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Community and Q&A

New home and ductless minisplits

Peter L | Posted in GBA Pro Help on

In the process of getting the Manual J done as per code requirement. The home is as follows:
Zone 4B – R-10 / R-30 / R-60 with R-8 windows – ICF walls – Slab on grade – 2 stories – 3,200 sq. ft. – Open floor design.

Looking to go with Mitsubishi ductless heat pump minisplits wall units (22 – 26 SEER). Two on the 1st floor and two on the 2nd floor.

How good are the ductless cassette units?

One room might get the cassette unit instead of the wall unit because of the room layout.

I noticed Mitsubishi cassettes only go to 16 SEER while Fujitsu cassette units go to 24 SEER. Why such a difference in performance?

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  1. Expert Member
    Dana Dorsett | | #1

    Let's just say the Fujitsu engineers have the jump on the Mitsubishi guys when it comes to mini-ducted designs & control algorithms. :-) But on the wall coil units it's a real race!

    For a zone 4B location you should be more interested in HSPF numbers than SEER, but there again the dedicated mini-duct units from Fujitsu have the edge.

    The latest-greatest cold climate single wall-coil type mini-splits (the -FHxxNA series) from Mitsubishi may have an efficiency edge over the Fujitsu competition (the -xxRLS2-H), but it's a moving target. Between predecessors of both (-FExxNA & xxRLS2, no "-H") the Fujitsu units outperformed the Mitsubishi units, but the specified operating temp range stopped at -20C, whereas the Mitsubishis were good down to -25C.

    The decision on which to install often boils down to how much local support you can get out of the distributors & installer networks, provided it has the capacity to cover your load at the 99% outside design temp. At the 99% outside design temps of most zone 4B locations there's still pretty good capacity with cassettes or wall-coil units from either manufacturer. Just try to avoid oversizing the capacity at your outside design temp by more than 50%, which leads to more on/off cycling during light load periods, which results in bigger room temp swings, more noise, lower comfort etc. At 1.25-1.5x oversizing there is usually a modest uptick in average efficiency, but beyond that it's pretty flat, and eventually falls so oversized that it's cycling on/off more of the time than it is modulating.

  2. Peter L | | #2

    There is a Mitsubishi Diamond Dealer just 15 minutes away so that is a huge plus. For the Mitsubishi ductless cassette the HSPF rating is 9.6

    On a tight energy home, for rooms that have a very tiny window (code min), if the adjacent room already has a mini split, will that room require a mini also or can it get by without it?

  3. GBA Editor
    Martin Holladay | | #3

    Q. "For rooms that have a very tiny window (code min.), if the adjacent room already has a minisplit, will that room require a minisplit also, or can it get by without it?"

    A. The answer depends on your family's expectations. If you leave doors open during the day, and you don't mind 5 degrees of temperature difference from room to room, you don't need a minisplit in every room. (A minisplit in every room is usually overkill, in fact.)

    For more information, see Rules of Thumb for Ductless Minisplits.

  4. Expert Member
    Dana Dorsett | | #4

    Code requires that the heating system be capable of getting every room to a minimum of 68F at the 99% outside design temp. That may take heating the adjacent room to 75F or more to get there, but if it can do it, you're fine from a code perspective.

    Limiting the window size and using only high performance windows (U0.20 or lower) in any doored-off rooms that have no dedicated heat source is usually key to making it work well with the doors closed.

    The higher the building envelope performance, the less the HSPF matters, as long as it has the capacity to cover your loads. An HSPF of 9.6 isn't terrible at all, given that an HSPF of 7 would still be legal to install. And depending on how optimally it is sized it is you may even beat that in real-world performance in a zone 4 climate. An HSPF of 9.6 only looks mousy when compared to the HSPF 11-12 performance of the Fujitsu's dedicated mini-duct cassette mini-splits. Taking the 1-ton as an example, the 12RLFCD tests at an HSPF of 11.5 , the tests at an HSPF of 10.0:

    That's 15% more heat per kwh of energy input, but it's ONLY 15% more. If your heating bill for the winter is only $400 due to the high performance building envelope, that's only about a $60/year difference. If you're feeling the carbon-emissions guilt for that extra power use, opt into to a "renewables only" power purchase agreement (if those are allowed in your state) and pay $70/year extra, or install some PV solar on your roof. (With any new construction it's advisable to consider site orientation, shade factors & roof pitches to optimize future PV, even if it's not installed on day 1. At the rate PV costs are falling it will be compelling long before the shingles need replacing.)

  5. Peter L | | #5

    There is only ONE window in the room and it is basically code minimum size for fire egress. The window has a U-Value of 0.16 and the SHGC is 0.23 (plus it is shaded from the roof and porch overhang).

    So even if the room shares an interior wall with a room that does have a mini split in it, the room without would still see a 5 degree variance from the other room? That seems high. I was expecting more like 1-2 degrees at most.

    I was told by a mini split installer that Mitsubishi is conservative on its ratings while Fujitsu is not. I don't know if what he says is true but if you look at the performance, they are neck to neck in the real world. It's like car manufacturers, Hyundai claimed 41MPG but real world results showed more like 34MPG. They cooked the books/numbers. Is that true with Fujitsu? I don't know...

  6. Charles Spalding | | #6

    I use mitsubishi and have had little to no trouble. Install the hyperheat models. The smaller systems tend to be more efficient. They have ducted models that do several rooms and be tucked into an attic space. I have installed a lot of mitsubishi and worked on a lot more fujitsu.

  7. Peter L | | #7

    The other option is the ducted MItsubishi unit which has a 16.5 SEER and 10 HSPF rating. The duct run would be very short, maybe 3-5 feet.

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