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Musings of an Energy Nerd

Just Two Minisplits Heat and Cool the Whole House

Carter Scott has built 18 homes in Massachusetts without any heat in the bedrooms

Carter Scott's first zero-energy house. From the outside, the only clue that this home is unusual is the large photovoltaic array on the south-facing roof. The entire home is heated by two Mitsubishi ductless minisplit units.
Image Credit: All photos: R. Carter Scott

Carter Scott was one of the first builders bold enough to build a cold-climate home heated by only two ductless minisplit units (one in the downstairs living room, and one in the upstairs hallway). Skeptics predicted that the unheated bedrooms would be cold and uncomfortable. Yet Scott was confident that the home’s excellent thermal envelope — with high-R walls, triple-glazed windows, and low levels of air leakage — would keep the homeowners comfortable even when the bedroom doors were closed.

Scott owns a construction company called Transformations in Townsend, Massachusetts. He built his pioneering two-minisplit house in Townsend in 2008; the inclusion of a 5.7-kW roof-mounted photovoltaic array made it into a zero-energy house.

The skeptics’ “cold bedroom” predictions were unfounded. “We have since built several houses in which the upstairs minisplit unit isn’t even being used until the outdoor temperature drops below 20 degrees,” Scott said. “Typically the response from homeowners is, ‘Wow, these houses have even indoor temperatures’ and ‘these houses are quiet.’ And the fact that there are no utility bill makes people excited.”

Carter Scott’s first net-zero energy house

A ductless minisplit is a type of air-source heat pump that can provide space heating as well as air conditioning. Most of the ductless minisplits sold in the U.S. are manufactured in Japan or South Korea; the best known brands are Daikin, Fujitsu, and Mitsubishi.

The first zero-energy house built by Scott’s company, Transformations, was designed by architect Ben Nickerson. Dubbed the Needham model, it was built at 18 Coppersmith Way in Townsend in 2008. Here are the home’s specifications:

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  1. homedesign | | #1

    Time for Micro & Nano-Splits
    The Mini-split concept makes a lot of sense.
    Moving heat in and out thru small copper pipes instead of ginourmous ducts.
    The problem is the smallest available system size.
    I know there are multi-head systems (like Daikin) out there ...but the piping can get pretty complex running back and forth to a central unit.....and also the smallest heads are not-so-small.

    What I would like to see are Micro-Split Systems (or Nano) small enough to handle an average size bedroom .. with a small inside head, a small outside unit and short run of tubing.

    Similar to task lighting...task heating and cooling.

  2. GBA Editor
    Martin Holladay | | #2

    Response to John Brooks
    Perhaps you're right that we need smaller units -- and perhaps the manufacturers will respond.

    However, it's worth pointing out that:

    1. Carter Scott's clients aren't complaining, even with no indoor units in the bedrooms.

    2. Oversizing is advantageous with ductless minisplits, because the oversized units operate at a higher efficiency than right-sized units.

  3. homedesign | | #3

    and Kudos to Carter for concentrating his research and development on Affordable & Buildable

    Most of the Passivhaus and even the Building America stratagems are Not-So-Affordable and Not-So-Buildable

    We need Stratagems for Everyman & Woman

  4. homedesign | | #4

    Response to Martin
    Most homes are not as simple as Carter's and many people like to shut their bedroom doors at night and for privacy.

  5. GBA Editor
    Martin Holladay | | #5

    Bedroom doors
    Carter Scott is a production builder. He is not pre-selecting his customers. He is building entire neighborhoods of houses for any interested purchaser.

    I can assure you that the homeowners are shutting their bedroom doors at night. Probably not during the day, but definitely at night.

    No builder wants to take a risk that home buyers will be uncomfortable or unsatisfied. Everyone hates callbacks. Carter Scott knows that these homes work, and he's willing to sell to all comers.

  6. UphillLarry | | #6

    Single mini downstairs
    Add us (Up Hill House) to the list of houses with a single mini downstairs. Yes, we're a simple rectangular shape, but we call that modern. Yes it gets hot upstairs in the summer, but not unbearably and not more than handful of days in the summer, ceiling fan works wonders. And yes, it was perfectly warm upstairs in the winter with single unit, although it was not a particularly cold winter. We made the right decision for us, but I can see how it's not for everyone.

  7. Marc Rosenbaum | | #7

    Comfort is subjective
    Carter's work is terrific and inspiring. Before others take his one-up, one down approach and apply it wholesale to their own work, it's worth pointing out that in addition to compact form, one aspect that keeps temperatures even in remote spaces is amount of glass - window area matters on cold nights with doors closed. Carter's home shown in the photos has very modest window area, especially in the second floor rooms, and that's a component in the success.

  8. GBA Editor
    Martin Holladay | | #8

    Response to Larry Burks
    Thanks for your post. I've added your house to the list of successful one-minisplit homes.

  9. GBA Editor
    Martin Holladay | | #9

    Response to Marc Rosenbaum
    Thanks for your comment; I value your input, and your point about window area is well taken. Designers always need to approach heating system design with whole-house thinking. There's no such thing as a cookie-cutter approach that will work with all homes.

    Many of us have been holding our breath and crossing our fingers as we watch Carter Scott building more and more two-minisplit homes, wondering whether he will eventually encounter a grumpy homeowner (or a grumpy building inspector) who lands him in hot water. So far, so good. We are all gaining useful information from pioneering superinsulated homes like those built by Carter Scott.

  10. homedesign | | #10

    List of successful one-mini-split homes
    do you have a link to "the List"
    Are there any from Mixed-Humid or Hot-Humid ?
    I know there is Pasivhaus example in Louisiana ... but I read somewhere that it may be having comfort/performance problems.

  11. user-1044812 | | #11

    Similar approach
    We built a smaller 2 story home for a local land trust that arranges permanently affordable housing. For a number of reasons, we did not install a HRV, but did use a 2 head minisplit - with a twist - because we were not confident that both upstairs bedrooms would be comfortable, we installed a ceiling cassette head unit that is ducted. We placed it in the lid in a back to back closet between the 2 bedrooms, making the duct runs super short. Our heating load was low enough that a single head unit would have been enough, and thought that using an HRV would help distribute the heat - but convincing local officials was another matter. In the end the 2 head minisplit plus electric baseboard in the bathrooms (hopefully never to be used) was what we installed.

    I look forward to more experiences like Carter Scott's so that we can forge ahead with smaller, more efficient systems. Of course it all starts with a good design and a great envelope....

  12. GBA Editor
    Martin Holladay | | #12

    Response to John Brooks
    When I referred to "the list," I was just referring to the last paragraph of the section of this blog under the subhead "When the temperature is below zero, Mitsubishi ductless minsplits still perform."

    Clearly, it's not a very long list.

    I invite any GBA readers with information on successful one-minisplit homes to add to the list.

