Mini-split sizing help

[Gunnert]

Detached garage: Interior dimensions 32' x 26'. Ceiling is a open rafters and come to a peak 17' high. 8' walls have 4" fiberglass insulation. From there, up, is 6"-8" blown foam insulation. 2 garage doors have 19R insulation. 4 windows, all double pane. I have 6' ceiling fan in the rafters. I'm located in northern Virginia where outside temps in the summer often go into the mid-90s and winter around mid-20s I know I'm in the 30k-36k range. Just can't decide which. I spent a lot of money getting the ceiling blown and don't want to buy a unit that is bigger than what I need. What BTU mini-split should I purchase?

Also, are there manufacturers other than MrCool that offer DIY mini-splits? By DIY I mean an HVAC specialists isn't required for charging...

 

[Black Oak]

Not sure if you sprayed the under side of the roof deck . If so , great move .I don't know of any other DIY units other than Mr. Cool . Pulling a vacuum isn't too bad , YouTube is your friend there . Your shop sounds pretty sealed up tight , I'd guess a good 18,000 btu that heats well would do . 24K at the most . Good doors are great .

 

[Gunnert]

Yes, sprayed foam underside of roof; open rafters in garage.

 

[ericm]

This purports to be a version of the Manual J load calc (with a bonus Manual S to size the equipment): https://www.loadcalc.net/

For Manual J you're supposed to select the 99% highest/lowest design temp , i.e. the high temp should be above the one you use in the calc on only 1% of days. The page has a lot of locations and temps (from the ACCA Manual J tables) but if you're in a microclimate between the nearest locations like I am you may have to interpolate a little.

 

[TangoFoxTrot]

On a garage application, I would always go up a size if it's a question.

As soon as you open the garage, you have to practically start all over. It's also probably not something that's going to be running all the time, you'll usually want it to either cool the place down or heat it up fast when you decide to work in there.

A house is different in that it's not opening up a "wall" to the outside and they are usually running all day and don't have the same load placed on it.

 

[Jackfre]

No more than a 24k unit.

 

[brihvac]

Remember DC inverter units ramp down. A 24K BTU will ramp down to about 10K BTU or so