Mini Split AC questions in Arizona garage shop

[Matt Przybylski]

Hello,
We just recently moved to Arizona and I am wanting to set up shop in our new home (to be completed in April). It's a 3 car tandem garage and has roughly 750 square feet with 9 foot ceilings. There is insulation in the ceiling as well as the exterior wall and garage door. The other wall contains the door (one large 20' door) and the rest of the walls are shared with the home. I was thinking of cooling it with a mini split AC unit but I don't know much about them so I was hoping you could shed some light on them for me. My questions are as follows:

1. What are the best, most dependable units to get (brand)?
2. How many BTUs will I need to cool this space in the Arizona summer?
3. How much should I expect to spend on the unit itself?
4. How much should I expect to spend on installation?
5. Is there anything I need to worry about in terms of wood dust and the unit itself?
6. Anything else I should be aware of?

Really appreciate any help offered on this. I'm really clueless about this so I'm hoping someone out there is/was in a similar situation and can help me out here.

[Alan Trout]

I have a similar sized shop in South Texas. I purchased a Gree 32,000 BTU unit. They will all typically last about 10 years. They will tend to leak at the flare connections a little. They may need a little refrigerant every year or two. I Have had mine going on 4 years and it has worked like a champ. I can keep it 65 degrees in the summer if wanted. It efficient and quiet. Keep the filter and coils clean and all will be well. I think spent about $1400 total and I installed it myself. I call it the single most important piece of equipment in my shop.

Alan

[Chris Damm]

I have a 1120 sq. ft. shop here in SC. It's very well insulated and a 1 ton unit works very well. A good tech will do a heat loss calculation before sizing your unit. If someone gives you a size without running one get rid of them. I had one guy come in, look around, didn't even ask about insulation and says I need a 2 ton unit. A one ton does the job! I have a Mitsubishi only because I couldn't find a Daikin dealer with 100 miles.

[Todd Mason-Darnell]

I am in Central Texas and I have a Mitsubishi Mr Slim unit for my home office that is an attic conversion for the last 8 years.

It has been great and I would highly recommend them (when my main unit fails, I am thinking abut converting the rest of the house to them).

I have not had Alan's problem of having to add refrigerant. As a matter of fact, I believe that Mitsubishi says that you cannot add refrigerant in the field.

[Charles Lent]

If going with a mini-split system, consider how you will filter the air going through it. If there is any moisture in the air' it will condense on the coils of the unit and will collect sawdust from the air very quickly. But with the 5-20% humidity in Arizona you may not have this problem. I have a 2 ton window style heat pump mounted high and through the North wall of my shop here in NC. I found that 12" X 20" X 1" furnace filters will fit behind the plastic front grill and completely cover the evaporator coil, so I buy the best (most pleats) filters that I can get in a 12 X 20 size and replace them often. Cleaning the coils of an air conditioner, heat pump, or mini-split is difficult, so you will want to keep them from filling with sawdust as much as possible. A mini-split system located anywhere else does not have the dust to deal with that one in a wood shop sees. In my opinion, it would be better to install a small split household system with short duct work that included apace to allow the installation of good quality filters to keep the sawdust out of the coils. Another option is the kind of unit that they put in hotel rooms, but they too are not designed for good air filtration.

Charley

[John K Jordan]

... Really appreciate any help offered on this. I'm really clueless about this so I'm hoping someone out there is/was in a similar situation and can help me out here.

Before I put HVAC for my new shop I used an on-line calculator to determine and double-check the BTU recommendation of my favorite company. The calculator required data for square footage, ceiling height, R-value of insulation in the walls and ceiling, number/size/type of windows and doors, etc. I ended up with a 1.5 ton Trane unit. I don't remember the cost but it was much less than I anticipated - I did wire and install everything myself to save a bunch of money - the HVAC company came afterward and soldered, charged, and programmed. I have no clue about specifics for Arizona.

Heat (in this area) and air in the shop allows effortless year-round spontaneous use, the best shop accessory I ever bought. In my old garage shop I had to plan an hour ahead in the winter and use fans in the summer.

I use a cyclone DC and get very little dust in the filters. I check them occasionally and blow any dust off outside, even change them occasionally for good measure.

