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Mini-Split and high seer HVAC approach

HartiganWill | Posted in Mechanicals on

I’m in the design process of a new house and weighing my options for HVAC solutions. This site and others turned me on to mini-splits and even ducted mini-splits which sound great for many reasons. Two items I’m a big fan of are the variable refrigerant flow and high seer, as well as the ease of eliminating HVAC outside the unconditioned envelope.

Fast forward a couple of weeks of research and I see two pitfalls.

1.) A lot of standard central HVAC equipment seems to be adopting VRF and has comparable seer ratings from what I can see. The sky-high seer on mini-split typically only comes from the wall-mounted heads; the ceiling-mounted cassette or mini ducted systems drop back down to 20ish seer which is no better than standard HVAC.

2.) The filtration on mini-splits seems pants compared to what you can do with a large air handler.

What’s typically the best approach to this? The house will be a quite large 1 story in Phoenix AZ so the biggest demand will be cooling capacity.

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  1. paul_wiedefeld | | #1

    There’s not really any pitfalls here, just different equipment for different applications.
    Your situation is easy enough: high seer ducted (minisplit or otherwise) gets you high efficiency, filtration, and an economic install. Important to note that you can mix and match - duct as much as you can inside the envelop and if there’s a room where that’s not feasible, add a ductless head there.

  2. jameshowison | | #2

    I agree that filtration is a problem with ductless mini-splits ("pants" hah, love it); it is also at least slightly difficult to design in with slim horizontal ducted (either a "bell" up to a large square for an inline filter, or various register mounted filtration options).

    Ventilation is still required (ie other than somewhat exotic approaches like Lunos HRVs, or Panasonic ERV spot ventilation). Not to mention bath fans, kitchen hoods. So there will be ducts ...

    1. PBP1 | | #3

      Great points, noting that Mitsubishi has a filter box FBL-1 for SEZ horizontal ducted air handlers that takes filter(s) on an angle without a "bell" up, FBL1-2 has 12"x34"x1" (actual dimensions 11.5"x33" about 2.6 sq ft of filter in a 6"x34" throat).

      For many reasons, I recommend separate filtration (PACs or other) automated with sensors (usually particulate sensors). It's possible to tailor some of the ventilation in relationship to sensor activity, the more sensors/types the better. Filters were originally to protect the air handler (mainly heat exchanger core), somehow they morphed into double duty (upselling?).

      Given a sufficient level of humidity, some wildfire smoke and other particles can be removed via condensation on coils during cooling season.

      And, how about directing air handler condensate to xeriscape?

      I may be that many equipment ratings do not adequately account for filtration, here's an article on minisplit and filtration (MERV11 electrostatic):

      "The result showed that the concentrations of small particulates could be reduced moderately with the full filter applied, however with 16.50% reduction in the cooling capacity and 13.24% reduction in EER."

      1. HartiganWill | | #4

        What separate filtration are you referring to? When I search on this topic every robust filter type looks to be designed to hook up to a large air handler.

        1. PBP1 | | #6

          National Institute of Standards and Technology (NIST) is a good resource for indoor air quality (IAQ) techniques/technologies. NIST has a lot of info on portable air cleaners (PACs), noting that various commercially available PACs have particulate sensors that automatically switch them on/off. Additionally, Dyson's compressor wheel tech (just like a turbocharger compressor wheel) in its PACs (and other products) is efficient, more so than squirrel cage blowers found in most air handling equipment.

          I worked in clean environments, facilities with spray dryers, biologics and asbestos and ran tests on dryer emissions where IAQ raised production, health and safety issues. It seems as if the residential home trade in the US got hooked into the bigger is better philosophy and started to borrow from industrial practices (usually with little notion of "green").

          A 5" thick pleated filter with substantial pressure drop in a heating/cooling air handler being used 24/7 is overkill, especially when IAQ can be adequately sensed and controlled. A heating/cooling air handler filter also demands user attention, as without changeouts/cleaning, efficiency drops, impacting heating/cooling (comfort and cost). And, when there's no heating or cooling demand? Running the heating/cooling equipment 24/7 merely for the sake of filtration is not a green solution, especially if IAQ is an issue in only a single room of a house (e.g., a kitchen).

          As the technology is available to put filtration equipment and ventilation equipment on autopilot, I believe we should be using that technology. Such an approach is aligned with the original purpose of heating/cooling equipment filters, to protect that equipment, not to clean the air.

          NIST continues to study HVAC and IAQ in light of the pandemic, some really good studies are emerging. The solution to the IAQ/ventilation problem should be "green". Perhaps one day all residential heating/cooling air handlers will be "low static" with efficient blowers/blower motors, saving considerable energy.

          GBA article on IAQ by Dr. Bailes (April 2022):

  3. jj1 | | #5

    This Carrier Infinity ductless mini split has a 42 SEER rating on the NEEP database:
    CARRIER Infinity Singlezone Non-ducted Wall Placement AHRI Cert #: 201753408
    Outdoor Unit #: 38MPRAQ09AA3 Indoor Unit #: 40MPHAQ09XA3!/product/26452/7/25000///0
    You could install 2+ units for multiple zones in a single story application? Carrier also has larger BTU cooling models albeit at slightly lower SEER. One limitation: All wall mounted units come with those small internal air filters, however.

  4. exeric | | #7

    I think there may be a misconception in the original premise of the OP's question. I generally learn by doing and I observed something interesting to me when I DIYed my own ductless mini split installation. There is no air exchange between the outside air and the inside air on the wall air handler. That means that there is none, zero, nada ventilation capability on that mini split. This means that the puny 1/8" thick filters don't even need to be there if you don't want to clean the existing air in the home. All those filters do is keep dust from gumming up the internal workings of the air handler as air get circulated while being heated or cooled.

    I also agree with the gist of Allison Bailes' article:

    If you need ventilation to remove polluting gasses, then the most efficient approach is to use an ERV or HRV. If you need to remove particulate matter then a good size stand-alone filter and fan unit to clean the inside air is recommended, Division of labor in those chores is the way to go and is the most efficient way. You can then turn them on or off as needed. Also, if one of them breaks you don't lose both ventilation and removal of particulate matter.

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