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Community and Q&A

Sizing minisplits for adequate heating & cooling in a cold-climate Passive House

tristcat235 | Posted in PassivHaus on

We are building a 2300 square foot Passive House in the midcoast area of Maine and are having trouble choosing and properly sizing our mini splits for both adequate heating and cooling. We live in an area where of course it gets below zero many times during the winter. So we are limited (I think) to the brand of mini-split we can choose from. (mitsubishi hyper-heat) I have been reading a lot of articles about the installation of oversized mini-split systems that short cycle and end up needing an additional thermostat to correct the problem….we are hoping to avoid oversizing our units, but from the articles I’ve read, it seems to be unavoidable as they all seem to have the same lower threshold. Secondly, there seems to be, not surprisingly, such a disparity between the heating and cooling calculations done by our Passive House consultant and our HVAC tech that we are having difficulties convincing our HVAC tech that the systems we want are sufficient (he wants us to sign a waiver if we proceed with less BTUs and/or smaller units than he is suggesting) – which in turn makes me nervous. My own concerns are more specifically about providing adequate cooling while not oversizing the heating units so that we are functioning within the passive house standards. Ideas, thoughts, suggestions would be greatly appreciated. The house is three levels and will have a ERV system as well.

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  1. GBA Editor
    Martin Holladay | | #1

    The heating and cooling load calculation made by the PHPP (Passivhaus) software is much more likely to be accurate than the estimates made by HVAC contractors.

    HVAC contractors routinely oversize equipment; this is a decades-old problem that shows no signs of going away. Trust your Passivhaus consultant.

  2. tristcat235 | | #2

    Using our Passivhaus consultant numbers we are still having a hard time finding heat pumps that are small enough but still have the heating capacity during our below zero winters.... our heat load is just below 9000 but if we install two 12000 but mini splits (one on first floor, one on the second) we will have more than twice the heat we need. BUT the recommended way to do it is to do just that.... install a heat pump on each floor since the cooling really doesn't well with just one unit. I'm looking for ideas / I overlooking something or do I have to suck it up and either use air cooling at night for the upper floors or settle for oversized units if I want them mechanically cooled.

  3. propeller | | #3

    Hi Cathryn,
    You can trust Martin advice. I'm a Passivhaus consultant and I often hear similar comments from contractors that don't understand building science and only go by experience with regular house. And even then they oversize it!

    Depending on your house plans, you could install two 9000 BTU units, one on each floors. The first floor unit would supply the vast majority of your heat in the winter while the second floor unit would provide you the bulk of summer cooling. And you get safety margin by having a second unit for the few extreme days of the year or if ever one unit goes defective.

  4. user-2890856 | | #4

    Unfortunately , the phenomenon which Martin describes is very true . HVAC guys have been oversizing equipment for decades and the likelihood that it will stop anytime soon is next to nill . Listen to your Passivhaus designer / consultant . His numbers are much more likely to be accurate than your HVAC guy's numbers .

  5. tristcat235 | | #5

    Thanks everyone......I can't seem to "find" a 9000BTU unit that has the below zero capabilities...any ideas? I'm going to put a call in to Mitsubishi monday but thought I'd ask here first! Thanks again.

  6. STEPHEN SHEEHY | | #6

    Cathryn: Check out the Fujitsu RLS3. I'm using them (9000 and 12000 btuh) in my new house, also in midcoast Maine.

  7. propeller | | #7
  8. jackofalltrades777 | | #8

    Welcome to the world of Manual J's and highly overstated load calcs, There is a built-in 30% over-engineering factor to the calcs. Can't escape it.

  9. GBA Editor
    Martin Holladay | | #9

    The problem isn't so much in the Manual J method -- there is no built-in 30% fudge factor, as you claim. Rather, it is due to the fact that HVAC contractors input the wrong numbers out of a false belief that they need to build in a safety factor. This is done (for example) by using the wrong outdoor temperature, the wrong air leakage rate, and the wrong window U-factors.

  10. tristcat235 | | #10

    Thanks Martin and everyone else. The problem I was having with the 9000 BTU heat pumps is that they are rated for cooling at 9000 BTU but it looked to me like they are rated higher for heating. Any thoughts about whether or not with a 9000 BTU unit the new HYPER HEAT makes sense or can we go with a normal mini split ......Our total house heat load is 9000 - so with two units (one on each floor for cooling purposes) we are already way over our heat load needs........but we do have some below zero days here... Any experience in New England or Maine with regular vs hyper heat units?

