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Sizing a mini split heat pump question

Debra_Ann | Posted in Energy Efficiency and Durability on

In my climate, both heating and air conditioning is important, though heating is much more dominant here in the mountains. I’ve completed a Manual J for heating a home we will be building this year.

Our outdoor 99% design temp is 16 F, giving us a heating load of 16,350 BTU at that temp. Our heating load at 47 F is around 7,000 BTU, and the few nights we drop to 5 F would be 19,700. We’ll have backup heat with a propane stove, so I’m not too worried about severe cold. I don’t believe I need a hyper heat mini split.

I’ve identified a few mini split models that I think would work well for us, and I would like some feedback. I’ll also be speaking with some HVAC contractors, too. My goal is to have our mini split be in a modulating state for most of our winter.

One of the models I’m looking at has a max heating capacity of 18,000 BTU down to 5 F, and a minimum capacity at 47 F of 5,150 BTU. I believe that would allow this mini split to be mostly modulating for my home with outdoor temps between about 13 F – 53 F. It would be going all out at around 10 F, and start cycling by 55 F or so. (I played with various outdoor temps in our Manual J to determine the heating loads at each point.)

Does that sound like a good size for our house? I’m just hoping I can find an HVAC contractor that won’t insist on over-sizing our equipment.

I would actually prefer a mini split that could continue to modulate at somewhat higher temps, but most models I’ve found so far that could do that would then be running all out at our 16 F design temp instead of continuing to modulate at a bit lower temps. Not sure which option is best, or if there are models that could actually do both. Any thoughts? Am I missing anything?

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  1. GBA Editor
    Martin Holladay | | #1

    It seems to me that you are approaching your sizing challenge with good logic.

    In my opinion, the most important aspect to a minisplit installation is to choose an experienced installer with a good reputation for service. That's more important than the brand. For example, if I were choosing between Mitsubishi and Fujitsu equipment, I would make my decision on the brand favored by a nearby reputable contractor rather than by the equipment specs.

    I'm not sure if the information will be helpful to you, but in case it is, you might want to read this article: How To Buy a Ductless Minisplit.

  2. user-1137156 | | #2

    If you want any amount of efficiency you'll choose the unit with the lowest minimum output period! EDIT: Maybe another"metric' is better and i should have said you MUST under sizes and choose from the under size candidates, the unit who's minimum is the smallest percentage of it's capacity.
    This deliberate under size means for some time your IDT will drop, however, you can choose to reduce or eliminate the drop by adding heat from your back up source. Anything else will spend less time in the high efficiency "modulating" mode.

  3. Expert Member
    Dana Dorsett | | #3

    What Jerry said. The Fujitsu 15RLS3 or 15RLS3H put out 18,000 BTU/hr @ +17F, and modulate down to 3100BTU/hr @ 47F, and would be a pretty good fit here..

    Sounds like you're looking at the Mitsubishi FH15NA?

    A PAIR of FH09NAs has more combined output, and each will modulate down to ~1600 BTU/hr (3200 total), and you have the option of turning one completely off during low loads stay in cycling mode.

  4. user-1137156 | | #4

    Unfortunately: Actual experience shared by another poster in another thread reported that in use his Fujitsu's actual minimum was 5000BTU. I recall seeing a minimum around 1900BTU in ,I think a Mitsubshi brochure. Thank you for bringing this up! From an efficiency perspective the ratio of minimum output to actual capacity in the operating range actually needed is the most telling criterion and why sometimes over sizing can be free of consequences. This happens because larger units modulate to a smaller percentage of their capacity and often loose a smaller percentage of their ratings at lower temperatures. The smaller load the greater the significance of matching unit to load becomes .
    EDIT another problem is,real output available at low temperatures is often greater than the ratings suggest, I have gotten test data from Midea that show, for example any of 9000 btu hyper heat units using the GMCC compressor family actually deliver 12000BTU way down to 0f so that they can advertise and deliver full rated performance at -5f these units have a spec'd min of 4000 and might be your best choice. Unfortunately the 12000 btu rated member of this family doesn't have the same level of over performance and has a 4500btu min rating. The 18000 would cover 100+% of your load, actually to well below design temp because it is delivering just shy of 20k@17f and 18k @-5f It may be best to avoid burning expensive propane and of the options this should do that best.
    The family wears brands of: Carrier, Midea,Pioneer,Hair,Gree and, no doubt, others AFICT all made by New Vision, in China, of course. So while you thought you didn't need/want "hyper heat" with a design temp of 17f. The over performance at mid temperatures to allow full rated output at very low temperatures makes a hper heat a better choice..
    What brand were you looking at that had the 5150 min? Midea's data sheet says 5500 min output @ 47f.