  13. JustHousing | | #13

    size of that rectangle
    Can you provide the size of the house's footprint (not including garage of course)? Is there a bedroom on the first floor or only on second floors? Great work, and timely. We've just sent out the construction docs for a similarly designed home to be built in Iowa, and we've specified mini-splits for each floor (including the basement).

  14. GBA Editor
    Martin Holladay | | #14

    Response to Rachel Wagner
    The article discusses several homes. The very first zero-energy home built by Carter Scott in 2008 was a model called the Needham. It has a master bedroom downstairs and two bedrooms upstairs.

    The Needham is 1,232 square feet. Its footprint is 24' by 32'.

    The Farmhouse model is 24' by 36'.

  15. user-626934 | | #15

    Wall mount mini-splits in central Virginia

    We've been pretty successful so far here in Central Virginia with wall-mount mini-splits for both heating and cooling, and we've had VERY hot summers in 2010, 2011 and most of 2012 so far. I'll outline them briefly here (one is my own house and I consulted on the other two), but they all share many of the key attributes of Carter Scott's houses: compact shape, extremely well insulated, very air-tight, and all have ducted ERV systems.

    My own two-story house in Charlottesville, VA (finished in 2008) has a single wall-mount mini-split (Mitsubishi 12kBtu) located on the first floor. Though we are certainly under shaded in the summer (I'll get around to remedying this one of these days), we typically experience temperature stratification of just 3-4 degrees in summer and 1-2 degrees in winter between the floors. We have 3 bedrooms upstairs, 2 of which are closed at night (children sleeping). We also benefit in the summer from a double-height, lofted space above the living room that the master bedroom overlooks. We run a high-efficiency ceiling fan on low-ish speed in this double-height space during the summer to help improve mixing between the floors. Finally, each bedroom also has a high-efficiency ceiling fan for improved night-time comfort. We use about 7,000kWh per year (all-electric house) and have been well into net-positive energy territory since we installed a 6.2kW PV system in April.

    The next house is the Lankford (certified) Passive House, built by Jobes Builders (, also in Charlottesville. This one is a little different, in that it has about 2,100ft2 total on three floors and two wall-mount units (connected to a single outdoor unit). We installed one unit on the top floor and another unit on the lower floor, nothing in the middle. While the house is not occupied (it's for sale - we measured temperature difference between various rooms (doors open) and floors at just 1F on multiple 100F days last summer.

    The last house is Daniel Ernst's recently finished (not yet certified) Passive House in Steeles Tavern, VA, and it may be the cream of this crop. Daniel has a main 2-story house with a mother-in-law wing connected to the main house on the lower floor. Total floor area is about 2,500ft2. While the mother-in-law wing has it's own wall mount unit, the 2-story main house has just a single wall-mount mini-split (Mitsubishi 9kBtu) located in a small double-height space + stairway. Temperature stratification throughout this summer has been no more than 3-4F between floors, according to Daniel.

    Hopefully, we’ll get more information on all of these houses up on GBA sometime soon!

  16. Expert Member
    Dana Dorsett | | #16

    The nice thing about superinsulated houses is...
    ... the time constant created by the stored heat in the interior building materials and the R-value of the exterior surfaces is sufficiently long that the mini-split need not carry the whole load at the temperature extremes to keep it comfortable.

    And that's a good thing, since there will be many times during the course of a winter that the outdoor dew point is close enough to the air temp that a mini-split will be spending as much time in defrost mode than actually heating, for a severe hit in output capacity (but not necessarily efficiency) during those critical hours. If you read the fine print next to the asterisks in the specs you'll find that even at a balmy +17F the Mitsubishi's are typically rated at only ~60% their nominal output when the outdoor dew point is as high a +10F, and if the dew point was even higher, say +13F (as often happens in MA) the output capacity would quickly fall to below half.

    Sure, in arctic dry air they can deliver the full rated BTUs even at 0F, but the air in MA isn't usually that dry- it's an issue, but only for homes with short time constants.

    The 99% outside design temp for Townsend MA (Carter's first Zero Energy house) is about 0F, so there will definitely be times when there is a BTU shortfall below the instantaneous heat load from the mini-splits, but with R50 walls the place could coast for hours at 0F without losing much ground, even if the mini-split was only delivering a third or half the load during that period. And as soon as the air temp rises well-above the dew point the full capacity resumes, making up for the shortfall that occurred during the coldest overnight hours.

    With less-well insulated houses with much shorter time constants, a significant capacity shortfall at or near the outside design temp quickly becomes comfort problem.

    It's usually better to spend the ~$35K difference in heating system cost between ground source heat pumps & mini-splits on building out a higher-R/lower-loss building envelope, since it's more comfortable, uses less power, and those low temp dew-point & capacity shortcomings of mini-splits that would otherwise be problematic become a non-issue.

    [Editor's note: For a further discussion of this issue, see information provided by Marc Rosenbaum in Comment #66 on 8/24/2012.]

  17. GBA Editor
    Martin Holladay | | #17

    Response to John Semmelhack
    Thanks for sharing your experience specifying ductless minisplits in Virginia. I have added your house to the list.

  18. James Steel | | #18

    Great article! It's nice to see a model that's affordable and reproducible. But I'm very curious what Carter Scott is using for ventilation in these tight houses. HRV/ERV? Small duct system to each bedroom? The $6000 minisplit system is encouraging. What's his take on an affordable ventilation system for the spec market?

  19. Mike Eliason | | #19

    scott homes recently built a
    scott homes recently built a house outside of Olympia WA that isn't PH (probably not far off, actually) that i believe has a single DHP.

    one of the PHs we're consulting on in seattle is a 1,500 sf home that recently broke ground - planning a single DHP as well. the house R-values are about the same as the carter scott NZ home (R-45 wall, R-58 roof, R-34 slab). surprisingly, it performs better w/ same specs if it were located in boston (even though 25% more HDDs than here) - meaning could probably get away w/ less glass to bring cost down slightly. the upgrade to intus windows and sub-slab foam (4" more than they normally build) end up being the only big difference in terms of their typical envelope.

  20. wjrobinson | | #20

    Great blog, great posts. Love
    Great blog, great posts. Love to see this hit a hundred useful posts. Personally would like to see inexpensive storage of the work Splits do developed.

  21. GBA Editor
    Martin Holladay | | #21

    Ventilation options for James Steel
    Good question. I have edited the blog to add information on ventilation.

    Carter Scott lets his customers decide what type of ventilation system they want. He said, “The baseline ventilation system we offer is a Panasonic exhaust fan in each bathroom. They cost us $250 each installed. The next step up -- an available option -- would be a Panasonic ERV that exhausts and supplies from the same location. That’s $500 installed. A little better would be the Fantech VHR 704 HRV with one exhaust location and one supply location. Better than that would be the Fantech SHR 1504 HRV, which could exhaust three bathrooms and supply all of the bedrooms. The next bump up would be the Lifebreath ERV, which is about $2,500.”

