Garage A/C

[danbgt]

This question has probably been covered too much already but I still have specific questions. When it comes to any electrical or A/C stuff I just go brain dead.
My garage is a three car stack. 19' wide x 39' deep with 9' ceiling. It has a fully insulated 7'x11' enclosed storage area in one corner leaving 664 sq. ft. I have insulated the entire ceiling and insulated the two overhead doors. That made a huge difference in the temps but I am going to go ahead and install a Mr. Cool mini split. Heres the question:
Will their 12,000 BTU unit running on 110 be enough? Or should I go up to the 18,000 BTU unit running on 220? If I go for the bigger unit I will have to have an electrician come and run the 220.
I apologize for he long question.

 

[justinjoyal]

Insulation R values for ceiling and walls?

 

[WordMan]

One thing to think about, you'll want the A/C to be oversized as, unlike with your house, you'll need the A/C to drop the temperature rather quickly (since it won't be running all the time like in your house).

 

[vavet]

One thing to think about, you'll want the A/C to be oversized as, unlike with your house, you'll need the A/C to drop the temperature rather quickly (since it won't be running all the time like in your house).

I’m in a similar situation as the OP. I’d considered this aspect, but I’d also considered that I don’t necessarily care about it getting as cool in my garage. I don’t need it to get to 74. I’d be happy to get it below 80.

 

[Lindy911]

The Mr. Cool website shows the 12,000 btu handles up to 500 sqft, and the 18,000 btu up to 750 sqft.

 

[danbgt]

Insulation R values for ceiling and walls?

Walls common to the house and storage room and the ceiling are R-30. Ceiling floor above garage is decked. Roof has radiant barrier. Garage doors are R-4.8 foam panels. Garage floor has SwissTrak type tiles. The longest outside wall is not insulated yet. I haven't exactly figured how the best way to do that would be. But that wall is protected and doesn't seem to radiant much heat.

 

[Jinks]

Go with the 18K, 220V unit. I have 12K in a 456 sq.ft. space here in Fla. The east & south walls are not insulated, but the other two sides are & it's under an air conditioned second floor. 12K does a fine job of cooling & dehumidifying, but takes a little time if I let the shop get hot before starting the unit. The other day I was running the unit & brought a sports car into the shop. The temp rose 2 deg. in about 5 minutes. Took 30/45 min. to get back to normal. I'm not unhappy with my 12K, but if it ever dies I'll replace it with an 18K DIY.

 

[WordMan]

I’m in a similar situation as the OP. I’d considered this aspect, but I’d also considered that I don’t necessarily care about it getting as cool in my garage. I don’t need it to get to 74. I’d be happy to get it below 80.

I have a 24X28 cinderblock garage. R-19 insulated walls and... whatever is the best I can get in the roof (haven't insulated the roof yet), and I'm planning on using a 2-ton window/wall unit mounted on top of the 10' tall wall.

 

[kj_mustang]

Required number of btus is always contingent on insulation and windows and doors and local temperatures. A generic 18,000 btus for up to 750 sq ft is nonsense. I have a 18,000 btu central ac unit cooling over 1700 sq feet.

 

[u3b3rg33k]

it's safe to oversize if you're using a minisplit, as they're modulating, and typically single head systems modulate down to at least 1/3 capacity (i.e. a 12k unit can go down to 4k). assuming the same, and 18k could modulate down to 6k, which sounds just fine to me. even a 24k could be A-OK (mod down to 8k) without being too big.

going too big will cause the unit to cycle more, and cycling = lower efficiency than running at a low modulation. best to check the spec sheet and see what the minimum is.

if you were putting in a single stage regular system, I'd be more wary of math-free casual oversizing. 2 stage standard splits are typically 2/3 capacity in stage 1, so not nearly as much wiggle room.

 

[danbgt]

Required number of btus is always contingent on insulation and windows and doors and local temperatures. A generic 18,000 btus for up to 750 sq ft is nonsense. I have a 18,000 btu central ac unit cooling over 1700 sq feet.

This is, kind of, my thought. I have two Air King fans ceiling mounted in the garage. Unless it is really hot, which happens here in Houston, it is not too bad out there. I plan to do some additional weather stripping around the garage doors. If I go 18,000 I will probably have to have an additional broker box installed to add the 220 because my box is maxed out now. Being a "tightwad" I don't want to spend that money.

 

[OccupantRJ]

When you avoid things with the tightwad aspect, you are setting yourself for failure due to bad decisions, in my opinion. I will spend money on infrastructure before anything else, and I have never disappointed myself by doing so. A sub panel should do it if your panel is full.

 

[sreeb]

I don't see why 220v requires an electrician and 110v doesn't.

Either should have a dedicated 12ga line (the same exact wire) and a disconnect. The only difference is which breaker you put in the panel.

 

[hippie2cams]

If your garage looks like your avatar,you didn't cheap out on looks, so why cheap out on comfort. It looks like an entertainment area to me, the more people in the space the more cooling it's going to take. My garage is 30x40 with ten ft. walls and I put a 31/2 T heat pump in it. winter time heat stays at 70* and summer it is set at 74* nice and comfy. Enjoy what it cost to be comfortable, it's really not that much.

 

[ducksface]

I went ahead and installed a central air unit, it's way too big 4 ton heat pump in a 21x21 garage. It still takes half an hour to drop the air temp.
The cost wasn't much more than a mini split and came with four down ducts from the ceiling.
You'll want it to cool fast.
Experience dictates. It'll take HOURS to drop the temp in your garage to use able with anything less.


I know everyone buys them, I just don't like minisplits except as the barest fix to a real problem... I believe them to be a mediocre compromise in almost all ways.

The picture represents opening and closing of outside ambient vents at specific times in the summer and winter in a well insulated (as yours is) garage.
The historic high temp you see is due to a long day of open doors and a fan to vent fresh air and is usually 10 degrees cooler, and the historic low is the same open doors but on a snow covered winter day.
It is 60 to 65 in here with not much other fluctuations almost any day of the year.
You can see the outside temp swings as noted by the historical high/low numbers.
This is in a different garage than the air conditioned garage. Nights in Havasu are often over. 100 degrees at 1 am in the summer. Ac was required to match the interior(much like your interior.)