AC for a shop.

[maccrazy2]

I’m looking into a mini split ac system for my shop. I have only been working out there on weekend mornings because after sweating at work all day I don’t feel like doing it at home in the evenings. Anybody have experience or recommendations on properly sizing a unit for the space? I have been trying the online calculators and getting size recommendations that are all over the place.

I installed central air in my house a few years back and used the recommended size unit and it is inadequate to keep the house cool. It will run all day without cycling off and the temp slowly rises throughout the day. I was going to oversize the system but was told by a couple companies not to so I followed their recommendations and I’m kicking myself for doing so. However, in that case the house has an addition without vents in it and according to “the book” you are not supposed to count that or the basement. I think not counting the basement was not the way to go. I did try putting up a sheet of plastic to block off airflow into the addition and it still couldn’t keep up.

The shop is 24 by 28 with 9 ft ceiling. It’s drywall’d and insulated. There is one man door and a double car wood garage door. It is currently shaded by a large tree but I think it may be coming down in a few years so I don’t want to count on that being there. If anyone has experience with these I would appreciate your input.

[Sparky_NY]

I retired from the heating/ac trade.    The double car wood door is a huge factor, they leak air wildly and have no insulation value.    You said the shop is insulated but not how much insulation for walls/ceiling,  makes a big difference also.     Yet another factor is if you plan on leaving it on always or only when you go out there to work.   If you plan on only running it while occupied, it will take quite a bit more \"muscle\" to pull it down in a reasonable time.     Ok, still another question, where are you located??  Climate makes a bid difference also.    Where I am, mid 90\'s is the norm for a couple months, sometimes pushing 100.   What about windows with the sun shining in?

 

You are talking a touch under 700 sq ft BUT I am guessing there is a bunch of machines and \"objects\" in the shop that hold the heat and change temperature very slowly.   They work somewhat like a heat radiator.   You cool the air but the objects warm it up just a fast.    That is the classic case of places like restaurants, rule  of thumb is each person \"radiates\" 300 btu\'s of heat.   A restaurant that seats  lot of people needs LOTS of btu\'s of AC for that reason.

 

Give us a bit more data and I will take a guess on a practical size.

[Dave]

I retired from the heating/ac trade.    The double car wood door is a huge factor, they leak air wildly and have no insulation value.    You said the shop is insulated but not how much insulation for walls/ceiling,  makes a big difference also.     Yet another factor is if you plan on leaving it on always or only when you go out there to work.   If you plan on only running it while occupied, it will take quite a bit more \"muscle\" to pull it down in a reasonable time.     Ok, still another question, where are you located??  Climate makes a bid difference also.    Where I am, mid 90\'s is the norm for a couple months, sometimes pushing 100.   What about windows with the sun shining in?

 

You are talking a touch under 700 sq ft BUT I am guessing there is a bunch of machines and \"objects\" in the shop that hold the heat and change temperature very slowly.   They work somewhat like a heat radiator.   You cool the air but the objects warm it up just a fast.    That is the classic case of places like restaurants, rule  of thumb is each person \"radiates\" 300 btu\'s of heat.   A restaurant that seats  lot of people needs LOTS of btu\'s of AC for that reason.

 

Give us a bit more data and I will take a guess on a practical size.

Really good questions/info!

 

I cool a 700 sq ft shop with a med sized 220 window unit. But its insulated well with a really good insulated 16ft door. R13 in the walls and around r45 in the ceiling, you can heat it damn near with a candle! St Louis area, cold winters, hot steamy summers!

[Sparky_NY]

Really good questions/info!

 

I cool a 700 sq ft shop with a med sized 220 window unit. But its insulated well with a really good insulated 16ft door. R13 in the walls and around r45 in the ceiling, you can heat it damn near with a candle! St Louis area, cold winters, hot steamy summers!

Sooooo true.    You just can\'t beat good insulation.     I live in upstate South Carolina,  its been mid 90\'s everyday for the last month. (it was 94 at 7:30 tonight)   Inside my pole barn shop I built a 20x20 room for a metal working shop.   I used 2x6\'s with R19 insulation, same with the ceiling at 8ft.    With the  AC off and not using it for a few days its usually about 83 in there but drops to 73 within 10 minutes when I turn the AC on.    Its a heat pump so it also heat it in the winter.    Its a old unit I scrounged, two different brands for inside and outside units, nothing special.   Its a 2 ton unit which is big for the job but was free.   It runs so little that I cannot notice any difference in my electric bill.

 

Uninsulated garage doors are a huge problem, might as well leave a window open.   When I lived in New York state my shop was a 1-1/2 car garage.  I put a 3-1/2 inch thick commercial insulated garage door on it.   In single digit temps you could sit right against the door and it was room temperature.    Not a cheap door but well worth the money.   Many of the home store insulated doors are very low R values and perform poorly.   The good ones have excellent seals between the sections and around the perimeter.

[Sparky_NY]

Really good questions/info!

