Detached Garage - Ductless MiniSplit

[Pettrix]

For my insulated detached garage (1,200 sqft). I was thinking of getting a 1 ton ductless minisplit unit to help cool it in the summer and provide a little heat in the winter, if needed. Would the 1 ton be undersized for an insulated garage? (R-30 walls/R-40 roof/R-8 car entry doors) I'm in Zone 4B (High Desert)

Which option should I go with:

A – 1-Ton - SEER2=24.00, HSPF2=9.00, Able to heat down to -22F = $1,200 for the unit

B – 2-Ton - SEER2=17.00, HSPF2 = 6.7, Able to heat down to 5F = $1,200 for the unit

The 2-ton unit is not as efficient as the 1-ton unit, hence the pricing difference. Which would be the best unit for the garage?

 

[Code54]

Look up load calculations for HVAC and run the Calc. You need a lot more info, like number of windows, directions of the windows, and most importantly the height of the garage as the volume of air matters. You need a large unit for a 16’ high 1200sq ft garage than an 8’ high garage.

 

[Code54]

HVAC Load Calculator - Manual J Calculation | ServiceTitan

Our free, easy-to-use HVAC load calculator will help you determine the necessary thermal output capacity (in BTUs) of any residential building.

www.servicetitan.com

Here is an example

 

[WM75Guy]

I don't think you will have enough capacity if 12,000 BTU = 1 ton, as I have been told. We have a 30'X40' insulated metal shop with a 14'X20' wood framed insulated room inside for stained glass work. The HVAC company put in a 12,000 BTU Daikin mini split for that room. Added on to the west side is our our 12'X20' shop office. It is insulated with metal walls outside and inside. The HVAC company used an 18,000 BTU Daikin mini-split because of the higher heat load. The main shop has a 50,000 BTU electric heater just to keep the plumbing from freezing in the winter.

 

[Pettrix]

Appreciate the info.

Last winter the garage never got below 58F.

There are NO windows and the height of the garage peak is 16' as a gable so it starts at 10' and peaks at 16'. There are TWO insulated (R-8) car entry doors (18x8) and an insulated side entry door (R-7).

The garage walls are high-mass (concrete) so the R-Value is R-30 but studies show the Effective R-Value can be as high as R-50.

I think I will go with the 2-Ton unit...

 

[mrmikey]

Nice garage. Looks like a long span for those beams, what size are they?

 

[FPGT72]

I think the real question is how bad is it in the summer. Mine hit -20 last winter, and the heat just could not keep up. I drained everything when I learned that cold snap was coming.

Mine has a loft now, and I store as well as work up there, I have a "large" 240 "window unit" in the shop and that keeps the down stairs nice enough to not sweat like a pig, my area is really getting the humidity out. But up stairs it is a different picture. You can have 75 down stairs, and well over 100 up. I am thinking of a small minisplit to help out when I am working up there.

 

[2manyrocks]

Maybe worth mentioning that some mini splits are rated to provide heat in temperatures as low as -13F where some others won't do that. If heating in very low temps is a concern, you'll want to check the specs before buying. There may also be a loss of efficiency in providing heat in very low temps. Last issue is the BTU capacity for heating is usually higher than the BTUs required for cooling. The BTU required to heat 0 degrees up to 72 is more than the cooling BTU required to bring 100F down to 72.

For a loft, heat rises upward so adding cooling at loft level should make it more comfortable.

 

[bdhsfz6]

I installed a 12,000 BTU Daikin mini split in my 900 sq ft insulated garage / shop with 10' ceiling and two insulated 8' x 9' overhead doors. It works fine here in the northeast US with temps from -10 to 95F.

I chose a WiFi model that I can control from the house. I turn it on a few minutes before I go out and it's at the perfect working temp.

If you plan to leave the unit on 24/7, a 12,000 BTU (1 ton) unit would work to maintain a constant temp. If you plan to use it intermittently, you might consider an 18,000 BTU model, or even the 24,000 BTU (2 ton) unit which will bring the building up (or down) to temp much quicker.

I'm no expert though, so this is just my 2 cents.

 

[3Ts]

You really need to do the load calculations. I have an 864 sqft cabin every well insulated that my wife does her crafts in and it is heated and cooled year round. The HVAC guys said a 2 ton unit would be required based solely on the sqft. The load calculations said about 13,000 btu so just over 1 ton. I ended up putting in a 1-1/2 ton unit and it is more than is needed.

Also, the air sealing of your doors is going to be very important. I found the air infiltration factor to be a major factor in the load calculations.