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Mitsubishi sizing

Colt Freeman | Posted in Mechanicals on

I’ve built a SIP Panel house, 6″ R24 walls and 10″ R40 roof, Low E windows .31 on the sliders and .29 on the standard 3×5 windows. Floors will be packed with cellulose between my to be finished basement at a later date. I’m currently

My plan calls for a minisplit in both upstairs bedrooms (I want a single unit that could be ducted into both with a short run of 6′ either direction but I’ve not seen on that will do that) 1 for the Master bedroom on 1st floor, one in the kitchen that would service both the kitchen and living room since it’s all an open area. I’d get (2) 3 port Mitsubishi M’s that would allow for me to add one to the basement at some point.

I will be installing a ERV that is capable of 50-140cfm and it will be ducted to the 2 bathrooms and 1 kitchen for the intake and will exhaust to the 3 bedrooms.

The problem I’m running into is that even with 9k units I’m going to have way more cooling/heating than I need. We will be heating and cooling solely with these units in zone 4, I’ll have a 20k BTU propane wall fireplace as well eventually but not right away. I have a wood fire forced air furnace from charmaster that I was going to install but I realized it would be incredible overkill at between 80,000-100,000 BTU’s depending on the wood that I would be using so I’ll be selling that on craigslist sadly.

I did the following Manual J:



Idea’s? Am I just going about this the wrong way? In polling the HVAC people in my area I don’t really trust any of them with SIPS calculations nor Minisplits as the only ones that are used are the tack it on the side of the house with a 1ton unit ‘remodels’. There aren’t any SIP houses in my area that I’m aware of. I’ll be installing them myself and having a local ‘charge’ them.

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  1. GBA Editor
    Martin Holladay | | #1

    I'll begin my advice by repeating an oft-heard phrase: "It's hard to provide advice unless you tell us your climate zone or geographical location."

    In Massachusetts, Carter Scott has had good success heating and cooling houses like yours with two ductless minisplit units: one upstairs and one downstairs.

    In your case, I would put the downstairs unit near the center entry door on the left side of your plans -- I don't know if this is the east, south, west, or north wall -- aimed at the Eating Bar.

    I would put the upstairs unit in the Sitting Area.

    If your family keeps the bedroom doors closed all day long, this approach may not be satisfying, although Carter Scott finds that all of his customers accept this approach.

    For more information, see these two articles:

    Just Two Minisplits Heat and Cool the Whole House

    Rules of Thumb for Ductless Minisplits

  2. Colt Freeman | | #2


    I did actually put the zone in but I apologize that I buried it in a paragraph. Zone 4, central VA. We have humid summers great shoulder seasons and a few weeks of single digits depending on the winter. I'll have a backup heat source with an in wall propane of about 20k BTU.

    House front (bottom of drawing) faces almost north (tad east rotation)

    Im having trouble locating drawings for ducted minisplits, I don't trust any of the pros around here to do it because none of them have ever designed for them. I'd love to use a single unit upstairs split to each bedroom, a unit in the master downstairs split to music room and one in kitchen/dining. That way bedrooms are covered. Does this seem feasible? My dad has done commercial HVAC control wiring years so we are fairly comfortable with HVAC just not specific to minisplits.

  3. GBA Editor
    Martin Holladay | | #3

    Sorry I missed your climate zone reference -- my mistake.

    If you do a little Googling, you can find installation manuals for ducted minisplits. For example, here is a link for the installation manual for the Mitsubishi SEZ-KD09.

  4. Expert Member
    Dana Dorsett | | #4

    First, the Manual-J output of 32,258 BTU/hr @ +17F is quite bit on the high side for a better-than-code 2410' house.

    Not all of the assembly substitutions used to in the calculation in lieu of a SIP make sense. An R38 between the joists ceiling is lower performance than an R40 SIP (by quite a bit), though that's not enough to make the difference. An R49 between-the-joists solution comes in at less than R39 "whole-assembly" after thermal bridging, including ceiling, roofing & air film R, and an R38 between joists attic is only ~R33 whole-assembly. An R41 SIP has some thermal bridging to the splines, but it also has roofing, ceiling gypsum, and air films, would be closer to an R49 roof (probably even a bit higher performance), than an R38 roof.

    The 2813 BTU/hr ceiling losses are really more like 2435 BTU/hr, or about 380 BTU lower than calulated, bringing the 32,258 down to 31,878 BTU/hr.

