Mini Split AC questions in Arizona garage shop

[Joe Jensen]

I believe the sizing calculators will always significantly understate the requirements for a garage shop, for three reasons.

First, garages have less effective sealing than home interiors, and so you are going to get more cold air escaping to the outside. Garage doors are always culprits. Also, there are usually penetrations and vents in garages to deal with CO dangers, etc. So, your mini-split system will have to combat all that additional hot air coming in from those penetrations.

Second, woodshops have enormous thermal mass. The concrete slab is a huge thermal mass. Stationary power tools have huge amounts of thermal mass. Lumber. Workbenches. And so on. If you keep your shop AC running 24/7, the thermal mass will be an advantage in keeping things cool when the outside temps get crazy hot. But if you are only turning on the AC when you are in the shop, it will take forever to cool the shop down given the thermal mass that holds all that heat, and on a hot day, it will be more than your "properly sized" mini-split can keep up with.

Third, all the electricity you burn in your shop turns into heat. Lights, power tools, dust collector, etc.

When I had my mini-split installed, my contractor used a sizing calculator and then bumped me up a size. I think it's still massively underpowered for my shop. On a hot day, my mini-split works as hard as it can just to keep the shop from getting hotter. So, I have to start the AC early in the morning so the shop gets cool before the day heats up. Then the AC can keep it cool. But if I let the shop heat up before I turn on the AC, the AC is not powerful enough to cool the shop off while it remains hot outside -- all it can do is keep it from getting warmer.

Overall, my recommendation would be to take what the sizing calculator says and say, "that's the number that I know is probably way too small, and the question is how much bigger I want

Excellent post, exactly my learning from putting AC in my shop.

[Alan Trout]

I believe the sizing calculators will always significantly understate the requirements for a garage shop, for three reasons.

First, garages have less effective sealing than home interiors, and so you are going to get more cold air escaping to the outside. Garage doors are always culprits. Also, there are usually penetrations and vents in garages to deal with CO dangers, etc. So, your mini-split system will have to combat all that additional hot air coming in from those penetrations.

Second, woodshops have enormous thermal mass. The concrete slab is a huge thermal mass. Stationary power tools have huge amounts of thermal mass. Lumber. Workbenches. And so on. If you keep your shop AC running 24/7, the thermal mass will be an advantage in keeping things cool when the outside temps get crazy hot. But if you are only turning on the AC when you are in the shop, it will take forever to cool the shop down given the thermal mass that holds all that heat, and on a hot day, it will be more than your "properly sized" mini-split can keep up with.

Third, all the electricity you burn in your shop turns into heat. Lights, power tools, dust collector, etc.

When I had my mini-split installed, my contractor used a sizing calculator and then bumped me up a size. I think it's still massively underpowered for my shop. On a hot day, my mini-split works as hard as it can just to keep the shop from getting hotter. So, I have to start the AC early in the morning so the shop gets cool before the day heats up. Then the AC can keep it cool. But if I let the shop heat up before I turn on the AC, the AC is not powerful enough to cool the shop off while it remains hot outside -- all it can do is keep it from getting warmer.

Overall, my recommendation would be to take what the sizing calculator says and say, "that's the number that I know is probably way too small, and the question is how much bigger I want."

I agree. I rarely ever put a lot of my expertise forward as advice but I have been a house inspector for 20 years. I deal with HVAC systems daily in my business and there are trends I have noticed over the last few years in new construction. Undersized systems are commonplace and don't always work out well especially in hot climates. When I installed my mini split system I intentionally oversized my unit for reasons you listed. I am glad I did. I have a 32,000 BTU unit in a 800sq. ft. shop Even on the hottest days it has no problem at all keeping up but yet runs enough to effectively dehumidify the air which in my climate is just as important as the cooling.

Alan

[Bill Dufour]

That unit is fairly quiet as they go. The Frederich units are the quietest indoor. It is about 50% quieter then the one you listed. For my home units I oversized by 50% or more. this means I run them on superlow or low and they are much quieter inside. Keeps things cool enough at the low speeds but it has not been over 110 since installed. Installation is easy: run 240 to compressor. run two copper pipes between units, run 3 conducter? wire between units. Flare tubing to connect, insulate the tubes(together) then either you or a hired man draws a vacumn and thats it.
I mounted mine on a concrete slab rather then more expensive plastic pad that sun may damage.
Only surprise is the tubes are run and insulated together as they are both cold.The link below has a
installation guide. similar for all units.
I think there idea of btus needed is in reference to a climate with much more humidity to be frozen out to make the air comfortable then you and I have in summer.
I installed my own because it gave me a chance to buy a tubing flare set, digital scale since I had to add refrigerant for the longer pipes. I already had a vacuum pump, gauge set and hoses. had to buy an adapter to connect to the high pressure line fittings.

Bill
Modesto, CA

https://www.friedrich.com/products/c...specifications