  5. Expert Member
  6. Debra_Ann | | #6

    Thanks for everyone's help and suggestions. I appreciate it. I haven't yet found a reputable installer for Fujitsu units in my region, but I'm still looking. Mitsubishi is a lot more common here. I would love to have 2 smaller units, so I can have better modulating control at either temperature extremes (especially for our fairly low air conditioning needs), but I doubt that will be in our budget. I'll ask, though.

  7. user-1137156 | | #7

    YMMV! It's a moving target specs, submittls and data sheets often reflect past reality. If the data source includes the AHRI # and these documents agree on @47max and model # and the AHRI status is active it's good data. The "source" keeps "refining" them!. The AHRI Cert. for the exact model of any brand is for units they are currently selling (the AHRI data base holds many "discontinued" models). & show's only @47f max. The "specification" min is ONLY @ 47 and is totally dependent on firmware. at other temperatures ? The only data I've obtained, i got from Midea and is max output. it includes neither model # or AHRI#, it agrees at5f and 47f with ratings in their "catalog" where the min is 6000. hopefully the min is constant over temperature, and lowered in the new model but WHO KNOWS?

  8. user-1137156 | | #8

    Perhaps there are considerations, other than efficiency, that could, or might apply to choosing your mini-split. Considerations like aesthetics (my wife hates the "wall warts", for me it must be out of sight) or comfort, even heat distribution,even in a highly compartment-ed house with the doors closed or the one you are struggling with available installation service.. You have to choose how to weigh these considerations you may want to choose system type. There are, essentially three types, slim duct, cassette , wall mount. If your choice is dominated by efficiency you must choose wall mount and all the discussed brands and models apply. Choosing a type, other than wall mount, means an efficiency hit, as In my case, excluding wall mount = fewer choices..

    Consider buying a unit from a "big box" home improvement store, Lowes, Home Depot etc. They offer, installation service through affiliated contractors.
    . .

  9. user-1137156 | | #9

    Here are, I'm sure, the two best fits for this situation with full data @ low temps.
    Available for online purchase or through lots of affiliated contractors.
    The Carrier 38GRQB18 would be my choice, however I doubt that a seasonal cost difference is even measurable.
    18 K model heating capacity 3755--24566 @47f
    Is as good as it gets and covers the Whole winter.
    With design conditions: Modulation only below ODT=57.6f
    or 12 model
    3071--- 18766BTU @ 47f, 12763BTU @ 6f and above
    MAY offer a little less cycling, but MAY require some occasional minimal amount of supplemental heatl It satisfies the design load down to 27.8 f to maintain design IDT costs propane!
    Modulation only below ODT=59.9f.

    Question: Is this considered a 10% oversize, (18,000/16350)?

    EDIT: I thought that this was the best match possible wrong again! By pairing a 2 zone multi split version of this compressor's models, which can be used with only one IDU connected with any style of IDU The heating output is rated as 2539--23475BTU @ 47f for modulation only @ ODT <61.6f
    Data sheet rated low temp output = 14,200BTU @ 5f
    Wall warts only modulation =ODT<62.6f clearly insignificant.
    based on current data from the lowest cost brand, Pioneer cost of ODU & 18K slim duct IDU & 50' lineset=$1521

  10. user-1137156 | | #10

    A fundamental question that must be asked is: What IDT and tolerances is "acceptable."? From all the data it would appear that the design IDT is 70f. What is the tolerance?
    Firmware that as Dana said " does the right thing" will end it's first heating cycle, at the design temp + the tolerance. It will resume heating when the IDT drops to the design -tolerance. If we were ale to tell the "firmware" that our tolerance is 4f the last system in the previous post is a PERFECT match to Debra's house. Absolute minimum number of cycles per year. But it is, by usual reckoning 22% oversize so"right thing" firmware's tolerable overshoot is 3f?