    But that's not nearly as large an error as the 100cfm "fresh air" load of 5655 BTU/hr, since you are ventilating nominally with the ERVs(?) with a ~75% sensible heat return. The real fresh-air heat load at a 53F temperature difference (70F in, 17F out) will be under 1500 BTU/hr even if you're giving it the full 100cfm. (100cfm x 60 min=) 6000 cubic feet per hour x 0.018 BTU per degree-F x 53F= 5724 BTU/hr, but with at 75% sensible heat efficiency from the ERV that drops to 1431 BTU/hr. Without looking up the actual sensible efficiency of the ERV, assume the ventilation load is about 1500 BTU/hr, not the tool-calculated 5655 BTU/hr, which immediately takes ~4K off the load.

    That lowers the 31,878 BTU/hr number down to about 27,878 BTU/hr.

    The 668 BTU/hr for the 21 square feet metal door is also high, unless it's metal clad solid wood rather than a run-of-the mill R4-ish insulated metal exterior door (recommended.) If it's a insulated metal door, cut that in half, knocking another 334 BTU/hr off the total.

    Now you're down to the 27, 544 BTU/hr.

    An R24 SIP with a wallboard interior is going to have a whole-wall R close to R22 (or even higher depending on siding) after factoring in the splines, headers, and window & door framing plus air films, no matter what siding you install. The 2x6 /R21 + R4 sheathing wall you simulated with is about R19-R20 whole-wall, not more. In reality you should subtract at LEAST 10% off the 5421 BTU/hr wall losses, call it 4875 BTU/hr (probably high- depends on the siding), knocking another 546 BTU/hr off the whole house load.

    So now you're at 26, 998 BTU/hr.

    If the floors are packed with cellulose and bigger than 2x6, you have a LOT more than R21 in there. If the floor joists are 2x8s (pretty skinny) you have about R27, and if they're 2x10s (more likely) it would be about R35. So the floor losses will go from 2805 BTU/hr, to about (R21/R35) 2805= 1683 BTU/hr, or 1132 BTU/hr less than calculated...

    ...bringing the heat load number down to 25,886 BTU/hr.

    The 25,886 BTU/hr / 2410' is 10.7 BTU/hr per square foot of conditioned space, which at least COULD right, but I would have guesstimated it to be lower still given the fairly simple shape (= favorably low exterior surface area to floor area ratio.) I don't know what other calculation errors may be lurking there, but the infiltration losses feel pretty padded for an easy-to-air-seal SIP house.

    You're probably looking at a true whole heat load in the 22K-23K range if you build it tight, well within the heating capacity of 2-tons of Mitsubishi M-series. And you're planning on TWO of them? The room by room load numbers would be important for sorting this out, but you're probably going to be fine with a 6K head in each bedroom (still overkill, but ...) and some other solutions for the rest. If you can get it down to 4 zones with a mixture of ducted & ductless, the 3-ton MXZ-4C36NAHZ wouldn't be extreme overkill for the loads, since the compressor unit can still throttle back to 7200 BTU/hr @ +47 in heating mode, and down to 6000 BTU in cooling mode, but a pair of 2-ton 3 head units would be cycling rather than modulating, literally ALL all the time. The modulation range of the compressor of just ONE 2-ton 3 head M-series is 12,600 BTU/hr in cooling mode, and 14,000 BTU/hr @ 47F in heating mode. With a pair of them the minimum modulated heat output for the two will exceed your whole house load the vast majority of the time. (x2)

    vs: (X1)

    If you really need more zones, the 5-zone 3.5 tonner would still do better than a pair of 2 tonners:

    You might want to consider doing the first floor with a 1.5 ton Fujitsu -18RLFCD (or even the 1-ton -12RLFCD) mounted in the ceiling of the basement, and pair of half-ton Mitsubishis upstairs. The Fujitsu mini-duct cassettes have much beefier blower than the Mitsubishis, and can handle longer runs. The any of the xxRLFCD units will throttle back to 3100 BTU/ hr in heating or cooling mode, and the MSZ/MUZ FH06NA Mitsubishi will modulate down to 1600-1700 BTU/hr. That gives you about the same modulation range as a 4+ zone Mitsubishi multi-split, but it would offer more flexible installation options and higher efficiency.

  5. Colt Freeman | | #5

    Thanks Dana that's giving me a lot to think about! That's a huge help.

    10" joists definitely, the doors will temp be insulated metal clad as you assumed. Eventually I'll be building a solid oak door.

  6. Expert Member
    Dana Dorsett | | #6

    A 2" solid oak door runs about U-0.5, which would make the original door loss number in the right ball park.

    It's possible to build or buy an insulated wood door that looks solid, with a lower U-factor than that.

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