 

I cool a 700 sq ft shop with a med sized 220 window unit. But its insulated well with a really good insulated 16ft door. R13 in the walls and around r45 in the ceiling, you can heat it damn near with a candle! St Louis area, cold winters, hot steamy summers!

Probably a 18,000 btu unit.   18,000 btu and up are usually 240volts.    To compare window type units to central type ones,  12,000 btu equals one ton.   So your unit is probably 1-1/2 ton which is about what I would guess for that size and area.   18,000 is a very common size.

 

I have a 12x24 shed, insulated with R13 insulation and a 18,000 window unit.   It gets real hot in the shed and that AC cools it down quick.  The shed was my shop before I built the pole barn,  I still use it for some things like reloading ammo.

[maccrazy2]

I’m in Denver area. The shop has no windows. I put exterior seals around the perimeter of the garage door to seal the sides. The door sections mate together quite well for what it is. I have heat in the shop already so I tried to seal everything up as good as possible.

The insulation is r13 in the walls. The ceiling does not have any insulation yet. I will be installing it this fall once it cools off a little so I can get up there and work. I was thinking if I’m going to do AC I will do it at the same time. It is a work in progress getting everything done.

[Sparky_NY]

I would say a 1-1/2 to a 2 ton unit.      The smaller one if you intend on leaving it on,  larger if only when occupied.       No insulation  in the ceiling is going to be a problem until you get that done.      I would do something with that door,  maybe some foil backed 4x8 foam insulation panels that can be rigged up for easy removal when needed.     To be safe, I would probably go with the 2 ton especially considering your experience with the house AC.    Your R13 is minimal,  the door a huge heat gain,   hopefully a lot of insulation to be in the ceiling.

 

You didn\'t say but hopefully the machines are just hobby use occasionally and not real big.    Some machines CAN generate enough heat to offset a large amount of AC.

 

Maybe somebody with a similar setup can tell us what they have.

 

  Those simple little online sizing calculators are basically a wild guess.   The calculators the pros use take a LOT more into account, even the roof color, shading on the house, etc and give a very accurate estimate on sizing.    Personally, I won\'t usually guess on sizing without seeing the house because so many factors make a difference.  Your experience with the house AC shows why details MATTER.   Over sized is not needed,  CORRECT size is VERY important,  two different things.

 

The advise on not over sizing is the standard of the industry.   Over sizing and choosing the wrong size are two different things !      A unit oversized will cool the air faster yes BUT at the expense of poor de-humidification.    Getting rid of the humidity is what really creates comfort.    A properly sized unit will get the humidity to around 50% after it has been on a few days.   

 

A DIY ac install often results in problems not because of the mechanical portion but the science of it.   

[bruski]

Pickup one of those cheap laser aimed temperature readers from Harbour Freight to pin point where your heat is coming in from so you can concentrate on a quick fix to get you through the summer.

 

bruski 

[maccrazy2]

I have a flir thermal heat gun I can use for checking for leaks and bad insulation spots. I will be doing the AC install and insulation at the same time.

I have looked into lining the door with the foil backed foam to try to better insulate it.

I generally only work in the shop for 3-5 hours in the evenings so I will only run it when needed. Typically I come home from work and spend 1-2 hours in the house and then get to work. So, I would turn it on when I get home and let it run 1-2 hours before I get out there.

[maccrazy2]

The machines are 3 12/36 lathes, 1 Bridgeport, 2 benchtop mills and several small tablesaw size machines for various other purposes. It is pretty filled up with stuff.

[Sparky_NY]

I have a flir thermal heat gun I can use for checking for leaks and bad insulation spots. I will be doing the AC install and insulation at the same time.

I have looked into lining the door with the foil backed foam to try to better insulate it.

I generally only work in the shop for 3-5 hours in the evenings so I will only run it when needed. Typically I come home from work and spend 1-2 hours in the house and then get to work. So, I would turn it on when I get home and let it run 1-2 hours before I get out there.

Yup,  that will take more \"muscle\" aka btu\'s to pull the temp down quicker.    Humidity levels will suffer.    For the occupied times you give, I would rule out the 18,000 btu / 1-1/2 ton size it just will not pull the temp down quick enough, it would just start getting nice when you are done.      Revised guess,  2 - 2-1/2 tons depending on the variables.

[bruski]

My machine shop is 1,100 square feet with an 8 foot ceiling all built with 2 x 6 walls and rafters with the I believe R-19 insulation for 2 x 6\'s. The split unit is a 4 ton I think and I keep it 78 degrees while it is 115 degrees outside. When I close up for the night, I raise the thermostat up to 82 degrees. My machine shop is built inside of a much larger Butler type of building so it is shaded but it\'s like an oven in the big shop. The unit cycles on and off normally.

 

bruski

[maccrazy2]

Humidity is not much of an issue here. It’s generally down around 10-30% all summer. I’m from Atlanta originally. There it was heavy all the time.