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Minisplit sizing, and ventilation question

Trevor Chadwick | Posted in Mechanicals on

So I’ve been playing with manual J, beopt,  and have neared a solution, I think…
To start it off, I’m in CZ 6 design temp -6°F 
1200 sf rectangle, whole wall comes out to R30 using reclaimed polyiso outboard of the sheathing, and R60 fluffy cellulose in the attic
That gives me a heat loss of 8877 btu.
First question is on ventilation, I’ve done lots of searching and reading, but it looks like there isn’t a widely agreed upon answer, so Per 2012 IRC I need 45 cfm.  The spreadsheet for manual J has an option for 50 cfm, this is an additional 4117 btu.    Am I in the ballpark, is it too much, or not enough?
I assume/hope that an hrv/erv will reduce that, but is it as simple as taking the efficiency of the unit, and multiplying to determine how much of that 4117 btu are saved?  (assuming you can get anyone to tell you what the efficiency is)

The next questions are the often asked “will this design be ok with a single minisplit head in the common area?”  and “which mitsu would you reccomend?”  (friend of mine works for a mitsu and LG dealer will do the commissioning for me, and recommends the mitsu out of the two)

I’ve attached a sketch of the floor plan with the room by room heat loss, excluding ventilation

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Replies

  1. Jon R | | #1

    > 2012 IRC I need 45 cfm

    Code minimum may allow that, but do you want the the worst you can legally build when it comes to a health issue? I'd also make sure to deliver at least 15 CFM/person to any occupied breathing zone (as in IMC Table 403.3.1.1).

  2. Expert Member
    Dana Dorsett | | #2

    With a heat load of nearly 2K for the master bedroom and almost no common partition wall to the main living/kitchen area it's not going to make it with a single ductless head at design conditions. If it shared a full common wall and perhaps a smaller bedroom window (= lower load) it might. The fact that the bedroom walls are mostly exterior increases the heat load, and separated from the main living area by the bathroom & closet means the conducted & convected heat transfer from the main area to the bedroom is very limited.

    Is there a crawlspace/basement or conditioned attic for running ducts? (Looks like there a stairwell...) A ducted cassette located fairly centrally under/over the master bedroom closet area could have very short straight duct runs to the bedrooms and still serve the main area.

  3. Trevor Chadwick | | #3

    Jon, There are only going to be 2 people living there so 62.2 calls for 51 cfm. I though I was playing it safe with 50 cfm. Will have to re evaluate I guess.

    Dana, There will be my hobby shop below and because I want to clear span it, the floor will be 24" trusses, so ducting should be relatively easy. Going to a code size exit window drops it to 1779, that and increasing the walls to R40 will end up at 1661. I'd guess that isn't enough change to make it work. I'm not surprised, that a single wall hung won't work, but thanks for reassuring me.
    Do you have a suggestion for a ducted cassette combo?

    1. Expert Member
      Dana Dorsett | | #4

      >"Do you have a suggestion for a ducted cassette combo?"

      Does it have to (or is it preferred to) be Mitsubishi?

      1. Trevor Chadwick | | #5

        Doesn't have to be Mitsubishi, but they are preferred. (its the I know a guy thing, he manages the local distributor for Mitsubishi, and LG and prefers Mitsubishi. Will give me a great deal on either, let me do the install, then come and vacuum it down and fill it up)

        1. Expert Member
          Dana Dorsett | | #6

          At maximum speed the Fujitsu AOU/ARU9RLFCD is good for 14,000 BTU/hr @ -5F (the lowest it goes in the extended temperature capacity chart) so even though it's not specified I'm pretty confident it will cover a ~9K load @ -6F. That series is specified at 0.4" water column static pressure, compared to 0.2" for the comparable KDxxNA Mitsubishi mini-duct cassettes (which also have lousier low-temp capacity), which makes the ductwork design a bit easier.

          https://www.fujitsu-general.com/us/resources/pdf/support/downloads/submittal-sheets/9RLFCD.pdf

          Another nice feature with the RLFCD units is that they can be mounted vertically, whereas most mini-duct cassettes out there are horizontal only. It's possible to make a 7-10 square foot "utility closet" to house it.

          To have enough capacity with the wimpy-blower KD/KA series Mitsubishi would call for the 1.5 ton unit:

          https://ashp.neep.org/#!/product/29056

          The beefier PEAD series mini-duct cassettes from Mitsubishi can support 0.6" w.c. static pressures, but it too takes a 1.5 tonner to have sufficient capacity at low temp:

          https://ashp.neep.org/#!/product/28991

          The full size air handler version would also have to be a 1.5 tonner:

          https://ashp.neep.org/#!/product/29008

          A full size 1.5 ton LG would likely cover it too:

          https://ashp.neep.org/#!/product/29554

          None of those have pan heaters on the outdoor unit and would have to be monitored for pan ice build up (and manually thawed/drained if the drain plugs with ice) during extensive use at colder temps.

          Fujitsu's even beefier RGLX cassettes (0.8" w.c. blowers) would work too, but the smallest is a 1-ton, but with a wide modulation range not necessarily overkill:

          https://ashp.neep.org/#!/product/25349

    2. Trevor Lambert | | #7

      Any system should have boost capability. What happens if you have a gathering of six people and your system is maxed out at 50cfm? It will not take long for the CO2 levels to start running away, as in a couple of hours or less. My house runs at about 90cfm with 2.5 people and a very small dog. If I leave it at that when the family comes over, the levels go from 500s to 800s within two hours (and still rising, but I've never let it get higher than that before boosting the rate).

  4. Trevor Chadwick | | #8

    Dana,
    Thanks for the list!!
    Trevor,
    (there aren't too many of us out there) I'll have to do some more thinking about the ventilation.. I've never even seen a HRV/ERV in person, or been in a house with one, so that part is pretty new to me.

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