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Sizing Split AC Unit for Small Garage


How do I calculate the size of split AC unit for my small workshop/garage? It is ~390 sq ft so a small unit I think. 9,000 or 12,000 BTU likely. This area is an addition to the two car garage but it is fully insulated 2x4 wall adjoining old garage and the rest 2x6 wall construction with fiberglass batt in all the 10' high walls and blown in insulation in the attic to meet MN code in 2006. Exposed concrete floor which also has 2" foam board underneath between cement and soil. Garage door is also insulated. I have 110 and 220 subpanel for power available in the room and it has a double 30+30 amp 220 breaker that is never used since I sold my big table saw years ago.

Comments

  • Jamie HallJamie Hall Member Posts: 12,947
    Size it just as you would any other heating or cooling unit. Figure out your heating and cooling loads and go from there.
    Br. Jamie, osb

    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England.

    Hoffman Equipped System (all original except boiler), Weil-Mclain 580, 2.75 gph Carlin, Vapourstat 0.5 -- 6.0 ounces per square inch
  • EBEBRATT-EdEBEBRATT-Ed Member Posts: 7,091
    edited May 27
    Are you going to shut it on and odd or let it run when it is hot outside? I would think the 9000 would do it. Alot depends on how well the garage doors fit
  • mattmia2mattmia2 Member Posts: 1,428
    And how often you open the doors.
  • ChrisJChrisJ Member Posts: 11,060

    Are you going to shut it on and odd or let it run when it is hot outside? I would think the 9000 would do it. Alot depends on how well the garage doors fit

    As @mattmia2 mentioned, those doors open so their fit isn't that important IMO. It needs to be sized so things can operate normally. Minisplits can throttle way back so slightly oversized isn't a huge deal.

    If the heatgain calls for 9K I'd go 12K maybe even bigger in a garage / shop. Not only do those doors open, but often hot vehicles, mowers etc are brought in.

    Single pipe quasi-vapor system. Typical operating pressure 0.14 - 0.43 oz. EcoSteam ES-20 Advanced Control for Residential Steam boilers. Rectorseal Steamaster water treatment
  • pecmsgpecmsg Member Posts: 1,450
    What are you doing in this workshop?
  • ChrisJChrisJ Member Posts: 11,060
    pecmsg said:

    What are you doing in this workshop?

    Single pipe quasi-vapor system. Typical operating pressure 0.14 - 0.43 oz. EcoSteam ES-20 Advanced Control for Residential Steam boilers. Rectorseal Steamaster water treatment
    HVACNUT
  • 59ctd59ctd Member Posts: 3
    I found an online cooling load calculator and it came up with ~10,400BTUs which I think is about right. I think I will go wit hthe 12,000 BTU mini split unit. I have one of those portable units that is I think 9K BTU and it does the trick but it is such a PITA to have to haul out the huge exhaust duct, etc.

    Shop is where my car gets to sleep and we do a fair bit of auto maintenance in here. Right now this is also my COVID WFH office.
  • HVACNUTHVACNUT Member Posts: 3,412
    Unless the long wall and roof gets southern exposure, IMO you'd be fine with the 9K.
    What's the roof pitch? Decent?

    The garage door isn't up and down all day right?

    Maybe a ceiling fan for the 10 ft ceiling if you keep the space purty.

    Being the head is in a workspace, keep the filters, coil, and blower wheel clean.

    And dont drive over the remote.
    Zman
  • 59ctd59ctd Member Posts: 3
    The long wall is north facing so no sun issue there. Door is not up and down all day - especially if the radiant heater or the AC is on.

    Roof pitch is an oddball and I can't recall exactly. The front and north sides are a common pitch but the back is a lower pitch. The roof was not easy to get it to work properly with the existing garage and all the weird angles introduced by a garage with five walls and angled like it is.

    I will look at the 9K units as well. If 9K would do the trick is a 12K unit going to have problems like icing up or...? The prices between them is fairly small.
  • ChrisJChrisJ Member Posts: 11,060
    59ctd said:

    The long wall is north facing so no sun issue there. Door is not up and down all day - especially if the radiant heater or the AC is on.

    Roof pitch is an oddball and I can't recall exactly. The front and north sides are a common pitch but the back is a lower pitch. The roof was not easy to get it to work properly with the existing garage and all the weird angles introduced by a garage with five walls and angled like it is.

    I will look at the 9K units as well. If 9K would do the trick is a 12K unit going to have problems like icing up or...? The prices between them is fairly small.

    A 12K minisplit won't have any issues.
    Single pipe quasi-vapor system. Typical operating pressure 0.14 - 0.43 oz. EcoSteam ES-20 Advanced Control for Residential Steam boilers. Rectorseal Steamaster water treatment
  • EBEBRATT-EdEBEBRATT-Ed Member Posts: 7,091
    With 2x6 walls and the insulation you have 1 ton for 400 square feet is pretty heavy. 9000 sound right unless your located in Africa. You have a 9000 now and it does the job.

    Why go up?
    HVACNUT
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