  11. Debra_Ann | | #11

    I met with one HVAC contractor today to discuss my options. I showed him my Manual J results, and my floor plan. Explained that I wanted a unit sized so that it would be modulating for most of the winter, and that with the 99% design temp of 16 F I didn't need a unit with more than 18-20,000 BTU.

    He immediately disagreed, reminding me that we just had a week with lows around 5 F. He "always felt better by providing customers with extra heating capacity so they'd never complain about being cold." Sigh... I managed to overcome that objection by explaining I'll have a backup propane heater for power outages, and that could cover any severe cold spells. No need to oversize the mini split.

    Then I asked him if it would be OK for my fresh air intake to be installed near the interior mini split head. (It's in a little traveled area of the great room.) He poured over my floor plan, and suggested placing it right over my direct vent propane heater instead. Huh??? Fresh air pulled from outside, just above the exhaust of the propane heater? I don't think so! I explained that code doesn't allow air intake within several feet of any exhaust vents.

    Then he said that he didn't think I needed any mechanical ventilation as long as I had windows in my house that could open. I had already told him that my house would be very tight. So I had to explain to him that code required mechanical ventilation in all new buildings within our state. Then he decided that the fresh air would be OK near the mini split head.

    For Pete's sake!! This is insane that a homeowner not working in the industry would have more knowledge about the system and code requirements than the HVAC professional. And they expect to be paid good money for their unique "training and experience". Argh!!

    And the scary part is that this company has one of the best reputations in my area!

    It's no wonder that I've been spending months educating myself on the various aspects of designing and building my home. I'm not sure I can trust hardly any of the contractors to do all of the details right.

  12. GBA Editor
    Martin Holladay | | #12

    Your frustration is justified. Last June, I wrote an article that included a discussion of the issues you raise. In that article, I wrote:

    "An industry in a perpetual state of crisis
    "The known scarcity of skilled HVAC contractors leads to an obvious question: What’s wrong with the residential HVAC industry? It’s hard to say. On a typical residential construction site, the framers, on average, are doing an excellent job. Framers work from good plans and execute the plans well. Framing failures are very rare.

    "Electricians and plumbers are also doing excellent work. Residential electrical system design and residential plumbing system design are both excellent, and these systems are installed well.

    "So what’s wrong with residential HVAC work? Why are contractors doing such a lousy job?"

    For more, click here: Fixing Energy Star Version 3.

  13. user-1137156 | | #13

    Where are you! I share the frustration but have it, not only for contractors, but with code enforcement officials as well, have you gotten any required permits?
    "For Pete's sake!! This is insane that a homeowner not working in the industry would have more knowledge about the system and code requirements than the HVAC professional."
    The problems of inadequate competence and excessive greed seem pervasive.

  14. gusfhb | | #14

    When I wanted my condensing boiler installed, all 3 contractors I had quote it wanted to measure the feet of baseboard in my house.


    I knew the last one and that he ran a quality outfit, so he ordered the boiler I instructed him too.

    I wish I had had the guts to go with the even smaller boiler, the one I speced is, by actual usage experience, nearly 3x oversize, the smallest would have been only 2x. It is half the output of the mooring that came out.

  15. user-1137156 | | #15

    It seems there is a wide range of modulation "range" in mini-split systems. Who knows why but multi-zone systems are,generally, offer much larger ranges. Midea and other brands offer mix and match systems that can use from 1 to all zones of a multi, of course,the capacity with one zone is the lower of the IDU or ODU. I found loads of info, some of it is suspect at:
    Where I also found :

  16. Expert Member
    Dana Dorsett | | #16

    Keith: "When I wanted my condensing boiler installed, all 3 contractors I had quote it wanted to measure the feet of baseboard in my house."

    As well they SHOULD!

    The amount of heat emitter needs to be sufficient to not short-cycle the boiler when operating at condensing temperatures, and any competent mod-con installer would need to know how much baseboard you have in order to design a system that both covers your 99% condition heat load and still not short-cycle.

    What they SHOULD do for rough sizing is 200 BTU/hr x feet of baseboard = the absolute upper bound for the minimum fire output of the boiler.

    What they should NOT do for rough sizing is dumb stuff like 500 BTU/hr x feet of baseboard = boiler output.

    But it still requires knowing how much much baseboard there is, and if multi-zoned, the amount of baseboard on each zone.

    This is of course just the first rough cut for starting the design, but it's an essential parameter to have for any hope of getting it right.


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