  22. user-600754 | | #22

    Great article Martin! We
    Great article Martin! We have built 8 super insulated homes in CT utilizing DHP's and have had great results with them. I would agree with you on most of your points except for your comment on tempertaure stratification of single level homes. I also used to think that was the case until we built a 2400 sq/ft single level home with 2 mini splits. Its temperature stratification was amazingly low; as good if not better than the 1600-2000 sq/ft two story homes we had built prior. While we dont have any hard data from monitoring rooms temps on an hourly basis, the customers have been very happy, which is really what maters the most. This home has R45 Walls, R80 Flat Ceiling, R45 framed floors, a high efficiency ERV, and infiltration rate of 1.1 ACH50. While a two story home typically has a better surface/ volume ratio, it proves that you dont have to stick to a square box in order for a super insulated home w/ DHP's to work.

    We currently have two homes entered into the 2012 Connecticut Zero Energy Challenge that utilize DHP's, you can follow their progress here:

  23. user-984364 | | #23

    Oversized minisplits?
    Martin, can you say more about the advantages of oversizing? I put in a 1-ton single head at the top of the stairs to keep our 2 hot weeks liveable. I sized it at what the heat load calc said, even though I knew it wasn't optimal distribution. I didn't want to go bigger, because the smaller units had higher SEER ratings. What's this about more efficiency from a larger unit?


  24. user-1087436 | | #24

    Oversize OK?
    Like Eric Sandeen, I'm surprised at Martin's remark about oversizing. On 8/1, in a separate thread, Keith Gustafson replied to a reader who wanted to put 7 mini-splits in a house:

    "Minisplits 'think' a lot, and they never shut off. They want to run at a very low level to be efficient. This means that if they are oversized for the space[as any 9k unit is for a normal bedroom] they are blowing cold air on you when you no longer want it. So you turn it off. then it is stuffy in the morning. Having a more central unit and making it work for a living is better for comfort, especially if you have some means of air exchange room to room."

    He's talking about cooling, obviously. But does this apply to heating as well?

    Wonderful article and postings, by the way. I'll definitely be printing this out.

  25. GBA Editor
    Martin Holladay | | #25

    On the efficiency of oversized minisplits
    Eric Sandeen and Gordon Taylor,
    I'll explain the efficiency story to the best of my knowledge. These inverter-driven ductless minsiplits have variable-speed compressors and variable-speed blowers -- features lacking on most conventional heat pumps and air conditioners. These features allow the ductless units to provide heating and cooling at part-load conditions -- more efficiently, in fact, than under peak load conditions. My understanding is that peak efficiency occurs when the compressor is operating at 1/3 of peak load speed.

    This applies to cooling performance as well as heating performance, as far as I know. According to this document, "MSHPs [mini-split heat pumps] can reduce the compressor speed to meet low cooling loads and have higher COPs under such conditions."

    Check out the graph below. It comes from page 10 of the document linked to above. Notice the COPs when the outdoor temperature is at 27 degrees F and 36 degrees F: this unit has the highest COP when operating at low fan speed, and it has the lowest COP when operating at high fan speed.

  26. homedesign | | #26

    Oversizing is GOOD with Mini-splits?
    I have only glanced at the document you referenced.
    Are you saying that with Minisplits.......
    other than the initial cost...
    Oversizing is "better" than Right-sizing?

    'Course I am wearing my Hot-Humid Glasses....
    and the first thing I think of is short cycling...
    and less water removal

  27. JoeW519 | | #27

    RE: conditiioned air distribution w/min-splits -- have y'all seen Allison's latest blog?

    He also has a webinar on min-splits coming up

  28. GBA Editor
    Martin Holladay | | #28

    Response to John Brooks
    Yes: with minisplits, oversizing is better than right-sizing -- especially if energy efficiency makes you smile. Oversized units running under part load conditions will have the highest COP.

    An oversized unit is also more likely to give you the heating output you need at low outdoor temperatures. Of course, if you are sizing a minsiplit for a house in a cold climate, you always want to look at the unit's output at the design temperature.

    If you are a production builder, however, you may not see any advantages to oversizing.

  29. user-1087436 | | #29

    So. Here we are again in Upper Nerdistan, and I'm lost. DHPs, according to Martin and the referred document, operate at peak efficiency when over-sized for existing conditions. But existing conditions change, and DHPs, it appears, are built to adapt to those conditions. Design temperatures, if I understand correctly, are extremes--average extremes--likely to be encountered in a given climate zone. Most of the time, the minisplits will not be operating in extreme conditions. Therefore, it would seem, if they are "right-sized" for the extreme conditions, they will, ipso facto, be over-sized for the day-to-day conditions that will be encountered. If that is the case, why would you over-size the unit for the design (i.e., extreme) temperatures, when most of the time they will be over-sized anyway?

  30. GBA Editor
    Martin Holladay | | #30

    Response to Gordon Taylor
    The main reason that it's useful for the residents of Upper Nerdistan to know that ductless minisplits operate most efficiently under part-load conditions is to remove sizing anxiety.

    Sizing anxiety may or may not be useful when it comes to sizing conventional furnaces and air conditioners. (That's a topic for another blog; it turns out the issue is not as clear-cut as many Upper Nerdistan residents think.) But fortunately, when it comes to sizing minisplits, there is no anxiety.

    Get your minisplit somewhere in the ballpark, or choose one that is too big if you want, and you're fine. Isn't it nice to be anxiety-free? The minisplit does all the work for you. It even operates better when it's too big. I think that is very useful information to know.

  31. wjrobinson | | #31

    Ok, we love Splits and the US
    Ok, we love Splits and the US of A. What is the split situation with US manufacturers? Toss some stimulus crumbs their way instead of Solyndra maybe ... Or not.

  32. user-1087436 | | #32

    It's all very cool.

  33. DavidButler | | #33

    Martin, I believe you're misinterpreting COP data.
    As alluded to by Gordon, a correctly sized system will operate a part-load conditions most of the time anyway. That's the advantage of having variable capacity. But COP is a steady-state efficiency metric, which mean it ignores cycle losses. Intentional oversizing increases cycle losses since these units have a limited capacity range (the minimum capacity is as high as 50% for smaller units such as the MUZ-GE09). Oversizing effectively reduces operating range.

    Although COP's may be higher at part capacity, in practical use, you won't realize those high COP's unless the unit runs long cycles. Oversizing works against that. Intuitively, we understand that the initial few minutes of each cycle are inefficient as the system overcomes thermal inertia. The shorter the cycle, the more those initial minutes weight on total efficiency. If you double up on capacity, then the system will operate at the minimum most or all of the time (depending on turn-down ratio). So when the load is less than design (most of the year), cycles become shorter and efficiency suffers.

