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Community and Q&A

Minisplit System for Apartment Over Garage

nynick | Posted in Energy Efficiency and Durability on

We’re building a new 3 car garage with a 2 bdrm apartment above. The footprint is 28×38. All construction is to code in CT with triple glazed windows and upgraded insulated garage doors. Slab is also insulated.

The HVAC contractor is quoting a 4 ton Mitsubishi Hyperheat system with 4 heads: one in the garage (since I want HVAC in there) and 3 in the apt: 1 in each bdrm and 1 in the kitchen living room. We’re discussing wall mounted units. He doesn’t think a unit in the bath is necessary.

He says this is the least obtrusive and cost effective design. I was under the impression a ducted install was better.

Any comments?



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  1. paul_wiedefeld | | #1

    Ducted for upstairs would absolutely be prettier, quieter and better sized. It’s what the vast majority of people install, leave the ductless for retrofits and 1 room buildings :). I’d be surprised if it was more expensive - ductwork is cheap and you can actually heat and cool all rooms.

    Can you use a separate ductless unit for the garage itself? I’d be worried about temperature swings and contaminants from the garage.

  2. Expert Member
    Akos | | #2

    That 4 ton hyper heat in your climate delivers 4.5 tons of heat. That is not just oversized but ludicrously oversized.

    I'm heating a bit above code min 2bed+den in colder climate with a single one ton ducted hyper heat unit.

    Instead of guessing, figure what your loads are and size from there. Coolcalc or the sizing tool are relatively straight forward to run since I doubt you'll get an accurate man J from a contractor that is so far out already.

    For an ADU, I would go with either a single wall mount in the living space and resistance heat for the rest of the rooms or a ducted unit as suggested above.

    For the garage bellow a dedicated wall mount will work.

    I would not put this onto a multi split shared with upstairs since the heads are never truly off on a multi split so you have heating the garage even when you turn the head off.

    Instead I would get a budget oversized no-hyper heat unit for the garage, this will let you heat up and cool down the place quicker. A standard garage heater would still be the cheapest to install but won't give you cooling.

  3. nynick | | #3

    Thanks to both of you. These situations are so frustrating. The GC's depend on the HVAC subs to know what they're doing and consider my opinion to be questionable. They have experience with these subs and trust their judgement. For me to try to then convince the sub of my way of doing things, along with substantially reducing the size of the units, is an exercise in futility. Worse, I have to then go find an alternate sub and foist them upon the GC.
    So frustrating.

  4. gusfhb | | #4

    For better or worse I have GC'ed my own projects and I do not regret it for a second
    All three subs I had quote my boiler install wanted to count the feet of baseboard in the house to determine the size.
    No please quote the install of this unit for me
    There are frustrations certainly, but the upside is instead of the contractor having relationships with subs, you have relationships with subs.

    1. nynick | | #5

      Understood, but that ship has sailed.

      1. gusfhb | | #6

        Still your house.....
        Look, nobody wants to act like a jerk, but contractors do not live in the place, they get paid, by you, to build it.
        'This is what I want, is there a problem?'
        I have run my own business for over 30 years, so expecting people to do what I say is pretty ingrained at this point.

  5. Expert Member
    DCcontrarian | | #7

    I highly recommend this blog post:

    It's from 2016 but it's still relevant. He goes over the results of 40 Manual J calculations and compares the results by square feet per ton. Quote: "The low number on that graph is 624 square feet per ton. The majority of the cooling loads shown here are above 1,000 sf/ton. Only eight are below 1,000 sf/ton... Those 40 results ranged from a low of 624 to a high of 3,325 sf/ton." And: "If you tell me your load calculations average 400-600 square feet per ton, I assume you’re not doing them correctly. "

    Your building has a footprint of 896 square feet, on two floors. At four tons that works out to 450 square feet per ton. That's assuming equal tonnage on each floor. With three heads on the second floor that floor could be a lot lower.

    The title of the blog post is "Manual J Load Calculations vs. Rules of Thumb." The introduction is: " When I talk to potential clients, a lot of them tell me their contractor wants to size their air conditioner using a rule of thumb. You know how they go: Install one ton of air conditioning capacity for every 500 (or 400 or 600) square feet of conditioned floor area. How far off are they? Let’s take a look."

    It sounds to me like your contractor is a rule-of-thumb guy.

    1. Expert Member
      Akos | | #8

      As much as rules of thumb are can be way off, we can still use it to see if the system is in the ballpark.

      In my climate, a high efficiency structure would come in under 10btu/sqft, something code min 10-15BTu/sqft maybe 20btu/sqft if there is a lot of galzing.

      Assuming code min and that units 54000BTU output, the structure should be about 4000sqft. Since you are less than half that, the system is nowhere near the right one.

      These units do modulate, but not that much, especially something like a multi split it needs to be closely matched to actual load. Installing something that is 2x to 3x oversized is recipe for high power use, comfort and humidity issues.

