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Community and Q&A

Sizing mini-split for cooling & humidty control.

Chris Harper | Posted in Mechanicals on

We are building a small cabin in CZ 6. It is a seasonal cabin that will see limited winter use so while we do want a mini split that will heat the space we are not concerned if it ends up being undersized. There is so much free firewood in the area that the Forest Service and private land owners often burn huge slash piles in the winter just to get rid of it. I also have access to hundreds of board feet of free hickory cut-offs from a local cabinet shop. So we’ll have a wood burning stove regardless.

As I talk to local contractors it seems some are stuck on sizing the unit for heating and thinking the cooling functions will be good enough.

I don’t believe any of the contractors will use Manual J methods so I plan to do that part myself. I have read the related GBA articles on that subject.

Cabin is 640 square feet and has about 6900 cubic feet of interior space. We are getting recommendations for 12,000 or 18,000 BTU mini splits. If we are indeed on the fence between those two sizes I want to make an educated choice between the two.

We are concerned about humidity control. I know there are mini splits with a drying modes but that aside I have a question about humidity control in cooling mode.

Specifically will more humidity be removed with a slightly smaller unit that will move more air across the coils or a slightly larger unit that will ramp down and move the air more slowly across the coils?

It’s possible this has been covered in a GBA article I do not have bookmarked. A link to an article that covers this would be appreciated.

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  1. Expert Member
    Dana Dorsett | | #1

    The maximum cooling capacity of a cold-climate 1-ton is on the order of 13.5-14,000 BTU/hr at a interior-to-exterior temperature difference of 15F (80F inside, 95F outside is the test condition). That's likely to be on the order of twice the cooling capacity required.

    Dehumidification is only happening when the thing is running, so the critical aspect is a mini-split that has a low-low minimum modulated output so that it runs almost constantly. Most zone 6 locations have 1% outside design temperatures well under 95F, many under 85F (well under mini-split test conditions), so to ensure that there is sufficient sensible load to keep the thing spinning the minimum-modulation has to be as low as possible. Most 1.5-tonners are going to have a minimum-modulated output that is more than half your peak cooling load, and even a 1-ton might run into that problem.

    You should probably be looking at 3/4 tonners, not 1-1.5 tonners. Cold-climate 3/4 tonners still deliver over 11,000 BTU/hr of cooling, and your likely peak requirement is almost certainly less than 9,000 BTU/hr (the nominal modulation rate at which it's SEER efficiency is tested.) Of the first tier vendors you should probably be looking at the Mitsubishi FH09NA, which modulates down to about half that of Fujitsu's 9RLS3H. The Fujitsu has more low-temp heating capacity, but both can probably handle the whole load down to +5F or so.

    Most mini-split can operate in a "dehumidify" mode, which maximizes the latent to sensible load ratio, but it still has to be running to provided that dehumidification. If your cooling load at the 1% condition is, say, 6500 BTU/hr, a the Fujistu would be cycling on/off most of the time, whereas the Mitsubishi 3/4 tonner would be running almost all the time.

  2. Brendan Albano | | #2

    Have you considered a standalone dehumidifier if you are concerned about humidity? Having simple, discrete systems seems simpler than trying to do it all with one device. That way you can size the mini-split just right for your heating and cooling needs, and you can size your dehumidifier just right for your dehumidification needs, and you don't have to make any compromises trying to get the mini-split to do both.

  3. Expert Member
    Dana Dorsett | | #3

    A house this size in zone 6 shouldn't have any problem at all doing both latent & sensible cooling with the same coolin equipment unless the cabin houses a hot tub or something. The key factor is right-sizing it for the load(s), and the minimum modulated output. Outdoor summertime dew points are only high in zone 6 locations during high sensible-load conditions, unlike the muggy gulf-coast (or even mid-Atlantic state) locations. In zone 6 ventilation can reliably handle the dehumidification when there isn't much sensible cooling load to speak of.

