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Correctly calculating an A-frame mini split load so I can weed out bad HVAC guys?

donarntz | Posted in Energy Efficiency and Durability on

I have an a-frame, almost perfect 32x32x32 triangle. Using online calculator I got about 22,400 cubic feet (this does not include the 3′ conditioned crawl space). I have 2 bedrooms upstairs w/ bath and 2 downstairs with a primary bath and an open plan with a great room that goes all the way up 27 feet that is roughly 384 square feet at 1st floor. I’ve spoken on the phone to the hvac people and gotten one wildly high estimate from one.  These guys can’t seem to wrap their head around the triangle shape I guess. I also have to mention I am required by code to do the ample roof / walls as roof insulation at what is that r38? 

So the estimate I’ve gotten so far is a 6 ton mini split system for $29000. Doesn’t that seem a bit high for a 1600sf house? He wants to put a unit in every bedroom when the bedrooms are only 150sf or so each? I have ample crawl space room, but from what I’ve seen mini splits are only ducted from the ceiling correct? I only have 8′ ceilings.

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  1. paul_wiedefeld | | #1

    "So the estimate I’ve gotten so far is a 6 ton mini split system for $29000. Doesn’t that seem a bit high for a 1600sf house?"

    That's about the worst load calculation I've ever seen. Hopefully you have other contractor options.

    "I have ample crawl space room, but from what I’ve seen mini splits are only ducted from the ceiling correct?"

    Nope, you can easily duct through the crawl space. That solves the first floor. The second floor may prove more challenging - perhaps you can duct the two bedrooms together, possibly through a closet or a lowered hallway ceiling. The second floor bathroom might be best served with a tiny resistance heater, which may be good enough for the two upstairs bedrooms as well.

    This is an existing home? You can calculate your true heat loss this way:

  2. donarntz | | #2

    Thanks for replying. It's a new build. If I can run mini split ducts through the crawl space, why can't I find much about it online? Do you mean with a mini split air handler, much like a traditional air handler? Would I be able to connect that air handler as a multi-zone unit?

    1. paul_wiedefeld | | #3

      "Do you mean with a mini split air handler, much like a traditional air handler?" Precisely - ductwork is ductwork. Mini-split or otherwise, you're looking for a ducted heat pump. The contractor prescribed the worst system for your house. Here's an example: I think nearly all minisplit brands have a version.

      "Would I be able to connect that air handler as a multi-zone unit?" Yes, but why? Since this is a new build, design ductwork into the upstairs bedrooms. Multi-splits (one outdoor to many indoor units) should be the last resort. Putting a ductless head in every bedroom will lead to poor comfort (they're louder than ducts and would cycle on and off a lot based on that insulation and how oversized that equipment is) and low efficiency. Even if only the first floor gets the ductwork, I'd consider either one outdoor unit-to-one indoor units minisplits upstairs (they modulate much better) or go with resistance heat up there if cooling isn't desired.

      1. donarntz | | #5

        Oh cooling is definitely needed. Because of the triangle shape, I know (from testimonies) the heat will concentrate up there. I am trying to avoid whole house ductwork, but I understand if it will be best for downstairs. What I am interested in is redundancy. I live in a smallish town and should the AC / Heat go down, having it fixed right away isn't an option. What I had originally hoped for was perhaps a single zone for the large expanse of the house and then a multi zone unit for the bedrooms.

        1. paul_wiedefeld | | #6

          That's the right idea - however, multi-zone units won't work well with small zones like bedrooms in a new, well-insulated house (which is of course how they're marketed and installed, but the manufacturers themselves advise against it). Maybe ductwork for the downstairs and then two additional ductless units upstairs - you get the redundancy, better load matching, higher efficiency, and it'll probably be cheaper. So three total outdoor units, which probably will come in less than half the size of what was proposed. Or combine the upstairs bedrooms if possible, and go with 2 outdoor units total.

          1. donarntz | | #9

            Thanks so much for your assistance! I think you are probably correct. I am taking advantage of a freelancer site to hire an HVAC engineer. Hopefully he will guide me to the right sized equipment to use. No more unexpected and ridiculous estimates.

  3. donarntz | | #4

    I just decided to post the job on upwork in the hopes of getting a qualified HVAC freelancer to sort this out for me. It may be easier in the long run to just tell a qualified installer what to do rather than rely on someone who isn't up to the task of proper calculations.

    1. Expert Member
      NICK KEENAN | | #11

      The job you want to quote is "Manual J calculation."

  4. greenright | | #7

    What zone? This will dictate your heating and cooling load... Off the top of my head I would say for zone 5 you are looking at 25-30k btu on design day (around 0-5 F)... so a 3 ton tops assuming average and code insulation for the A- frame...

    1. donarntz | | #8

      According to the GA residential field guide I am in zone 3.

  5. Expert Member
    NICK KEENAN | | #10

    Immediately cross off anyone who quotes a size without visiting the site with a tape measure in hand.

    There are no rules of thumb in this business. The proper process (called a "Manual J") is to estimate the heating load on a day that is colder than 99% of the days in your location (the "design day") and the cooling load on a day that is hotter than 99% of the days. The way the estimate is done is to go through the house room by room, and look at amount of the room exposed to the outside, and what those walls are made of. Then look at all the doors and windows and what they're made of. Then look at any sources of heat inside the house. Then calculate the angle of the sun and how much heat is going to come in from solar energy. Ideally then you measure the leakiness of the house using a blower and estimate how much heat is gained and lost from air infiltration. It takes some cooling to dehumidify in summer, so you estimate that too.

    Once you've done every room, you add up all the rooms and get numbers for the whole house. Then you design the system to deliver the right amount of heating and cooling to each room.

    If it's an existing house and you have actual utility history, that can be used and will typically give you a more accurate estimate.

    I hope you see this process is more involved than just sizing based on floor area or even building volume.

    1. donarntz | | #12

      Well at this stage there is no building... concrete shortage is preventing the foundation from going in quickly. I do have detailed measurements and floorplan.

      1. paul_wiedefeld | | #13

        Related to the supply shortages: I think you’ll get better sizing this time around and hopefully a lower quote but I wouldn’t expect it to be that much cheaper

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