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Musings of an Energy Nerd

Rules of Thumb for Ductless Minisplits

How to design a house that can be heated with one or two ductless minisplits — and how to operate the units once the house is occupied

Alex and Jerelyn Wilson heat their two-story Vermont home with a single ductless minisplit unit. The minisplit head (a fan-coil unit) is located on their kitchen wall.
Image Credit: Alex Wilson

Since 2008, when Carter Scott built a pioneering Massachusetts house that was heated and cooled by just two ductless minisplits, GBA has endeavored to publish reports from the field to guide people designing homes that are heated and cooled by ductless minisplits. We’ve learned a lot on this topic since 2008.

My article on Carter Scott’s approach to heating and cooling was called “Just Two Minisplits Heat and Cool the Whole House.” Since that article was published, builders, engineers, and researchers have shared their minisplit experience and data. Carter Scott has given technical presentations at several conferences (including the Westford Symposium on Building Science and ); energy consultant Marc Rosenbaum has written several valuable articles on the topic for GBA (including “Minisplit Heat Pumps and Zero-Net-Energy Homes” and “Practical Design Advice for Zero-Net-Energy Homes”); and researchers Kohta Ueno and Honorata Loomis have published useful monitoring data ().

We now have enough information on the use of ductless minisplits to heat and cool cold-climate homes to set out some rules of thumb. The nine rules of thumb that I present below are based on the work of Scott, Rosenbaum, Ueno, and Loomis, to whom I am indebted.

1. Design your building to have an excellent thermal envelope

If you want to heat and cool your building with just one or two point-source heaters, you want an above-average thermal envelope. That means that the building needs a very low rate of air leakage; above-code levels of insulation; and high-performance windows.

2. Consider snow loads when placing outdoor units

If you live in snow country, your outdoor unit needs to be protected by a roof — but not a roof that inhibits…

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17 Comments

  1. Scott Tenney | | #1

    air transfer grill
    I've read about air transfer grills with a low wattage fan that kicks in when there are temperature differentials between rooms. Does anyone have experience with these? Or, is there any research re how effective these are?

    I'm wondering if this might be an alternative to supplemental electric resistance heaters in the bedrooms - that might also be effective in the cooling season.

  2. User avater GBA Editor
    Martin Holladay | | #2

    Response to Scott Tenney
    Scott,
    There are two main problems with your proposal. The first is the specific heat of air, which is 0.0182 Btu/cf/°F. That means that you need a high volume of air flow to make much of a difference.

    The second problem is a low delta-T. If your bedroom is 60°F, you may want to heat it up. But the adjacent living room is probably at 70°F, so you only have a 10 F° delta-T to work with. (That's a much smaller delta-T than you get when you move air from the warm air plenum of a furnace.)

    When you do the calculations, you have to move large volumes of air to raise the temperature in the bedroom. Such a fan has its own drawbacks -- noise, for one, and the problem of drafts. It's simpler just to install a small electric-resistance heater.

  3. Antonio Oliver | | #3

    Heat pump oversizing
    Martin,
    I'd heard about oversizing heat pumps before to increase efficiency. But I still don't quite understand why it works. Is it that compressors use less energy per cycle when they run at a slower speed? And if that is the case, does this apply only to a unit that is equipped with an inverter. Or should one expect a similar result for a conventional heat pump capable of running in a lower of two or three stages?

  4. Eric Habegger | | #4

    Martin, are you sure that the
    Martin, are you sure that the specific heat of air enters into the problem or at least in the way you explained it? My understanding, which is imperfect, is that a home with a given interior volume, outside air temperature, and existing level of insulation and air sealing whatever that is, will need a certain amount of BTUs. A heat pump will supply that heat, mostly in the form of warm air rather than radiant heat. If you have 2 bedrooms that aren't getting warm but your minisplit is putting out 10000 BTUS and heating the living/kitchen area to a comfortable 70 degrees then that specific heat of air is already included in that 10000 BTUs.

    Obviously to heat the bedrooms you will need more BTUs coming from the minisplit to supply them and the main living area. So if you increase the output to 15000BTUs from the single minisplit and use the in- wall fans, assuming your whole house requires 15000BTUs to heat, then I don't see why .the specific heat of air would enter into it. In other words, the specific heat of air is already included in the total of 15000 BTUs required for the whole house.

    It seems to me, and I could be wrong, that the in- wall fans would really help distribute heat. You can't ding them for poor energy efficiency when you would have to count the extra BTUs for heating the bedrooms that would always be required. It seems like its double counting.

