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Community and Q&A

Undersizing a Minisplit Heat Pump

crc1 | Posted in Energy Efficiency and Durability on

Lots and lots of threads and recommendations not to oversize.   Not clear why, just the implication that it is BAD.   Naughty bad.   Don’t ask bad.  I’ve searched a bit but all the hits are about oversizing etc.

Occasional mentions cautioning against undersizing.
Can’t keep at 70F on coldest day of the year?  So what?  It only hits 68F for a day or two or what?
Can’t keep at 75F on the hottest day of the year?  Same questions.

Some threads suggest that efficiency suffers.  How does that work if the minisplit is supposed to work continuously for best effect?  Does it not work well at stated capacity?

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  1. Trevor_Lambert | | #1

    "Can’t keep at 70F on coldest day of the year? So what? It only hits 68F for a day or two or what?
    Can’t keep at 75F on the hottest day of the year? Same questions."
    For most people, this is actually a big deal. Also have to figure in sizing error. If you knowingly undersize it by 10%, and your actual load is 10% higher, then your actual undersizing is pretty big.

    "Some threads suggest that efficiency suffers. How does that work if the minisplit is supposed to work continuously for best effect? Does it not work well at stated capacity?"

    Your best bet is to look at the actual performance tables for the particular unit. Most of them operate best at rated capacity rather than maximum capacity. At the lowest temperatures you're most likely in the maximum capacity range rather than rated capacity range.

  2. Jon_R | | #2

    Say it's 15% undersized (for 0F) and it's 0F outside for an extended period. It's going to be about 60F inside, which is uncomfortable and far outside of ASHAE, ACCA and code (IRC 303.10 says 68F) requirements. But some non-portable supplemental resistance heat will fix this.

    Say it's 15% undersized and it's 100F outside. It's going to be about 80F inside.

    With inverter mini-splits, minor over-sizing (say <= 25%) has little effect and may even be more efficient. And larger over-sizing isn't as negative as most suggest. See here (Fig 2-5, upper line) where latent performance is almost unchanged when 400% over-sized!

    Far more important than avoiding any over-sizing is latent performance for the specific model. Some (eg, Daikin Quaternity) 50% over-sized systems will provide much better comfort than most perfectly sized ones.

    1. PBP1 | | #3

      Great points. A link with some good info:

      I live in a relatively dry zone (smoke filled summers due to wild fires), heat pump sizing was for winter, heating design load of 28 kBTU/h. Cooling design load was 12.7 kBTU/h. Heat pump system with three ducted heads, cooling 27.4 kBTU/h (indoor 80 dry bulb/67 wet bulb) and 27.6 kBTU/h (at 47 F, indoor 70 dry bulb, 60 wet bulb). Installers said heads help to remove some smoke particles via coil condensation, though I've never measured how much condensation they produce.

      Anytime I've said "undersized" or "oversized", that's for heating and not cooling. I guess if I lived in Florida, I be thinking the opposite. In Florida, humidity/latent heat sure is a concern, particularly for low-load homes as explained in the DoE study. Not sure if any units actually measure condensate volume as an indicator of RH/latent? Seems like it would be quite easy using something like a Blue Diamond reservoir/float triggered condensate pump.

      From DoE study: During higher cooling loads, air-conditioning runs at a higher capacity for longer periods and removes more moisture from the air. On a very hot and humid day, 105 pounds (12.6 gallons) of condensate was measured. Condensate removal correlated very well with outdoor temperature, as shown in Figure 2-30. * * * To address high latent loads in low-load homes, extended run time of the cooling system is required during low-load hours.

      1. Jon_R | | #10

        If you think that extended run time provides good latent performance, review the very poor SHR of the Fujitsu 12RLS when running continuously at 50% load. It would do much better if it cycled.

        Page 30:

        It's all about CFM/ton - not sizing.

  3. crc1 | | #4

    Well, when I say undersized it is not my meaning that the unit is not rated for the outside temperature. Apparently heating can drop off at low temps but we would be well above minimum operation temps. The hyperheat type systems produce good heat well below zero F and it would be highly unlikely that we would see temperatures below 10F for years, or decades. With climate change we may not see it again in our lifetimes, so the ability to make heat will not be a question and, to my knowledge there is no real upper temperature limit to squeezing heat out of the house during summer time.

    Therefore, my question is what happens when a minisplit runs at full capacity for extended periods, say days at a time once or twice a season. Does efficiency somehow plummet in comparison to a larger unit or is the downside an ability to maintain the last few degrees during the coldest hours?

    The other issue and the question this likely overlaps with regards the recommendations to maintain a constant temperature setting in contrast to reducing production in off hours and then ramping back up during times of use/demand.

    1. PBP1 | | #7

      Looked at your other post on backup heat. I'm in Montana, with -15 F (and lower) not uncommon (nighttime low). The heat pump still keeps the house above 65 F. The installers (was new construction 2018) do residential and commercial and said they have never had a problem with Mitsubishi HyperHeat (even slightly undersized) in Montana winters.