    Unfortunately, there's no way to estimate this trade-off, but it's a myth to think that an oversized unit will necessarily be more efficient. Perhaps to a point, but certainly not if double-sized. Also, when sizing to the heating load, it's important to consider what happens in cooling mode. You could easily end up with 3 times more capacity than the design load. Not good when you consider what happens with cycling losses at part load conditions.

    My own design guideline is to avoid selecting a system with a minimum capacity exceeding 60% of design load. This most often comes into play with multi-zone (single compressor) systems, since the compressor's minimum now becomes the minimum capacity for each zone during single zone calls, even though the individual head may have a lower minimum when multiple zones are calling.

    Finally, I think you're confusing VRF with inverter technology. Just because a heat pump has an inverter driven compressor doesn't mean it has variable refrigerant flow. VRF is a method for managing refrigerant distribution to multiple heads in commercial multi-zone systems, such as Mitsubishi's City-Multi model line.

  34. GBA Editor
    Martin Holladay | | #34

    Response to David Butler
    Thanks for your detailed comments. And thanks for the correction on variable refrigerant flow; I have removed the phrase from my earlier post.

    I feel confident on the truth of the statement that ductless minsiplits operate more efficiently at part-load conditions than peak-load conditions. However, you have suggested several other factors that call into question whether an oversized unit will save energy over the course of the year. I intend to do more research on this issue and report back.

    In the meantime, I always welcome more comments.

  35. Marc Rosenbaum | | #35

    I don't intentionally oversize minisplits even though they are variable output. That's because if they are sized to handle the peak load, they'll be running at less than peak output almost all the time, and therefore take advantage of the efficiency gains available when the compressor and fans ramp down from peak capacity. As has been stated above, these products are variable in nature, but nonetheless have minimum output below which the compressor is off.

    Somewhere above is a discussion about dewpoint and capacity. I hope what follows is clarification rather than the opposite!
    Heat pumps are rated at 47F and 17F. Most of the minisplit products lose capacity as the outdoor temperature drops, and all do at some point. The principal reason is that the work that needs to be done to extract heat from progressively colder air increases, and the output of the unit decreases (some units use more power as the temperature drops, to keep capacity up, and others use less power but drop off in output - in both cases COP drops). There is a secondary reason, which is that there is a point where the condenser coil runs below freezing and therefore has to occasionally run in defrost mode to keep the coil from excessive icing.
    Mitsubishi, in engineering literature for their S series City Multi (true VRF product), has specific information about output reduction vs. outdoor temperature, as well as output reduction due to defrost. The maximum defrost output reduction is 12% at 32F wet bulb; at colder temps the defrost capacity reduction is less, because there is less moisture in colder air, so it rises to a 5% output reduction. So the outdoor temperature is the dominant capacity determinant (at least on equipment I'm familiar with, so not to ay others don't operate differently).
    My personal observations living with a Fujitsu minisplit is that this is borne out, and the unit is in defrost mode more around freezing outdoor temperatures than when it's really cold out (say, 10F). At any time i doubt the unit is in defrost mode more than 10% of the time.

    I hope this helps and I haven't inadvertently pushed anyone's Flame button :-)

  36. GBA Editor
    Martin Holladay | | #36

    Response to Marc Rosenbaum
    Thanks very much for your helpful comments. I especially appreciate the information on defrost output reduction.

  37. James Steel | | #37

    Re: Ventilation
    Thanks for adding information on Scott's ventilation strategies to the blog. I suspect that an exhaust only strategy comes up way short. But a conversation best had in another thread. Thanks again for the info.

  38. Expert Member
    Dana Dorsett | | #38

    In-re oversizing and dew-point/capacity
    Thanks to David Butler for the confirmation on the limits of oversizing. I've never attempted to formally model it, (with insufficient performance data to work from), but it's pretty clear that the standby losses will eat your efficiency lunch when it's duty-cycling at min-speed most of the time rather than modulating. Being 1.25=1.5x oversized at the design condition, most would still be modulating near their low end of the average winter load giving a slight boost to average efficiency over a unit that just barely made it at the design load, but beyond that point it slides off an every steepening cliff.

    The defrost derating is a function of how close the outdoor dew point is to the actual outdoor temp, which will vary widely by climate and from day-to-day. Even though they'll spend more time in defrost when outdoor temps are near to but above the freeze point than at colder temps, capacity derating isn't as severe. The raw efficiency is substantially higher at 35F then at 17F, and when it's still above freezing the heat pump is reaping the full benefit of heat of vaporization of that water, while only giving up the heat of fusion of that water during defrost. While the amount of moisture in the air is less when it's colder, there are be conditions where the dew point is close enough to the air temp to put a dent in capacity that it wouldn't otherwise experience.

    Mitsubishi H2is have "rated capacity" at 17F of about 60% of the "maximum capacity" at that temp (which is usually close to the nominal nameplate output). The "rated capacity" is dependent on a specific wet-bulb temp at which it is valid- there's a little asterisk and number at the various "rated capacity" values at different outdoor temps, and next to the +17F is a "*3". (Clipped from a spec I have on my computer) the "*3" conditions are:

    *3 Rating conditions (heating)-
    Indoor: D.B. 21.1º C (70º F), W.B. 15.6º C (60º F);
    Outdoor: D.B. -8.3º C (17º F), W.B. -9.4º C (15º F)

    A wet-bulb temp +15F, and a dry bulb temp of +17F corresponds to a dew point of +10F, which is not a rare mid-winter dew point in southern New England.

    Putting it in real terms, the popular MUZ-FE18NA is nominally a 1.5 ton, and can deliver more than ~18KBTU/hr in dry air even at +0F. (They're rated for 21.6KBTU/hr at +5F)

    But even at +17F, with a dew point of +10F it's only rated for 11.7KBTU/hr.

    While heavily derated conditions don't occur every day, they DO happen multiple times every winter, and can sometimes persist for hours. But those hours are usually when most people are in bed. If say, your outside design temp is +10F and your design heat load is 18KBTU/hr @ 10F, the unit makes pretty handily on nominal numbers, but there will still be a shortfall at even the +17F heat load when the dew point is that high.

    In a very-well-insulated house you'd sleep through it and never notice, whereas in a small barely insulated summer cabin with the same +10F heat load you might feel the chill first thing in the morning. The lower thermal mass and shorter time-constant of the smaller less insulated house will see a more rapid drop in temp when the shortfalls occur.

    After daybreak the capacity improves rapidly with outdoor temperature due to the greater separation from the dew point. Dew points move much more slowly than air temp at the start of the day, but more than the raw outdoor temp it's the separation between the outdoor temp and the dew point that gives it the increased capacity. The wider the separation in dew point and outdoor temp, the more closely the capacity approaches the "maximum capacity", which is it's bone-dry-air capacity. But in freezing-fog conditions where the outdoor temp has pretty much hit the dew point, the output crashes.