  5. Jerome Lisuzzo | | #5

    Mini-Split Oversizing Causing Short-Cycling
    Hi Martin - Thought I'd offer up a personal experience pertaining to mini-split sizing and short-cycling. Bear with me on some background...Over the past year-and-a-half, I've been finishing a Passive House in southeast PA (certification is currently in-process). My wife and I moved in last October. The house is two floors @ 1,000 sqft/floor plus a 1,000 unheated basement. We've tested .3 ACH @ 50 Pascals, and our Passive House heat load calculation is about 8,500btu/hr. The house is heated with two single-source Mitsubishi 12,000btu mini-splits; one on the first floor and the other on the second. I had two units installed precisely for the reason stated in your post; concern about summer cooling.

    In December I installed an energy monitor on all the main circuits, and one of the first things I noticed was that the heat pumps were short-cycling most of the time; often at three to five minute intervals. This occurred whether I was running both units or only the first floor unit (I've attached a screen shot showing one example).

    Mitsubishi concluded that the problem was occurring because the heat pumps are oversized, but added even the smallest mini-splits (9,000 btus) would have resulted in the same issue (because the lower limit on both the 12,000 btu and 9,000 btu units is essentially the same). The proposed solution, which appears to have largely solved the problem, was the installation of remote thermostats on both units.

    I offer all this up to make several points. First, apparently (at least from my limited experience) oversizing may be a potential issue with mini-splits in exceptionally efficient homes. Second, the problem may not be clearly evident to occupants who aren't monitoring the actual electrical usage. And third, maybe a ducted mini-split is the better option when it comes to extremely efficient building envelopes, even though they are somewhat less efficient (as was suggested by Mitsubishi).

    I haven't been able to find any other reporting of this short-cycling issue, and didn't see anything about it in the Ueno/Loomis report. It leaves me wondering if anyone else out there has experienced the same problem.

  6. User avater GBA Editor
    Martin Holladay | | #6

    Response to Eric Habegger (Comment #4)
    Eric,
    The amount of heat that a fan can move is calculated using this formula:
    Heating BTUs = (cfm of the fan) x (delta-T) x 1.08
    Where does the 1.08 come from? It's the heating BTU multiplier at sea level, determined by multiplying the number of pounds of air per cubic foot times the specific heat of air times the number of minutes in an hour (60). Another way of expressing this: the factor (1.08) is the volumetric heat capacity of air (in BTU * minute / hour * cubic feet * °F).

    We learn from this formula that the smaller the delta-T, the fewer BTUs the fan can move. The specific heat of air matters because a cubic foot of air at 70°F holds less heat than a cubic foot of water at 70°F -- which is why hydronic heating pipes have a smaller diameter than ducts.

  7. User avater GBA Editor
    Martin Holladay | | #7

    Response to Jerome Lisuzzo (Comment #5)
    Jerome,
    Your observation has been made by others; the solution to the problem you describe is, indeed, to install a remote thermostat.

    For more information on this issue, see Mitsubishi minisplit behaving very differently with external thermostat vs. without.

  8. User avater GBA Editor
    Martin Holladay | | #8

    Response to Antonio Oliver (Comment #3)
    Antonio,
    The high efficiency of ductless minisplit units under part-load conditions is a direct result of the fact that these units are inverter-driven. Single-speed air-source heat pumps without inverters don't share this feature.

    For more information on this issue, see .

    Quoting from that report: "When looking at the full results from the steady-state tests, the fully variable speed nature of the [inverter-driven ductless minisplit] equipment becomes apparent. The variety of compressor and fan speeds combine to offer a huge range of input powers and output capacities to meet the space conditioning load. A clear trend from the data shows that the higher the output capacity, the lower the efficiency. Equipment performance is maximized when the loads are small. This is true for both heating and cooling. Additionally, the ability of the equipment to run at a very low speed will greatly reduce the amount of time the equipment must cycle on and off in low load situations. This on-off cycling is a performance penalty for single speed heat pump systems but, in comparison, is largely avoided with the DHP [ductless heat pump] variable speed equipment."

  9. Keith Gustafson | | #9

    oversizing
    I could not see any advantage to oversizing a mini split.

    First, since they are selected for design heat load, they are already oversized over 90 percent of the time. This is added to the fact that simple heat load calculations usually over size equipment, and pretty much every assumption a HVAC contractor is going to make is going to err on the side of 'bigger is better'

    Second, in many cases in a well insulated house, they smallest available unit is already oversized

    Third, Larger units have a lower SEER rating for the same series, IOW, a 9000 btu unit may have a 27SEER, while the 12000 btu unit of the same series will have a SEER of 22. So while the unit might run at a higher efficiency percentage, it is of a lower number, thus you are not really saving actual electricity.

