Mini split vs. forced air for 2000 sqft shop – WWYD?

[bugman-74]

I’m trying to decide mini-split vs. conventional ducted forced air for my new shop. I’ve read through tons of posts already, so let me start off with: this is not a request to size a unit, or advice on brands. I simply want to figure out what type of system fits my application the best. My “hands on” experience with mini splits is limited, whereas my forced air (heat pump) experience is based on the unit on my house (which pulls the temps down reasonably quickly).

My shop is primarily used for mechanical work – cars, motorcycles, that sort of thing. Going into this project I thought I would run the AC primarily on the weekends (scheduled), maybe a little during the week to keep the edge off. But it would be nice to be able to cool the space somewhat quickly for those days I play hooky from work.

Shop location is Tucson, AZ. Drawings are in work. It will be 2x6 construction, stucco’d exterior, drywall interior, on a concrete pad. Shop is 2000 sqft total. RV bay is 16'x50'x16' ceiling height (with a 12'x14' overhead door). Other bay is 30'x40'x12' ceiling height (with two 12'x10' overhead doors). It is 1 big clear open span inside. The overhead doors face South. There are two man doors (one on north side, one on east side), four double pane skylights, and potentially a few small clerestory windows (haven’t decided yet).

Walls and ceiling will be open cell spray foamed (better than R19 in walls and R30 in ceiling). Overhead doors are insulated. I’ve done an initial Manual J calc and come out to about 3-4 tons of cooling - more refinement will be done as the design progresses.

My big question is: What would you do, and why? What makes the most sense for a larger space like this?

My intuition says a forced air heat pump system will cool the space quicker, filter better, and distribute air better… I don’t have much experience with mini splits, so my concerns are:

• Would I need to run a mini split 24x7x365?
• Will the recovery time of a mini split be terrible if I open an overhead door?
• Are mini split interior units be able to blow air across a 30/40/50 foot space?
• How does one get good distribution inside a large space without ducting (lots of fans?)

 

[124ci]

I am in the process of completing my shop, in phoenix. I do hvac for a living. Not that means anything. I chose to put heat pump system in instead of mini splits. I did not want to deal with cleaning the coils of a mini split when they get dirty. I also chose a 3.5 ton for a 28x36 shop. The a/c will probably only be used a few times each summer, a few hours at a time and it will always be a hot pull down. [ meaning everything in garage is saturated with heat ] A few times for heat also. I wanted extra capacity to make it happen quicker. I also installed an evap cooler. Lots of people like the mini splits. I like ducted sytems.

 

[CraigBlackCoupe]

For that large of a space, I would not recommend a mini split. I have a single mini split in my 30 x 36 garage and it does not cover the space well. It works well for what I need (the single bay side where the lift will go) but it does not cool the entire garage well. My garage is 2x6 stick build with R-19 in the walls and Icynene up top.

Open the overhead door and the cool air rushes right out. Multiple wall units may help cool the large space quickly, but my experience with mini-splits is they work best in normal size rooms.

 

[bugman-74]

BTU is not intrinsically a unit of time, though I see that when it comes to heating and cooling, it is typically expressed as BTU/h. One would think a mini split with the same BTU rating as a heat pump would pull the temp down approximately the same amount of time (ignoring differences in airflow, ducting, etc between the two). But this seems not to be the case?

Follow up: What would you do for ducting? Go with spiral?

 

[Fueler]

Mini split, maybe 2. I use one 24k for a 40x40. Cold as I want it in the summer.

If you are in cold country figure a secondary heat source.
I use overhead tube heater but it only comes on in the worst cases if the split can't keep up.



Mini splits run continuously but don't let that freak you out. Once the room reaches temp it throttles back and just maintains it. It's actually cheaper to let them run than to buy into the shut it down to save money mentality.

 

[theoldwizard1]

Walls and ceiling will be open cell spray foamed (better than R19 in walls and R30 in ceiling).

Why open cell ? Open cells retains moisture (okay you are in AZ, so maybe this not an issue).

Usually it is cheaper to spray about 1"-2" of closed cell and than fiberglass batts/rolls of insulation. Similar R rating.

My big question is: What would you do, and why? What makes the most sense for a larger space like this?