    [Editor's note: For a further discussion of this issue, see information provided by Marc Rosenbaum in Comment #66 on 8/24/2012.]

  39. user-1087436 | | #39

    One lingering question,
    getting back to the Carter Scott heating strategy. I ask this because I can see a skeptical HVAC contractor (or home owner, or building code official) asking the same thing, and I'd like to have an answer ready. Suppose you have a minisplit head mounted high on a wall in a hallway outside a series of bedrooms. It's daytime, and the doors to the bedrooms are open. The house, of course, is compact and well insulated, and the air handler has a heating capacity adequate for those bedrooms. You know the strategy is going to work, because it has worked for others. But the question remains, and we all know a skeptic is going to ask it, how can that minisplit head sense the heating/cooling requirements of those bedrooms when its only connection to them is a series of 30"x80" openings?

    Of course one answer is going to be: Have faith in the Second Law of Thermodynamics. And the second answer is going to be: Put electric baseboards in the bedrooms for cheap insurance. Still, I was looking for a bit more than those answers, and if somebody has something else to offer, I'd love to hear it.

  40. GBA Editor
    Martin Holladay | | #40

    Response to Gordon Taylor
    Like most heating systems, a ductless minisplit is controlled by a wall-mounted thermostat. It will turn itself on and off (or will modulate its output) based on the temperature of the air near the thermostat. If you want, you can place the thermostat in a bedroom or in a hallway; it's your choice. [Later edit: This isn't true for most ductless minisplits - they actually monitor the temperature of the incoming air. Sorry for the mistake. See further comments below.]

    When a house has superinsulated walls, triple-glazed windows, and low levels of air leakage, the temperature variations from room to room are less than in a conventional home. Let's imagine that a hallway is maintained at 72 degrees F. That means that the partitions and doors (whether open or closed) are also at 72 degrees F. These partitions aren't insulated. The thermal mass in the house (including the framing and drywall) act like a flywheel. Everything inside the thermal envelope of the house tends to stabilize at the same temperature.

    Of course, heat loss (especially heat loss through windows) at night affects the temperature of each room, and heat gain (especially heat gain that happens when sun hits a window) also affects the temperature of each room. Proper window sizing, window shading, and glazing specifications can minimize problems from these types of heat loss and heat gain.

  41. homedesign | | #41

    A hallway may not be the best location for a ductless unit
    A couple of months ago I was having an email exchange with David Butler..... where I proposed locating a single Minisplit in a hallway near the bedrooms....

    David Butler's advice :"Hallways are not the best place to put a ductless unit. Better to put in a room with large load. "

  42. Expert Member
    Dana Dorsett | | #42

    Wall thermostats?! Remote controls!
    Wall thermostats are probably the least common way to control a ductless these days. Most use infra-red remotes (similar to television remotes) that communicate with the head, and the head tracks the temperature of the incoming air to determine the room temp.

    Most manufacturers support some sort of wireless or wired wall thermostat as an option, though it's usually company-proprietary, not a generic third party T-stat. (Recent Mitsubishi models support Honeywell RedLINK wireless thermostats, and it's at least theoretically possible to use a non-Mitsubishi version.) Most people seem to be comfortable with using the remote though.

    With R40 exterior walls and ~R2 partition walls, in a blind room (no windows) the temperature difference between the heated hallway and the bedroom could be roughly calculated by the R-ratio against the hallway temp & outdoor temp. But the heat loss out the windows could easily be 2x that of the rest of the wall, making the calculation more complicated, but still a number that can be roughly calculated on a bigger napkin (ignoring mass effects and solar gains, etc.). The simpler model balances the heat flux between the hall & R2 partition at the hall-to-room delta with the heat flux out the exterior wall/windows to solve for bedroom temperature at a fixed outdoor temp.

    But of course, in an occupied bedroom you would also have the ~400-500BTU/person-hour (awake, not particularly active) or 250-350BTU/person-hour (sleeping) either supporting the heat load or adding to the cooling load of that room. In a high-R home and a low-loss bedroom it makes a small but measurable difference in where the room temperature stabilizes.

    Bottom line at all but the temperature extremes (or in some cases high solar gains), the room to room differences will be pretty small. It's OK to bump the setpoint of the ductless up/down a few degees during colder/hotter weather to bring the doored-off areas into a more comfortable range. Most people can tolerate a 5F or greater delta between the warmest & coolest rooms in a house, and most of the time adjacent spaces will have deltas much lower than that.

  43. DavidButler | | #43

    is this what clients really want?
    Dana wrote:
    > Most people can tolerate a 5F or greater delta between the warmest & coolest rooms in a house

    That's pretty much status quo for conventional building practice. It's easy to save energy if we push occupant tolerance limits. However, in my experience, people who build (or retrofit) high performance homes are usually motivated by a desire to *improve* comfort, sometimes even more so than realizing a positive payback. On the other hand, I've had clients who willingly push their tolerance limits to great effect. Bottom line: I would never specify a central ductless heat pump unless the client fully understands and embraces the trade-offs.

    In practice, temperature variation may be acceptable in cold weather since people generally prefer a cool bedroom for sleeping. But in areas with significant cooling loads, especially homes with upstairs bedrooms, it's hard to get around the need for ducted air distribution. Especially if any bedroom windows are exposed to afternoon sun in summer.

  44. DavidButler | | #44

    pay attention to location of largest loads
    If a mini-split head is installed in a hall with little if any intrinsic load, it's going to short cycle. In fact, it will hardly ever run unless there's a way to induce air changes with adjoining rooms. If a remote stat is installed in a bedroom, the unit will over-condition the hall, especially when bedroom doors are shut. In fact, the blower should never be separated from the sensor by a closed door.

    While it's true that temperature swings will be smaller in a super-insulated, super-tight home, it's important to look at where the loads actually occur. The key to minimizing temperature variation is to deliver conditioned air in the rooms with the largest loads.

    A lot of folks advocate the use of ERV/HRV's to help distribute conditioned air, but this strategy makes no sense with a ductless mini-split as primary heat/cool source, since the ventilation system does little to drive internal air changes. In fact, it creates additional loads in rooms with fresh air supply vents. Moreover, ERV/HRV blowers are extremely inefficient air handlers. It takes a lot of power to push air through the dense heat exchange core. Even models with ECM blowers consume 5 to 10 times more power per CFM than a conventional ECM air handler.

    A ducted head is the most logical and effective approach to addressing bedrooms loads. But it's important to pay attention to available external static since these heads have very little fan power. For example, Mitsubishi's SEZ-KD ducted head can only handle up to 0.2"WC. I listed some design tips in my comment on Allison's mini-split blog (

  45. zimmerdale | | #45

    Boost fan?
    A few Passive House projects (my bungalow remodel included) are experimenting with using the combination of an ERV to exchange air and a ducted minisplit to condition air. They share the same duct work. I would like to add a small Fantech inline boost fan to supplement the wimpy minisplit fan and allow for more manageable duct sizes. Any experiences or thoughts on syncing a secondary fan with the built in fan?