  11. GBA Editor
    Martin Holladay | | #11

    I think that you are overthinking this. If you are worried about oversized equipment, just turn off your upstairs unit during the winter, and turn off your downstairs unit during the summer. If you can stay comfortable with just one minisplit, then there is no reason to operate both.

    If you discover that you actually need both units to operate, just flip your circuit breaker and turn the idle unit on.

  12. tristcat235 | | #12

    Thanks Martin. Truth. I overthink everything. I'll go order my units now. :-)

  13. STEPHEN SHEEHY | | #13

    Cathryn: I looked at the 9, 12, & 15000 btu units. There is very little difference among them with respect to maximum heat output. You can get detailed specifications from Mitsubishi that will give you coefficients of performance (COP) for the various units at different set points and different outside temps. With your very low heat load, you might think about the 9000, because it runs a bit quieter than the 12 or 15 at low speed.

    I'd opt for the hyperheat. As far as I know, there is no downside, other than a small increase in price. I don't know where you are in midcoast Maine, but here in Alna it got down to -18 on a few nights, so the better low temp performance seems worth it.

  14. tristcat235 | | #14

    We are building in Alna too Stephen (Down on the Head Tide end of West Alna!) Small world. The only downsize I come across with the hyper heat is that you can't use multiple heads --- you need one base unit for each head.........We were hoping to use the upstairs unit with two heads (one for the attic loft) -- Maybe we'll use a hyper heat downstairs and regular upstairs....

  15. kbtstone | | #15

    Mitsubishi now has multi-zone hyperheat units available. Here is a link to the brochure:

  16. STEPHEN SHEEHY | | #16

    Catheryn- we'd love to see your house and show you ours. We're at the interior painting stage. If interested, send me an email:
    Stephen at tidewater dot net.

  17. Expert Member
    Dana Dorsett | | #17

    The heating capacity of mini-splits is not constant- it falls with outside temperature, so it's important to look at the calculated heat load at your 99th percentile tempertature bin aka "99% outside design temperature", and compare it to the capacity of the minisplit AT THAT TEMPERATURE.

    The 99% design temp for say, Portland ME is +2F, for Bangor it's -2F. (see ) These are not very limiting design temps that narrow you down to Mitsubishi or nuthin'. The venerable Fujistu RLS2 series has specified output capacity down to -4F, the RLS2-H and RLS3-H series have rated capacities at -15F. eg:

    (Consult the installer or manufacturer for the extended temperature range capacity tables.)

    The Mitsubishi FH series is a good choice, but not the only choice.

    Short cycling with mini-splits is almost never due to oversizing. It's far more likely to be a function of where/how the ductless head is placed tucked too tight to the ceiling, between cabinets, corners or other restrictive air pockets, causing rapid localized temperature swings of the intake air near the head that exceed the average air temp in the room. Retrofitting a wall thermostat is usually a good solution in those cases, since the head no longer is responding to the incoming air temp at the head. Both Mitsubishi & Fujitsu have wall thermostat solutions.

    Up to a point, oversizing is GOOD for efficiency! Mini-splits are modulating systems, and typically have much higher efficiency at part load/lowest speed than when running flat-out at the max. But the turn-down ratios are not infinite, and if you oversizing factor is more than 1.5x they will begin to cycle on/off even when it's still somewhat cold outside, leading to bigger swings in room temperature and somewhat lower comfort. This also means it's spending more time in idle mode, with a modestly lower efficiency with increasing oversizing factor.

    The maximum capacity at -2F of a -FH09NA or -9RLS3 (or 3H) may be more than 9000 BTU/hr (it's actually close to that, even though they put out about 11,000 BTU/hr @ +5F at max speed). But they have high turn down ratios- the FH09 can turn down to 1600 BTU/hr even at +47F outside temps, the 9RLS3H can turn down to 3100BTU/hr even @ 47F, and the min-modulation output also falls with temperature. That level of oversizing is not a problem- they are NOT oversized. If the true heat load @ -2F is 9000BTU/hr @ -2F they are not even optimally oversized for efficiency, and don't have much margin for when the weather hits the 99.5th percentile temperature bin.

    The FH12NA and 12RLS3H are not oversized enough to cause excessive cycling during the shoulder seasons, and would still be modulating at part load at -2F. That would deliver measurably higher seasonal efficiency than the 3/4 tonners (!) It would also give you a bit more margin to work with for those days when it hits -15F.