      The wiser backup heat can be a different fuel if electric goes out (like Spokane, Texas, etc.). Dana is an expert and I see he weighed in too (he's not a fan of gas). Given your build details (double stud walls, etc.) and climate zone, I can't imagine a need for supplemental electric heaters that are permanently installed. And, who cares about a bedroom, water lines are where heat is needed (had to service a house that had 1,000,000 gallons of water run through it due to frozen pipes). My plumber would not install water lines on any exterior wall.

      Back to heat pumps, they generally work best when working near capacity all of the time. You have clarified that you are in a dry climate with primary concern for heating. I have 2100 sq ft with a heat load of 28 kBTU/h and a Hyper Heat 30 (name plate) rated at 27.6 kBTU/h heat. I can provide power usage details if needed. I do have a direct vent fireplace (around 25 kBTU/h) but it's not meant for heating, yet that's my non-electric backup (and run mainly for aesthetic effect for an hour or two in the evenings on some days in the winter). Running that fireplace, is more than ample supplemental heat if want 75 F when it's -15 F outside. Another option is adding a heating coil/water heat loop (radiator/baseboard) to the on-demand water heater (if you have one) and if it's gas, then you also have diversity in fuels.

      As to running all the time in humid, hot climates, that's good too, as explained by the DoE study. If you need to remove a hundred pounds of water from your house, undersizing/right sizing is best - not oversizing. Running all the time with the coldest coil possible means removing water all the time, which will increase comfort in humid, hot environments.

      As to playing with the thermostat, remember that you are heating or cooling more than air. Nearly the entire structure (think tile floors, granite/quartz countertops, etc.) need to recalibrate. When you keep the temperature constant (like I tell someone time and time again), you get that pseudo-steady state (pseudo because outdoor is changing, but usually slowly). Working toward steady state is good.

  4. mgensler | | #5

    Constant cycling is harder on equipment than longer run times. If you undersize, then do some more air sealing and add insulation as a way to "right size" the equipment. If you are severely undersized, then add another system. That's the awesome thing about mini-splits is you can buy them in 3/4 ton which then modulate way down unlike traditional split HVAC systems.

  5. Expert Member
    Akos | | #6

    It depends on the unit. If you look at:!/product/34552

    At higher load the COP drops slightly but it is pretty even across the whole operating range.

    Some units do have much better COP at rated VS max:!/product/25312

    Look at heating at 47F and cooling at 95F. I would expect similar trends at other temperatures.

    Under sizing a bit only creates minor comfort issues and maybe a small energy penalty for a small portion of the year. You would get higher efficiency the rest of the year and much better humidity control during the shoulder seasons.

    My advice to size it so it about covers your expected 99% load. Avoid the temptation to oversize.

  6. crc1 | | #8

    "My advice to size it so it about covers your expected 99% load. Avoid the temptation to oversize."

    This is exactly the approach we would like to take. It's a bit of a winger. Our HVAC contractor Manual J came in at 36,000 btu but reviewing the inputs suggests they have been lazy, using "typical" for insulation and airtightness. Who knows what other fudge they have cooked into their calculations but then that's the current state of HVAC. I'm guessing they are oversized. Now we are left to make guesses for true capacity needs while the GC strongly "disagrees" with any of our efforts to deviate. At this point, with seasons about to change my window is closing; I am going to have to make an (un)educated guess. Two 15,000 units are on order, one up, one down. If my estimate is grossly inadequate they may be for sale next spring...

    1. Jon_R | | #9

      If they won't fix it, I wouldn't replace guesses with different guesses. It's not that hard to do a correct Manual J yourself - especially if they provided most of the input values needed.

    2. PBP1 | | #13

      Don't know if a HERS score is required (was in my town), you could hire a rater to get a second opinion ( As to one up and one down, an advantage can be for the week or so where you want cooling upstairs and heat downstairs. A single unit with multiple heads (Mitsubishi) should have all heads on either heat or cool. Also, I think the Mitsubhishi nameplate 36 kBTU/h is rated for heating (47 F) at 45 kBTU/h (well above 36 kBTU/h on heating).

  7. GBA Editor
    Kiley Jacques | | #11

    This article from contributor and expert HVAC consultant Allison Bailes might interest you: My Undersized Ducted Minisplit Heat Pump.
    He purposely undersized his minisplit system—for reasons you most likely aren’t dealing with, but it’s interesting to learn about.

  8. Expert Member
    Akos | | #12

    A well sealed double stud house with solid roof insulation, if your climate is close to the rest of Oregon, you are looking at probably under 10BTU/sqft heat load at design temperature.

    Your 30000BTU of heat pumps would cover probably a 5000sqft house, more if you have some backup space heaters sprinkled in the perimeter rooms. I would get a ducted unit for the 2nd floor, this is usually the best for an area with a lot of closed off rooms.

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