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We have a 12,000 BTU Mitsubishi mini-split cassette in a two-car garage in Houston. The garage is insulated on two sides and not insulated on the wall that faces the sun. The door is insulated. The ceiling above the garage is not insulated.

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The temp is currently 94 outside. We set the mini-split to 74 at noon. Five hours later, it still can't get below 79.

Is this normal for a mini-split?

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    what is the color of the walls? What is the color of the roof? How drafty is the garage and what's the humidity like? Jul 30 at 22:59
  • And what does "insulated" mean, in terms of R-value or U-value? Probably not that much...
    – Ecnerwal
    Jul 31 at 0:47
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    You are trying to run a heater(the sun) and a air conditioner at the same time. Proper insulation will probably pay for most of the extra power costs you are using right now.
    – crip659
    Jul 31 at 12:47
  • @crip659 Amen to that comment! without proper insulation the OP is fighting a losing battle, esp. considering the uninsulated wall exposure to the sun. + Jul 31 at 14:40
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It would be normal for any A/C system unless tremendously oversized to try and get to set point with the inadequate insulation you have. Mini splits are very efficient but can't overcome the lack of insulation. Get it insulated and you'll have much better results as well as a lower energy bill.

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    The high humidity in Houston will be major load on the unit. Jul 30 at 23:05
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    I'm surprised it got down to 79. +1
    – JACK
    Jul 31 at 0:50
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It's 12K BTU/hr, and you've given it an uninsulated wall in the sun (R1, MAYBE) and a 20 degree differential, perhaps more due to sun striking the wall/roof. 600 square feet of uninsulated area would use all that up at 20 degrees differential. Given an uninsulated ceiling that's probably 400 square feet (average two car size, since you haven't clarified) and an uninsulated wall, hardly a wonder that you're maxing it out. The "insulated" walls are not perfectly insulated, so there's more load, but you've not given anywhere near enough information to detail how much load they add.

I use a pair of 12K units in a 2000 square foot building and they (or even just the upper one) cool it trivially to 68F at 90+ outside - because it's well insulated - R35 or so for walls and roof, R17 for the garage door. I only have two because they also heat it, and that's a bit more work (-15F out and 65F in has been seen.)

You need at least SOME insulation everywhere (because where you have none, even a little bit will cut your losses by a huge factor - going from R1 to R5 is 20% the load.) That's a very large return on not much insulation. Getting it up to something halfway respectable like R19 will let the unit cool the place effectively AND save you a good deal on your electric bill.

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