    The FH15NA has too high of a min-modulation output at 20F which means it's not a great candidate. But the min-modulation output of the 15RLS3H at +47F is the same 3100BTU/hr as the 9RLS3H, so despite the much higher max capacity it would still work just fine.

    If your 9000 BTU/hr load number was at the 99.7th percentile temperature bin rather than the 99% outside design temp, you probably need the 1-ton (or even 1.25 ton unit) to have sufficient capacity. So, what outside temp was used in the load calculation?

  18. STEPHEN SHEEHY | | #18

    Dana- according to Fujitsu, at -5, the 9RLS2 max output is 14000 btuh, the 12RLS2 is 15,000 and the 15RLS2 is 16700. Not much of a difference.
    The smaller nominal capacity units have slightly better SEER and HSPF numbers, but the larger units have a better COP at very low temps.
    All things considered, I think either the 12K or 9K units should work fine in Maine.

  19. Expert Member
    Dana Dorsett | | #19

    Curiously, 14,000BTU/hr is about what the 12RLS2 delivers when running flat out at 0F, according to third party testing. See Figure 4, p.9 (p17 in PDF pagination):

    That would imply that the 9RLS2 has more output than the 12RLS2 (not very likely...), or there have been a lot of incremental improvements in that model in the past 5 years without changing the model name (possible.)

    Or did you mean the 9RLS3 (not 2)?

    Does Fujitsu publish their extended temp capacity tables in some secret hiding place on the web, or do you have to bug the distributor?

  20. wjrobinson | | #20

    Downstairs is the main heat unit, go FH 1ton.
    Upstairs is the main cooling unit, go non FH and get two heads to cover loft.

    Site the outside units out of snow and away from your ears.

  21. STEPHEN SHEEHY | | #21

    Dana: I got the Fujitsu temp data from the installer, who got it from the distributor. I tried to attach it, but it didn't attach. Maybe the file is too big.
    The data I posted was for the RLS2. I suspect the numbers are similar for the RLS 3, which are going in my house.

  22. Expert Member
    Dana Dorsett | | #22

    In a maritime climate you still have to derate a bit to allow for defrost cycle times, but even without that factored in I'm a bit surprised the 3/4 ton is delivering 14K @ -5F.

    Specified capacity on the FH09 @ +5F is about 11K, but zooming in on the low-res curves published in the RLS2-H brochure it looks like they're claiming 11K @ -15F for the 9RLS2-H (and it looks like they have 14K @ -5F too.):

    The 12RLS2s of 5 years ago had substantially less sub-0F capacity than that, as tested by the EcoTope engineers & Jon Winkler at NREL.

  23. tristcat235 | | #23

    This was the article I was referencing about short cycling mini-splits Dana. -

  24. Expert Member
    Dana Dorsett | | #24

    Did you read down to the part where it said:

    "At any rate, last Friday the Mitsubishi tech showed up and installed the two remote thermostats. These thermostats (product MHK1) are manufactured by Honeywell specifically for the Mitsubishi mini-splits, and it appears that they resolved the issue. "

    They also noted that even the 3/4 ton units were oversized for the load. But the problem isn't the oversizing. If it were due to oversizing, the wall thermostat would not have had ANY effect, since the units are still oversized for the load.

    The house has substantial thermal mass to tolerate oversizing without inducting short-cycles. But localized air temperature feedback from where/how the head is installed can cause even under-sized or right-sized mini-splits to short-cycle.

  25. tristcat235 | | #25

    Thanks for the clarification Dana.
    Stephen - sent you an email.....also Amy at the Alna store knows where to find me. :-)

  26. user-626934 | | #26


    Regarding Fujitsu expanded performance data - you need to set up an account here:

    Once you're in, look for the "Design + Technical Manuals"

  27. user-1135248 | | #27

    Okay, I'm finding it hilarious that Fujitsu thought to implement
    a baseplate heater to prevent condensate water from freezing and
    piling up inside the outdoor unit. Having just wrestled with this
    over the past winter, I came up with my own aftermarket solution
    for the Daikin unit that is likely very similar, shown starting
    about halfway down the page

    where even a modest 40-some watts will bust the bulk of this problem
    and keep things from icing up in destructive ways.

    It totally makes sense, and is clearly another step in the evolution of
    air-source heat pumps as complete solutions for cold-ambient climates.


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