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  1. #1
    Join Date
    Nov 2019
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    Tucson, AZ
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    2000 sqft shop A/C advice - mini split?

    Hi All, I've been on the fence for a while about having either a mini split or conventional ducted forced air put in the plans for my new shop.

    As soon as I have a few more posts, I will attach some plans and renderings. For now, a verbal description will have to do: Location is Tucson, AZ. It will be 2x6 construction, stucco’d exterior, drywall interior, on a concrete pad. 2000 sqft total. RV bay is 16'x50'x16' ceiling height (with a 12'x14' overhead door). Other bay is 30'x40'x12' ceiling height (with two 12'x10' overhead doors). It is 1 big clear open span inside. The overhead doors are all on the same wall and face South. There are two man doors (one on north side, one on east side), four double pane skylights, and potentially a few small windows. Walls and ceiling will be open cell spray foamed (Some initial manual J's came back at around 4 tons). The space is primarily used for mechanical work on cars, fabrication, etc.

    The general concept was to use 2 mini split systems (2 ton each) with a total of 2-3 total interior units.

    In General my primary question is: Would you use a mini split to cool this type of space, or something different?

  2. #2
    Join Date
    Nov 2019
    Location
    Tucson, AZ
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    Thread Starter
    My concerns are:
    Recovery Time: It would be nice to be able to pull the temp of the space down reasonable quickly.
    Air distribution: Will mini splits be able to blow air across a 30/40/50 foot space, or are lots of fans needed for air distribution?

  3. #3
    Join Date
    Nov 2019
    Location
    Tucson, AZ
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    Thread Starter
    My secondary question is: Given my overall geometry, what is the preferred way to configure the 2-3 interior units to cover the square footage? Would it be best to put them on opposite walls to help "mix' the air and cover the space a bit better (versus putting them all on the N wall)

  4. #4
    Join Date
    Nov 2019
    Location
    Tucson, AZ
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    Thread Starter
    For background: Here's an exterior rendering and a rough interior layout
    Attached Images Attached Images   

  5. #5
    Join Date
    Jan 2012
    Location
    Broomall, PA
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    Id be concerned about dust and maintenance. Ducted system with return filter grills is easier to maintain.
    Even a RTU.

    Also, will you be painting, or any other issues with VOC's?
    If I do a job in 30 minutes it's because I spent 30 years learning how to do that in 30 minutes. You owe me for the years, not the minutes.

  6. #6
    Join Date
    Dec 2005
    Location
    Cincinnati, Oh
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    Quote Originally Posted by STEVEusaPA View Post
    I’d be concerned about dust and maintenance. Ducted system with return filter grills is easier to maintain.
    Even a RTU.

    Also, will you be painting, or any other issues with VOC's?
    That was my concern.
    Those units do not stand up well to dust, as they have the absolute minimal filtration systems on them.
    You may want to consider some type of fresh air.

    Minisplits are probably the most misused pieces of equipment in our industry. They are just seen as a catch all for everything.
    "Better tell the sandman to stay away, because we're gonna be workin on this one all night."

    "Dude, you need more than 2 wires to a condenser to run a 2 stage heatpump."

    "Just get it done son."

    Dad adjusted

  7. #7
    Join Date
    Nov 2019
    Location
    Tucson, AZ
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    Thread Starter
    Filtration does come to mind as an additional concern - I understand the mini splits have only a basic screen filter. I dont mind cleaning them regularly, and thought that perhaps if the interior units were up fairly high (9 ft?) it would reduce the particulate load. I primarily work in metal.

    If I am spray painting it will generally take place with an overhead door open. Also plan to do some small scale powder-coat, and I also have a 20 gallon parts cleaner (to be honest I don't know the VOC details on the cleaning solution). I plan to have a portable fume extractor for when I want to weld with the doors closed (or spray paint something small).

    Would a conventional forced-air heat pump offer an advantage with regards to VOC's?

  8. #8
    Join Date
    Dec 2005
    Location
    Cincinnati, Oh
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    Quote Originally Posted by bugman-74 View Post
    Filtration does come to mind as an additional concern - I understand the mini splits have only a basic screen filter. I dont mind cleaning them regularly, and thought that perhaps if the interior units were up fairly high (9 ft?) it would reduce the particulate load. I primarily work in metal.

    If I am spray painting it will generally take place with an overhead door open. Also plan to do some small scale powder-coat, and I also have a 20 gallon parts cleaner (to be honest I don't know the VOC details on the cleaning solution). I plan to have a portable fume extractor for when I want to weld with the doors closed (or spray paint something small).

    Would a conventional forced-air heat pump offer an advantage with regards to VOC's?
    Its not so much that you have to clean it alot. Its more so that if you are working with something more than just basic household dust, it will pack the coils.

    That, and the coils on them are not exactly designed to hold up to extremes. Even basic off-gassing of drywall in a tight room will cause them to fail.
    "Better tell the sandman to stay away, because we're gonna be workin on this one all night."

    "Dude, you need more than 2 wires to a condenser to run a 2 stage heatpump."

    "Just get it done son."

    Dad adjusted

  9. #9
    Join Date
    Nov 2019
    Location
    Tucson, AZ
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    9
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    Thread Starter
    Its more so that if you are working with something more than just basic household dust, it will pack the coils.
    Ah got it. Is this something a light blast with compressed air would solve?

    Even basic off-gassing of drywall in a tight room will cause them to fail.
    wow.

  10. #10
    Join Date
    Jul 2000
    Location
    Northern Wisconsin
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    Ducted furnace connected to spiral duct. GOOD filtration with returns located both high and low. I'd suggest a two stage split system AC. First stage for de-humidification and second stage for fast cool down.
    Use the biggest hammer you like, pounding a square peg into a round hole does not equal a proper fit.

  11. #11
    Join Date
    Aug 2008
    Location
    Hibbing, MN
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    767
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    I’ve had a Mitsubishi mini split in my shop for 5 years. I paint, cut, grind, weld, and blow lots of stuff with an air gun. I clean my filters regularly, and have no problems. It heats and cools the shop year round and the coils are not all plugged up.

    It’s really all about regular maintenance.
    If God didn't want us to eat animals... He wouldn't have made them out of MEAT.

  12. #12
    Join Date
    Nov 2019
    Location
    Tucson, AZ
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    Thread Starter
    Quote Originally Posted by firecontrol View Post
    Ducted furnace connected to spiral duct. GOOD filtration with returns located both high and low. I'd suggest a two stage split system AC. First stage for de-humidification and second stage for fast cool down.
    I like this concept.

    Spiral ducting is exposed, correct?

    Why returns high and low? I'd suspect the low returns to be extremely susceptible to debris

  13. #13
    Join Date
    Sep 2005
    Location
    Atlanta GA area
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    Quote Originally Posted by firecontrol View Post
    Ducted furnace connected to spiral duct. GOOD filtration with returns located both high and low. I'd suggest a two stage split system AC. First stage for de-humidification and second stage for fast cool down.
    Agree with this... definitely do the 2 stage AC!

    Whether you use gas heat or HP... ducted conventional split (or package) units would work better for what you are doing!
    GA-HVAC-Tech

    Your comfort, Your way, Everyday!

    GA's basic rules of home heating and AC upgrades:
    *Installation is more important than the brand of equipment
    *The duct system keeps the house comfortable; the equipment only heats and cools (and dehumidifies)
    *Cheap is not good, good is not cheap; however expensive is not a guarantee of quality!
    Choose your contractor wisely!

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