's.

Starting at 80 inside, I can drop temps by running 2 units approx 2deg/hr

Current draw ranges from 11.6A each at full output to just over 3A when holding temp.

I put in the Daikin wireless controllers so I can remotely adjust temperature. If I don't plan on using the garage, I can turn it off.

I run 2 diagonal opposite corner units & alternate them to keep the run times equal.
If I get an excessively hot day, I can start a third unit, or when pulling a hot car in to work on, I can start a 3rd or 4th unit to offset the extra heat. 2 are adjacent to my rack so I have the advantage of cold air movement that flows under the car on a lift.

I was nervous at first, but really happy with the outcome... my entire electric bill was $88 for the month of August which includes a 7.5hp compressor & 5000 watts of light.