    I am using the Samsung 1-ton ducted model EH035 and the UltimateAir RecoupAerator.

    In Kansas, we have significant latent loads during the summer, so I will have stand alone dehumidifier. Several HVAC contractors have also recommended the option of increasing air circulation--which would not be part of the ERV cycle, so as not to increase indoor humidity. The boost fan could be used for this task as well, without running the minisplit.

    Our house will be very similar to the ones Carter Scott is building--compact design with around 2000 ft2, heat load of 8000-10000 Btu/hr, Passive House levels all around.

    Perhaps I should have just used a single or double-head ductless minisplit, or waited for a magic box, but I have the ducted model already, so I want to try to use it.

  46. user-1087436 | | #46

    @Jason Miller
    New question at Q&A?

  47. user-1087436 | | #47

    Getting back to Carter Scott...
    This is all great stuff from the best in the biz, and worth studying at length. But could we get back to the Carter Scott houses? Martin has written that CS uses two minisplits, one on each floor of his two-story houses. Now presumably, if his second floors contain bedrooms, they must have something resembling a hallway. And I have to assume that his 2nd minisplit is located there, which David Butler says is not a good idea. If so, how does Carter Scott handle the problems we're talking about? Does he do anything special with thermostats, for example? Or does he just open up the bedroom doors and let the air handler take over?

  48. DavidButler | | #48

    answered at EV blog...
    @Jason, I posted extensive reply to your question which you double-posted in Allison's mini-split blog.

    @Gordon, I can't answer for Carter, but the fact that his homes are in a heating dominated climate mitigates the issue somewhat, as I noted in my last comment. Also, if upstairs unit is located in a loft area or balcony that communicates with downstairs (as opposed to a confined hallway), at least the over-conditioning issue would go away.

  49. GBA Editor
    Martin Holladay | | #49

    Ductless minisplit locations
    I am attaching an illustration from one of my Fine Homebuilding articles that shows the locations of the minisplit indoor units in Carter Scott's first net-zero-energy home. The upstairs unit is in a hallway at the top of the stairs, pointing down the stairs.

  50. GBA Editor
    Martin Holladay | | #50

    Response to several objections to Carter Scott's approach
    Several GBA readers have questioned aspects of Carter Scott's approach:
    John Brooks has noted that a hallway is probably not a good location for a ductless minisplit unit.
    David Butler has noted, "If a mini-split head is installed in a hall with little if any intrinsic load, it's going to short cycle."

    These comments are all fine and good, up to a point. However, Carter Scott has completed 18 homes using this approach, and the owners of the homes are apparently happy.

  51. user-626934 | | #51

    Carter Scott's approach
    Carter's approach is definitely climate and building-specific, as I think Martin mentioned in the lead-in to the article. The client is the ultimate arbiter over whether something "works" or not (at least to the builder/developer, if not the energy geek). Clearly, the hallway mini-split seems to be working for Carter's clients. Some ideas as to why...

    1) In the winter, the downstairs unit is probably doing the bulk of the work...perhaps carrying 100% of the entire house heating load over 80-90% of the heating hours (educated WAG). The hallway unit is nearly superfluous in the winter. At least one of Carter's clients doesn't even use the hallway unit.

    2) It looks like the 2nd floors on Carter's houses have relatively little glazing, the glazing generally has "OK" shading, and the bedrooms are probably modest...the bedrooms probably have modest cooling loads.

    3) Massachusetts has a short cooling season. Some short cycling over a short cooling season, although not ideal, is probably not a big energy penalty.

  52. DavidButler | | #52

    I'm all for pushing the envelope, but...
    Much of my design practice involves specifying systems and design strategies that run counter to the experience & training of most mechanical contractors. I always try to encourage these guys to push the envelope in their own homes to give them more confidence in the (mostly) correct ACCA design procedures. So here I find myself in the odd position of being the one arguing against outside-the-box thinking.

    One thing we must be careful about is not to propagate contrarian designs that might work in a specific case. Designing HVAC for high performance homes requires experience and judgment, not cookbook strategies. As a designer, my responsibility is to meet my client's expectations. A homeowner who expects a cool bedroom at night in summer may be very disappointed with the approach being discussing here. Personally, I know it would never fly in my home.

    That being said, I would be genuinely interested in seeing data (e.g., temperature logs) that demonstrate actual performance of a ductless head in this configuration.

  53. user-1044661 | | #53

    HRV return air cools bedrooms
    Carter's experience mostly mirrors our own. We've designed and installed mechanical systems (originally lots of mod con gas boilers and now mini splits too) for dozens of homes without heat in the bedroom (or often any heat upstairs) and have virtually never heard a complaint from a client. And we're in a more challenging heating climate than Mass.
    The couple of houses where we have gone back to add supplemental heat have all been homes with an HRV or ERV. I wonder if the absence of whole house ventilation contributes (unexpectedly) to the balanced heating of these homes. On a design day in Maine, even the most efficient HRV is bringing in fresh air to the bedrooms at 5 degrees or more below room temperature and though the flow rate is modest, if the bedroom doors are closed I think this can contribute significantly to a cooling of the bedrooms and a desire for supplemental heat.
    Recently, I've been encouraging clients to consider a small electric boost heater in the HRV return duct (1000 W) that runs only at very low ambient temperature to eliminate this issue. But maybe exhaust only or the Panasonic ERV is a good option (though I can already hear our ventilation partners protesting).
    Finally, just because piling on SHW seems to be the topic of the year, I feel the need to respond to that as well. In my opinion, the viability of SHW (compared to on demand or HPWH) depends on load and incentive environment and defies a one-size-fits all answer.

  54. GBA Editor
    Martin Holladay | | #54

    Response to Fortunat Mueller
    You're the second person today who has posted a comment referring to the fact that during the winter, HRV fresh air grilles tend to cool the room in which they dump fresh air, especially if the room is unheated. (The other person was Jesse Thompson, on this blog.)

    Of course, you're both right. I wrote a blog on the issue over a year ago; here is is: A New Way to Duct HRVs. But it makes no sense to deliver outdoor air to the coldest rooms in your house.

    The solution is fairly simple: deliver the fresh air to the warmest room in your house (in the case of Carter Scott's designs, that means the living room and the upstairs hallway) and exhaust stale air from the rooms without any heat (the bedrooms). That tends to even out the room-to-room temperature variations.