      Instead of guessing, I would ask the contractor do a man J/S to justify sizing, our building department here requires it so it must be done anyways. There are ways to skew the results on a man J so post it here to see if there is a heavy thumb on the scale somewhere.

      1. Expert Member
        DCcontrarian | | #9

        One of the lines in that post: "It’s easy to get whatever load you want when you do a Manual J load calculation."

      2. nynick | | #10

        He told me code requires Manual S.

        1. Expert Member
          DCcontrarian | | #19

          He should do four calculations:
          Manual J to calculate heating and cooling loads
          Manual S to select equipment
          Manual D to design ductwork
          Manual T to select registers, diffusers and grills to terminate the ductwork

          D and T can be skipped if going ductless.

  6. nynick | | #11

    So I took the time this weekend to run a quick analysis on the BetterBuilt software to see what kind of HVAC loads it says I would need for the apt/garage project I'm building. Bear in mind that the first contractor said I needed a 4 ton Mitsubishi unit to condition both the apartment and the garage.
    I've attached a screenshot of the results. Any comments?

    The software came up with 27K Heating and 11K cooling.

    1. Expert Member
      Akos | | #16

      How does that split out, how much for the garage and for the ADU?

      The heat loss looks to be in the ballpark, definitely far from the 54000BTU the 4 ton unit can supply, infiltration seems on the high side but than again it is very hard to make garage doors air tight.

      1. Expert Member
        DCcontrarian | | #17

        Garage 14.5K heating, apartment 12.2K.

        1. Expert Member
          Akos | | #18

          I guess I should have read that report a bit more closely.

          In that case it can be done with something like a 2.5ton multi split such as:

          This unit has better turndown than most but in case if only the ADU is being heating, the system won't be modulating most of the time.

          I would go back to my original suggestion and put the ADU on a 1 ton on-to-on slim ducted unit:


          Get a budget 1.5 ton wall mount for the garage.

          1. nynick | | #21

            I think this is a good way to go.

  7. walta100 | | #12

    Are you willing to give up the space required for duct work?

    Will your code enforcement allow one forced air system for both the garage and the living spaces?

    When we read sad stories about miny splits they almost always involve oversized equipment with multiple heads.


    1. nynick | | #14

      Yes, in fact I want ducted. Plenty of ceiling space in the garage.
      No, the forced air can't be shared in both spaces. They must be separate.
      I'm trying to avoid any sad stories.

    2. Expert Member
      DCcontrarian | | #15

      Walta, what do you think of a 2-ton compressor, one ton head in the garage, half ton in the apartment bedroom and half ton in the living room?

  8. gusfhb | | #13

    1 ton wall mount in the garage[Find one like my Pioneer that has a low temp 'away from home setting]
    1 ton ducted upstairs. Find a closet ceiling or drop ceiling you can hide it in
    I think your building inspector might disagree with your contractor. almost 20 years ago mine said 'you need heat in this bathroom' I put one of those wall mount electric units in, and it was great. Sitting on the throne an turn it up to warm up. Even better than electric radiant IMO.
    You won't need AC in the bath unless you have a big south facing window in there.

    Similar to mine for garage with low temp setting:**LP+-+Shop+-+Pioneer+-+Mini+Split+-+Ductless+-+Wall+Mount&utm_term=4579878248076278&utm_content=shopify_US_8631561360_33168702352+%7C+Pioneer+12%2C000+BTU+21.5+SEER+115V+Ductless+Mini-Split+Air+Conditioner+Heat+Pump+System+Full+Set+10+Ft+%7C+%24893

  9. nynick | | #20

    Very helpful everyone! Thank you!!!

    I have another HVAC company coming out Thursday to quote the project, this time a Mitsubishi Diamond dealer. Should be interesting to see what they come up with. I'll keep you posed.

  10. gusfhb | | #22

    Again, rather than
    'what do you think I should do here?'
    more like
    'this is what I want'
    Mitsubishi makes several 12k ducted units, varying by performance and low temp heating.
    Having an example model number would not be a bad idea.
    FOr example with low ambient which many don't:

  11. walta100 | | #23

    For the way I would use garage heater a heat pump is a poor option.

    I would rarely heat the garage and when I do, I would want a large heater that would quickly warm the space. If heating the garage is a rare event then a cheap large electric heater is the low-cost option.

    If the garage is a working shop every day with several cars coming and going gas radiant heater or an oversized gas unit heater are a good option.

    If the goal is to keep the garage just above freezing so daily driven cars can rust away quickly then a heat pump would be perfect.

    I do like the idea of 2 separate systems


    1. nynick | | #24

      The garage will hold 3-4 cars at all times. Some of them are collector types/classic. We're also on the water in coastal CT, so it can get be pretty humid. My thinking was to keep the cars 50-60 degrees in the winter so I can work in there if i want and low in humidity in the summer with AC.

      The apartment above is pure living space. 2 BRMS, 1 Bath.

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