    Typical cooling/floor area ratios for smaller houses even in the humid southeastern states with high latent loads and high 1% outside design temps are usually on the order of a ton per 1500' or a ton per 1000'. This house is 640', and most likely has a cooling load of about a half-ton. See Allison Bailes' compiled Manual-J cooling load ratios plotted against house size (most of those houses are in zones 1A to 3A):

    The absolute worst-case house in Bailes' graph had a load ratio of a ton per ~650' which means if THIS ~650' house were a barely insulated place with a lot of west facing glass in a Louisiana swamp it might actually need a full ton of cooling. But most 3/4 ton mini-splits can actually deliver a full 1-ton of cooling when running at maximum speed, so you're not really giving up much by going with a more appropriately sized 3/4 tonner.

    Most reasonably tight code-min houses will have heat loads under 15 BTU/hr per square foot of space at the 99% outside design temperature, maybe 20BTU/ft for very small houses. While there are many exceptions to prove the rule, that implies the 99% heat load of a 640' cabin will run somewhere around 10,000 BTU/hr, possibly less, but also probably not more than 13,000 BTU/hr. A cold climate 3/4 tonner will typically deliver 10,000 BTU/hr or more at 0F, and there's a wood stove to deal with anything colder, if needed. The Fujitsu's have a "Minimum Heat" mode to keep the place at 50F, and the Mitsubishi's can be be programmed to a 50F setpoint by using the "Smart Set" mode, if that's desired. (Most people would just drain the plumbing before winter and let it freeze.)

    If the mini-split is going to be a cooling-only type, pull the submittal sheets on any of the proposed units, and only pick those that modulate down to 2000 BTU/hr or less. Again, lower is better, independent of the max cooling. The 1-ton Mitsubishi MSY-GL12NA-U1 & MUY-GL12NA-U1 can throttle back to 1500 BTU/hr :

    But the 3/4 tonner in that series can only back off to 3,800 BTU/hr making it a less desirable choice for the application despite being more "right sized" from a maximum cooling perspective:

  4. Chris Harper | | #4


    We are absolutely considering a stand along dehumidifier. I had that written out in a longer original post but decided it was best I get to the question sooner and deleted much of my post. I am simply interested in the scenario where if truly on the threshold of two sizes would one be better off going with the smaller size if dehumidification was more important than heating.


    As far as outside design temperatures, the cabin is in the beginning of a canyon with strong orographic lift, or so I'm told. As I understand it this contributes to warmer winters and cooler summers. Or maybe the cooler summers are due to the 4400' elevation.

  5. Expert Member
    Dana Dorsett | | #5

    Can you be at least a bit more specific about the location, to get a better handle on the latent vs. sensible load estimates? The nearest listed city in the Manual-J manual might be useful- the moisture levels won't be very far off, even if the temperature data is.

    A smaller/lower minimum modulation is the key factor, independent of the maximum capacity, as previously discussed. That pretty much rules out all 1.5 ton mini-splits, and most 1 ton minisplits. Pull the submittal sheets on the specific models of any proposals to make apples-to-apples comparisons, since the minimum & maximum modulated output are done at a standard set of conditions, and can differ quite a bit from the "nominal" capacity, which is the modulated level used for calculating seasonal efficiency.

    Cooling-only, or heat pump?

  6. Chris Harper | | #6

    Cabin is located between Hill City and Keystone, South Dakota. If the nearest weather data is from Rapid City then I'm guessing data collection point is 1200' or so lower than our cabin.

    I would like the mini split to have a heat pump but am not concerned with it being able to meet all of our heating requirements. As mentioned earlier cabin will have a wood stove and the 100 square (850 cubic) foot mechanical/bathroom will have electric cove heaters.

  7. Expert Member
    Dana Dorsett | | #7

    Latent loads are low to negative in that location. Take a look at the dew point graph at the bottom of this page:

    The mean outdoor dew point even in JULY is a bit under 55F. With 55F dew point ventilation air at a comfortable & cool 75F indoor temp becomes 50% RH, so a mini-split running in "dehumidify: mode might even OVER-dry the place. Most of the time simply bumping up the ventilation rate would be able to bring the humidity under 50%RH, and outdoor dew points over 55F only happen when it'spretty hot out, at which point the AC would take care of it, even without resorting to dehumidify-mode.

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