  55. user-1026988 | | #55

    HRV return air cools bedrooms
    In super insulated ,airtight ( and by airtight > 1 ach/50) A properly sized and BALANCED ventilation system will aide greatly in distribution and elimination of atmospheric thermoclines. Distribution will be fine with the unit at the top of the stairs. In fact at higher levels of airtightness like closer to PH the Coanda effect itself will aide with distribution allowing the air to cling to the ceiling until it drops off at the other side of the room or space. It should also be noted that the evaporators that are ceiling mounted have wide ranges 110degrees or so and are continually oscillating. The floor mounted evaporators are less efficient I think because of this. They can not use surface tension for distribution.
    Lastly if there is a delta T of 5degrees F from the air from the ventilation system versus room temp then you are NOT using the most efficient units. At 7 degrees F exterior ambient temp we are getting supply air at 66F with a room temp of 68. That is with units with 92% (real 92% not some Stirling tech enthalpy wheel marketing BS) There are no north american manufacturers (that I know of) that can achieve this.
    Lastly 2- small windows suck for daylighting
    Lastly 3- Why two separate units in Carters homes? Why not use a MXZ-2b-20na hyper heat unit with 12000btu down and 9000btu up. Works great. One condenser and 2 evaporators. 20000btu capacity COP of 3.9ish.
    Oh yea edit- remember these thing push like 150- almost 400 cfm.

  56. user-1026988 | | #56

    Ahh snap. Exhaust air from bedrooms? 1 king size bed, two adult humans, two children's, two dogs,15 fish, 4 plants, .7ach/50....................................wont I suffocate?

    ASHRAE no likey.

  57. GBA Editor
    Martin Holladay | | #57

    Response to Christian Corson
    OK, your bed sounds crowded. Four people and two dogs? Well, eventually the two kids will probably sleep somewhere else. As for the dogs, it's just a matter of training.

    You wrote, "ASHRAE no likey." You're wrong, Christian. ASHRAE 62.2 specifies the ventilation rate for a home, but has no distribution requirements. Plenty of people have installed ASHRAE 62.2-compliant exhaust-only ventilation systems with no fresh air to the bedrooms. (I'm not saying that is necessarily a good idea.)

    I'm suggesting something better than that: A ducted HRV system that pulls exhaust air from your bedroom. If you want 30 cfm in your bedroom, there's no reason you can't have it. Adjust the exhaust air flow from your bedroom to 30 cfm. Obviously, air in = air out. You'll get 30 cfm of fresh air. It will be pulled in from the hallway under the door. And you know that the hallway air is fresh, because that's where you are dumping your supply ventilation air.

  58. user-1026988 | | #58

    Yes, its a party in our bed every night! One budding artist and another budding paleontologist. You should hear my wife read Goose and Grim in German. Getting boiled by a which sounds WAY scarier in German. Good night moon.........not so much. The dogs were trained until we had kids, now all bets are off. Plus they are so old they deserve to do whatever the hell they want.
    As far as ASHRAE goes you are correct I should have referred to the EN ISO recommendations of 18 CFM per person at rest , but was trying to keep it simple and I couldn't remember the standard off the top of my head....EN ISO 13790? Any way, that is actually where ASHRAE falls short. IMHO. Thanks for keeping me in line. Ill take 36 CFM in all night long in my room, Ide rather have 35 out in the bath.( I only have 22) But I get what your saying. My hallway stays fresh as I blowing 150 CFM through my minisplit.

    Oh and exhaust only ventilation is a bad idea. Just throw a wood stove in for good measure.

    What did my Grandfather say? ......'doing something wrong three times doesn't make it right.' Scary thought coming from a surgeon with a cigar hanging out of his mouth.

  59. ecdunn | | #59

    Have been building homes with just a central space heater here in Flagstaff, AZ for years. Yes, with great insulation (Oak Ridge National Laboratories standards developed in the 70s and 80s) Our bedrooms have no heating system. Bedrooms are no more than 4 degrees cooler than the main living area. Cooling is not necessary in Flagstaff, but a lot of homes with inadequate ceiling insulation are uncomfortable several days in the summer. Our homes are very nice inside. Does not matter if we use Straw, cob, foam or superinsulated stud walls. It works. A tight, well insulated envelope is the foundation for a properly designed passive solar home.

    The Department of Energy adopted the ORNL standards a few years ago.

  60. DavidButler | | #60

    no bedroom heat here either...
    I live in southeast AZ, but nearly a mile above sea level. Every winter I close off my master bedroom supply (actually, I tape cardboard over the inlet inside the cubical boot). I can't tolerate a warm bedroom. I turn down the stat to 64 for the rest of the house. The cats don't seem to mind (not that they have a voice in the matter). Since I'm a night owl, my wife is usually up long before me. A motion sensor outside the bedroom automatically returns the stat 69 for her comfort. However, the bedroom stays cool.

    The only time I ever had to open the register in winter was in Feb 2011 when we saw two consecutive nights in the single digits. That's also the only time my heat pump ever needed supplemental heat.

    In Ed's climate in northern AZ at 7000 msl, I wouldn't hesitate to forgo direct supply to bedrooms, provided the house is tight and well insulated.

  61. Zolton | | #61

    Wall hung heaters.
    Someone earlier in this thread mentioned the concept of "task heating and cooling." That's an interesting way of looking at things.

    Has anyone ever done anything with simple wall hung gas heating console units like those made by Empire? I have one in my garage shop that outputs about 10k BTUs. It is a direct-vent unit that intakes combustion air and exhausts through the same concentric pipe. As simple and as dumb as dirt.

    But it has a thermostat and is almost completely quiet - other than some small expanding sheet metal noises when it first fires up. There is an optional fan, but I never bought one.

    My wall hung heater was about $550 a number of years ago; I'm sure they're probably double that now. The efficiency rating is 80%; not great compared to modern day furnaces, but at a small fraction of the cost.

    So, put a couple of these heaters in a house and a couple of window A/C units into wall openings and you've got the output of a mini-split at very little cost. Is this an idea worth exploring?

    The only drawback I see is that I believe "space heaters" can't usually be used as a primary heat source. But maybe that point could be finessed...


  62. user-829148 | | #62

    One Mini-Split, 2 Indoor Units, One Ducted, One On The Wall.
    Kudos to Carter Scott on his use of Mini-Splits and the Envelope to make them work. I've been doing a similar approach for several years in renovations - first in California's central valley and now in the Blue Ridge Mountains of Virginia. I totally focus on renovation of existing housing stock, usually foreclosures. In 2009 I reported in Home Energy on my first of what we call the Mini Split Hybrid approach. Hybrid because the single outside unit drives two indoor units, one conventional unit on a wall in the main living space and one running an air handler and a small duct system to feed the bedrooms. I usually keep this ductwork inside the envelope, in a lowered hallway or stairwell ceiling. I use Fujitsu, but some others make an indoor duct able system. My initial results were outstanding with the owners reporting good comfort and very low bills. I've been invited to participate in a DOE program and two houses are now in a year long monitoring program, one in California, one in Virginia. These are both about 1700 square feet and have 2 ton units driving 2 12,000 BTU indoor units, one ducted, and one not. I always use the 2 ton. It's always oversize, but the smallest they make. As has been stated here, oversizing is not such a bad thing with inverter Mini-Splits.