My intuition says a forced air heat pump system will cool the space quicker, filter better, and distribute air better…

Quicker because it has better distribution.

For a space like yours, you would need multiple mini-split air handlers. Better yet, multiple mini-split units. The big win for mini-splits is much higher efficiency. Multiple units can be "slaved" together and will "recover" better. (Mini-split with multiple air handlers are less efficient.) Mini-split air handlers are not great at filter air. If you are going to paint, you booth will have its own filtration system.

It is kind of a classic case of "pay me now (for the equipment) or pay me later (for the electricity)". Anyone building in AZ is crazy to NOT install solar. Another "pay me now or pay me later".

 

[bugman-74]

Why open cell ?

Primarily due to cost vs. performance (total r-value achieved). The installation method of closed cell (under-filled cavity) vs. open cell (fully filled cavity) results in a very similar total R-value, and open cell foam is much cheaper.

For a space like yours, you would need multiple mini-split air handlers.

I didn't want to get too much into a "sizing" debate in this thread, but I had planned on 2-3 interior blower units if I went mini-split. My initial manual J calcs call for roughly 3-4 tons, so I thought a couple 2-ton units or maybe three 1-ton units.

Quite frankly I believe I can do the mini-splits myself for considerably cheaper than traditional heat pump... Just not sure which option would be best for this space with respect to speed/recovery time and distribution of air throughout the space.

Assuming the comparison is between appropriately sized traditional heat pump, and appropriately sized mini split (with 2-3 interior units), WWYD

 

[dcg9381]

Would I need to run a mini split 24x7x365?

I'm in Texas, 2400 sqft shop. 16' walls. 3 14x14 doors (poor insulators)
Foam insulation, but maybe R-12 walls and R-19 ceiling.
2 x 24k BTU mini splits.

Do not need to run it 24x7 and you can often set timers.
If it's 110 degrees out, you may need to run them. But during the summer, I simply turn mine on one hour in advance.

You an over-size mini-splits more than ducted units.


Will the recovery time of a mini split be terrible if I open an overhead door?

If it's hot, it's not going to be good. Opening and closing the door, you'll be fine. Leaving it open or setting up cross-venting doors, it'll set you back.

Are mini split interior units be able to blow air across a 30/40/50 foot space?

NO! you need fan(s)

How does one get good distribution inside a large space without ducting (lots of fans?)

In my case, 2 x 24K units were a lot less expensive than a single 48K unit, so to some degree you can choose your cooling location. If I was in my shop a lot more, I'd probably add a 3rd unit.

You should use a BAF (Big *** Fan) or other "move air around" system.


Mini-splits seem to be more efficient and more flexible than a ducted system, but I can say that 3 systems will fail at least 3x more frequently as one system... And the parts like air handlers, scrolls, etc of these mini-splits are not as well built as hamster-wheel ducted units.

 

[bugman-74]

Thanks for all the great input! Another question:

Assuming I went mini-split, and approximately 4 tons, How many interior wall units would you shoot for, and where to place them for optimal performance? (I attached a rough floorplan).

More than likely I would have 2 outdoor units near the corner on the N exterior wall up near the W wall of the RV pay protrusion (best option to shield exterior units from the sun). This would make 2-3 interior units on the N wall the best option plumbing-wise, although I wonder if 1 interior unit on the E wall and 1 on the W would help "mix" the cool air better.

Thoughts?

 

[dcg9381]

4 tons worked out best for me financially with 2 x 2 ton units.
Going to a 3 ton unit was more expensive.
Going to a 4 ton unit with multi-head was a LOT more expensive.
I'd "point" these things to where you are most likely to be working or need them. Yes, they'll probably cool all of it (well insulated) but the air flow makes a huge difference being in front of them directly. You also need to consider the position of the compressors.

 

[CarFire]

I am facing the same question in the southeast, but our shop in Phoenix AZ was very similar to OPs. It was about 2200sq ft with OC sprayfoam and 13'6" walls. I put a 5 ton package unit on the roof and ran spiral duct myself. It worked great and could keep up.

Now I am looking to either do it again or go mini-split. Bigger shop about 36 x 72 or so with 13' 4" walls. Having a hard time believing the mini splits will cool it down!