JKJ

[Matt Przybylski]

Before I put HVAC for my new shop I used an on-line calculator to determine and double-check the BTU recommendation of my favorite company.

You wouldn't happen to have the link to that calculator would you?

[Matt Przybylski]

After some research I'm leaning towards purchasing this unit: http://a.co/1HoOFCP

The price is right and it seems like it should cover my square footage well. The reviews are also very favorable. Thoughts?

[John K Jordan]

You wouldn't happen to have the link to that calculator would you?

Sorry, it was four years ago. I think I found it with a google search for something like "HVAC load calculation"

JKJ

[Joe Jensen]

Hi, I put a minispilt in my 3 car garage shop in the Phoenix area. The garage has a little insulation in the ceiling, maybe R13. The sidewalls are uninsulated. The garage door is rated R17. If I start the AC at night in the hottest parts of the summer it will cool down to maybe 76 by morning. When I start using the machines and the day heats up the AC can't keep up. By the hottest part of the day it might get up to 85 which is workable.

I did some web surfing. One ton of AC capacity equals 12,000 Btu/h. One Btu/h is equal to 0.293 watts so, one ton = 3.516 kilowatts. So one ton of AC can remove 3.5KW from a space.

I have a 5HP dust collector and my big tools are 3-5HP each. I also have 36 four foot fluorescent 32W bulbs in the shop. All that stuff turns into heat. A watt of power consumed by a motor turns into a watt of heat. A watt consumed by the lights becomes a watt of heat. My lighting with losses in the ballasts consumes 1700 watts. The dust collector draws 27 amps at 230V or 6200 watts but only runs maybe 10-20% of the time. Same for the tools.

So my shop with the lights on consumes about 1/2 ton of cooling capacity. Run the tools twenty percent of the time I'm in there and that's about 10,000 watts but only 1/5th of the time so that's about 1/5th of 3 tons. So the tools consume 1/2 ton or so plus the 1/2 ton of lights. Also with the poor insulation.

Due to wiring considerations I was limited to 20A at 230V for the minisplit. The most efficient one I could find was a Sanyo KHS2472 which is since discontinued. It's rated at 24,200 Btu/h. One disappointment was that it seems all the minisplit systems stop cooling above 115F and they throttle how hard they work down as it gets hotter to protect the system. (This is per my AC guy).

So I have 2 tons, and when working my lights and tools require 1 ton to offset the heat they generate. With excellent insulation I think 3 tons would be ok in a 600 sq ft shop. My system is under sized. Also there is a lot of thermal mass in a shop, tools, lumber, etc. Starting the system a day before you need it will help get everything cooled down so the mass in the shop will be at a cooler temp when your tools start heating it up.

[Bill Miltner]

I could have been this way one: loadcalc.net

[Matt Przybylski]

Thanks everyone for the great replies. Maybe I should consider a larger unit than what I linked to above.

[Alan Trout]

Matt,

I oversized mine for a reason when I installed it. We get hot in South Texas, Not as hot as Phoenix but pretty dang hot. My shop is well insulated and I wanted something I did not have to run continuously. My shop is 800 square feet. I can go home turn it on and have the shop cool in about an hour. However most of the time in the summer I just turn it up to about 78 degrees and let it stay on. I usually work with the temp at about 75 degrees. My 32000 BTU unit works off of a 20 amp two pole circuit. I chose my Gree unit as they build many of the units for some of the bigger name brand companies. The Mitsubishi, and Fujitsu units are probably a better units however they are also over twice the price. Mine is also a heat pump unit. It gets run in heat pump mode about 2 times a year but works wonderfully. A true split system has an advantage that it is easier to keep the coils clean. But you have ducts and more electrical complexity. Mini splits are just awesome for simple installation and overall efficiency. Good luck with you choice, it is wonderful to work in comfort.

Alan

[Richard Barker]

Go to https://www.acwholesalers.com. they have a sizing calculator. I purchased mine from them then installed it myself and had a local HVAC guy do the final purging and hookup. Most local companies wanted in excess of $4000 to sell and install. I spent around $1800 doing it this way. I bought a Mitsubishi because you can get repair parts easier..

[Kelby Van Patten]

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."