    Lastly, while my personal experience is mostly using them for AC, almost all of my complaints with comfort or setpoint control relate to [in my opinion] the units being oversized.

    I think the small ducted minisplits are going to excel in being sole heat source for efficient houses.

  10. User avater
    John Semmelhack | | #10

    Load Calculations
    Martin - Perhaps, in a better world, this would go without saying....but folks ought to be reminded to get an accurate heating/cooling load calculation before considering their equipment selection. Anyone who peruses the GBA Q+A forum will notice that it's quite common to see questions about equipment sizing for a particular application in the absence of a load calc. The question from the "first responders" (typically Martin or Dana Dorsett) is always: "What are your heating/cooling loads?"

  11. User avater
    John Semmelhack | | #11

    A caveat to #9...
    ...as always, rules of thumb come with caveats...here's one for #9:

    For homes with reasonably strong solar gains, and/or for any well insulated home on a mild winter day, it can be pretty easy to exceed the setpoint temperature (even with an external thermostat). In these cases, the mini-split will start to short cycle, adding small amounts of heat to the house at quite low efficiencies. For those willing to actively manage the device, there are some energy savings to be had (albeit small in most cases) by shutting it off until the sun goes down...

  12. User avater GBA Editor
    Martin Holladay | | #12

    Response to John Semmelhack
    John,
    You wrote, "Folks ought to be reminded to get an accurate heating/cooling load calculation before considering their equipment selection."

    I agree. That's why I wrote (in Rule of Thumb #6): "Of course, it’s still important to perform a heating and cooling load calculation before specifying a minisplit."

  13. User avater
    John Semmelhack | | #13

    Whoops...
    ...I read too quickly, Martin.

    Carry on with the fine work. ;-)

  14. Alec Shalinsky | | #14

    Ductless minisplit and ERV
    Hi Martin,
    No mention was made to the use of a Ductless Minisplit in conjunction with a high efficiency ERV/HRV (like a Zehnder 350).
    I presume there wouldn't be any conflict, but is there any evidence that the HRV would help evenly distribute the warm/cool air flows. You mentioned that ideally a heating unit should be used on the lower floor and a cooling unit on the upper. Wouldn't the HRV eliminate the need for 2 units?
    Thanks
    Alec

  15. User avater GBA Editor
    Martin Holladay | | #15

    Response to Alec Shalinsky
    Alec,
    As you probably know, a ductless minisplit serves a different function from an HRV or an ERV. The ductless minisplit is used for heating and cooling, while an HRV or ERV is used for ventilation.

    The air flow rates required for ventilation are low -- generally in the range of 50 cfm to 90 cfm, or perhaps 10 cfm to 20 cfm per room. These low rates of air flow aren't capable of transferring enough heat from one room to another to equalize indoor temperatures.

    For more information on this issue, see Choosing HVAC Equipment for an Energy-Efficient Home. In that article, John Straube is quoted as saying, “Ventilation air doesn’t do much to move around heat. Ten cfm of 72 degree air to a 65 degree bedroom won’t make any difference to the temperature in the bedroom at all. Open doors work better than HRV ducting.”

  16. Tom Bassett-Dilley | | #16

    Alec, I concur with
    Alec, I concur with Martin--we thought your idea would work in a Passive House we did a few years ago (Chicago suburb, Climate Zone 5), but there just wasn't enough air flow to even out the temps. The other interesting thing we noticed in the summer is that, as the ERV runs, it dumps air just a little warmer and a little more humid into the bedrooms, so they get progressively stuffier; get a few kinds jumping around in that room, and they start complaining about the temperature. So yes, keep those bedroom doors open whenever possible, and watch the solar gain carefully for summer performance. Ultimately the owners chose to replace the Zehnder unit with a CERV to provide conditioning to the ventilation air-- it helped comfort considerably.

  17. Kohta Ueno | | #17

    Oversizing of MSHPs in Heating
    Martin--great column as always; thanks for taking our research work and making sure that a wider audience gets to see it!

    Unfortunately, we never got to study short cycling effects directly in our study--due to cost constraints, we set up our kWh meters to record 5-minute total power use, and record that. Also, we didn't do anything on recording output. James Williamson and Robb Aldrich at SWA did a nice study that I *think* you summarized earlier (). As a result, we couldn't directly measure short-cycling effects.

    But I believe that Marc Rosenbaum and Robb Aldrich might have more useful information on this front.

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