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Community and Q&A

Does the new Mitsubishi ductless minisplit really work efficiently/reliably for zone 5A (Boston)?

Beatgen | Posted in Energy Efficiency and Durability on

I have a 270 degree glass sun room that I am joining to the adjacent kitchen.
The main house is 1968.
I am beefing up wall insulation to R19
Ceiling & floor to r38

Can this really work efficiently sub 25 degrees F? Is it reliable?

My alternative is to extend the existing gas hot water system to this new 300 sf room via radiant floor
6000BTU and remainder forced hot water baseboard heat. Nice & reliable, benefits from low us gas prices.

I still need an additional AC solution if I don’t do the heat pump. Just hate the idea of using electricity for heat.

Contractor suggests radiant floor main with ductless for “backup” heat.

Thanks for your help!


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  1. Expert Member
    Dana Dorsett | | #1

    Yes, the Mitsubishi Hyper Heating units work just fine in a zone 5 location, even in zone 6 locations, and can be properly sized for the loads at temps as low as -15F.

    At Boston's natural gas & electricity rates heating solely with the ductless is comparable in cost as heating with 82% efficiency gas, and slightly higher than condensing 95%+ gas. But that's the seasonal average. When temps are above 35F it's cheaper to heat with the mini-split than with a condensing gas boiler (!).

    But every good heating/cooling solution begins with a careful load analysis. I'm also curious how you're intending to increase the wall-R, and where it's starting out?

  2. GBA Editor
    Martin Holladay | | #2

    Glass sunrooms are difficult to keep cool in sunny weather and warm in cold weather -- regardless of what fuel you are using for heating and cooling. The heating load for such a room will be very high, as will the cooling load. Be careful.

    Many sunrooms are walled off and abandoned after a few years because they are uncomfortable and expensive to heat and cool.

  3. Beatgen | | #3

    Thanks. We love the room and need the space. if we need to eliminate glass will do it. Exterior walls that are not glass will be filled with closed cell foam and 1" rigid high density foam board over that.
    Plan to put 3M film on windows to cut solar heat gain.
    Will likely change late 70's Anderson double pane to something better.
    Have not done the load analysis... Btu estimated need comes from a vendor that saw the space.

  4. Expert Member
    Dana Dorsett | | #4

    Closed cell foam between studs is a total waste of high-R foam, an unless it's one of the very few products using low global-warming potential (GWP) blowing agents, it's a crime against the planet! Most closed cell foam is blown with HFC245fa, which pound for pound has over 1000x the lifecycle GWP of CO2.

    A typical 16" on center studwall has a 25% fraction of wall area that is thermally bridging framing. A 2x6 wall can only take 5" of R6/inch foam since you can't easily trim it, and at a 25% framing fraction that's only R16 (if you add R1 for the sheathing, siding and gypsum combined.). The same 2x6 wall with R20 cellulose or open cell foam (blown with water, not HFCs) comes in at R14.4. That's a VERY expensive way to get an R1.6 improvment!

    Polyisocynaurate is blown with pentane with a GWP of ~ 7x CO2, and runs about R6 to R6.6 /inch. Putting 1.5" of polyiso on the exterior of a 2x6 cellulose insulated studwall raises the whole-wall R to about R23.5-R25, and has sufficient R value exterior to the sheathing to protect it from wintertime moisture accumulation, eliminating the need for interior vapor retarders (1" wouldn't quite be enough for Boston in 2x6 construction.)

    Rigid XPS (pink, blue green) is blown with HFCs with even higher GWP than those used for closed cell spray polyurethane, and is slightly lower R/inch than polyiso, but a higher $/R. EPS (the bead board stuff of cheap coolers, packing materials & coffee cups) has lower R/inch, but is blown with the much more benign pentane, and has comparable cost to polyiso in terms of $/R. (Where you best use the different materials usually has to do with their water vapor permeance and differing hygroscopic characteristics.)

    But in a sunroom it's the glazing that kills it, even at a code-min R14 the walls are a tiny fraction of the heat load, since code-min glazing is about R3.3, losing. With a 68F interior, at Boston's +12F 99% outside design temp you're looking at a heat loss of ( 68F - 12F )/R3.3= 17 BTU/hr for every square foot of glazing, as opposed to (68F-12F)/R14= 4 BTU/hour for every square foot of wall. In other words, a square foot of window has the heat loss of 4.25 square feet of wall. If the windows are even 1/4 the total wall area, they dominate the heat loss numbers. Even with the 3M film it'll be nowhere near R4, but you might bring it up to R3.5.

    The output capacity of a typical pretty-good mini-split @ 12F is about 15,000 BTU/ton, and costs about $3500-4000/ton (installed price.) Run yourself a crude heat load calculation on the amount of area that is wall and the area of window. Use a U-factor of 0.3 BTU/per square foot per degree-F for the windows, and a U-factor 0.07BTU per square foot per degree-F for the walls. If you only set it to 60F or 65F at night, use a 50F as the multiplier. If you keep it 68F or so, use 55F as the multiplier. eg: Assuming 300' of window area and 400' of wall area and a 50F delta:

    Windows: 300' x U0.3 x 50F= 4500BTU/hr

    Walls: 400' x U0.07 x 50F= 1400BTU/hr

    add it up and you're at 5900BTU/hr . Give a ~1000BTU allowance for air leakage and ceiling/floor losses and you're at ~7000BTU/hr which is, easily handled by a 3/4 ton mini-split, but could be well over 1/4 of the whole heating load of the house. The window losses of the sunroom alone are likely to be 20% of your total heating bill for a tight ~1800' house of that vintage (with modes insulation & window improvements over the 1968 single panes & R9 in the attic).

    But if you traded 200' of wall for another 200' of window you'd be at:

    Windows: 500' x U0.3 x 50F= 7500 BTU/hr

    Walls: 200' x U0.07 x 50F= 700 BTU/hr

    Add a 1000 for the ceiling & floor losses and you're at ~9000BTU/hr, still well within range of a 3/4 ton mini-split, but a hefty uptick from 7000BTU/hr, and a VERY lossy room, even if you made that 200' of wall R-infinity.

    But in both of those cases a even if 1/3 of the total is east or west glazing it could tax a 3/4 ton mini-split in the cooling season.

    You don't have to live in the dark to have a room with good light and nice views, but you have to rationalize every square foot of glass, and tune it's solar gain for the side that it's on. South windows can be fairly high gain with good energy use benefit if it has reasonable overhangs that shade it from mid-summer sun, but a lot of high-gain east & west windows can roast out and give a large amount of glare, even with window-films.

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