    Because this is not new construction, I cant get as good an envelope as Carter is providing. Typically we dense pack the walls and add R-60 above. On the most recent project in Virginia, we drywalled the basement ceiling and dense packed the first floor as well. This one has a short hard pipe duct system currently exposed in the top of the stairwell. Initial results (summer) are surprisingly low watt draw on cooling. I'll report more results as they become available. DOE and NREL will report also.

    The Hybrid approach of Ducted and Ductless addresses the concerns about even temperatures in all rooms. The installation cost is about the same as two separate MSHP systems. So if you dont have the renovation budget for a deep retrofit with new rigid foam and triple panes, ... consider running ducts to the bedrooms and let a single unit handle the main living area (which you've opened up in the renovation anyway)

  63. GBA Editor
    Martin Holladay | | #63

    Response to Zolton Cohen
    Your approach works fine, and is one I advocated in my article, Heating a Tight, Well-Insulated House. I like systems that are "simple and dumb as dirt." And as many people have repeated, once you have an excellent thermal envelope, it hardly matters how efficient your heater is, since you will be using very little fuel.

    Add me to the list of people who live in a house without any heat in the bedrooms. I've never seen it get any colder than -38°F, but that's cold enough. I have one wood stove in the living room -- that's it. If I ever go away on the weekend during the winter, I keep my house warm with a single gas-fired Empire space heater in the living room, just like the one you describe. That keeps my pipes from freezing just fine.

  64. Matt Berges | | #64

    Installing the heads pre drywall?
    As shown in one of the photos at the bottom of this article, I have recently had a contractor who wanted to install my mini split heads on the "rough in". I had previously had 8 installed in other homes (with other contractors) and had never been asked to jump out of sequence like this. It was explained to me by this current installer that this was the best and only good way to do it (for sake of making the proper connections), but I am not comfortable installing my finishes on the rough in. I would like to hear advice from the GBA community on this sequencing issue. My concerns are dust, damage, added difficulty, added risk (theft or other). I like to have the mini split heads go in at the very end, and be commissioned and tested at that point.

    Wow- this article has generated a lot of postings!

  65. Marc Rosenbaum | | #65

    Back to minisplit capacity and dewpoint
    I'm going back to post #38 about minisplit capacity and dewpoint. The information in that post contradicted what I understood to be correct, so I checked in with my engineering contact at Mitsubishi. Here's what he wrote:
    "The only relevant factor in determining the available heating capacity is the outdoor wet bulb temperature. This graph below represents guaranteed heating capacities at various wet bulb temperatures."
    This graph shows that the Hyperheat models FE09 and FE18 produce 100% of their nominal capacity down to about 3F wet bulb, and the FE12 down to about 12F wet bulb (I'm eyeballing this graph). In the case of the FE18, I take this to be the capacity they publish at 47F dry bulb, 21,600 BTU/hour, which they show as the maximum capacity in their submittal sheet at 5F dry bulb as well. This graph states that it includes the correction for defrost.

    He goes on to say:
    "Rated capacity is the capacity as the equipment was tested in a lab setting to obtain the published efficiency values, maximum capacity is the actual capacity that the unit is capable of producing as it operates in the field, you should always use maximum capacity when selecting/sizing equipment for an application."
    So I don't believe people need to be concerned about dewpoint of the outdoor air when sizing the Mitsubishi Hyperheats. Of course, at temperatures approaching 0F, the wet bulb and the dewpoint get closer together than they do at higher temperatures.

  66. GBA Editor
    Martin Holladay | | #66

    Response to Marc Rosenbaum
    Thanks for taking the time to contact Mitsubishi to clear this issue up. So, the outdoor wet bulb temperature is relevant; dewpoint isn't.

    Bottom line: check the heating capacity of the unit you are considering at the outdoor temperature you are concerned about, and you should be all set.

  67. user-2078048 | | #67

    Needham Temp / RH data, exhaust from bedrooms
    I worked on the Needham design with Carter and we did some temperature / RH logging, which we can share.

    This data includes the week without power due to the ice storm that hit Central MA in Dec. 2008.

    If anyone is interested, send me a note at mduclos (at) and let me know if you want the HOBO files or .csv

    As Martin has described in the reference above, exhausting from the bedrooms is an interesting concept.

    During the design we were concerned about the bedrooms getting too cool, so Carter decided to try this in the Needham, this from Carter:

    "On this home we exhausted the three bedrooms and supplied the fresh cooler air in front of each of the two mini-splits to be heated up.
    We used return air grills to allow the air to get to the bedrooms even if the doors were shut."

  68. Expert Member
    Dana Dorsett | | #68

    Thanks to Marc Rosenbaum from me as well!
    The "rated capacity" without a graph or an explanation of what it implied (the output at which the stated COP at that temp was valid) led me astray (and leading me to spread dis-information with this confusion- mea culpa!)

    Is the more detailed data published on the web anywhere?

    I find the real explanation very comforting relative to my long-standing misinterpretation of the spec, since some of the people I've recommended them to (some of whom have in since installed them) had some degree of capacity-anxiety related to those numbers.

  69. DavidButler | | #69

    dew point vs wet bulb
    Marc wrote:
    > So I don't believe people need to be concerned about dewpoint of the outdoor air when sizing the Mitsubishi Hyperheats.

    Dew point and thermodynamic wet bulb temperatures are both non-relative indications of now much moisture is contained in the air. The HVAC industry standardized on wet bulb because it can be directly measured. On the other hand, dew point is calculated, not measured. But is commonly reported in climate archives. Online calculators are available to convert from one to the other, at a given dry bulb (typically assuming standard atmospheric pressure).

    Since the outside coil is acting as an evaporator in heating mode, both wet bulb (dew point) and dry bulb affect capacity. I've always been puzzled why Mitsubishi only references wet bulb. This should not be construed to mean that outdoor dry bulb doesn't independently impact capacity, as your contact apparently believes. If you look at any expanded table for US heat pumps, they only reference dry bulb. Dakin's engineering books reference both wet bulb and dry bulb.

  70. Expert Member
    Dana Dorsett | | #70

    But the result is the same, from a practical point of view
    If Mitsubishi is saying that you'll get at least the full nominal capacity down to where the wet-bulb temp is +3F independent of how close it is to the dry bulb temp it's easy to spec a unit with some confidence based on only the nominal output rating in areas where outside design temps are in positive single-digits F. That covers most of US climate zone 5.

    I suppose you'd need the graphs to spec one for sub-zero design temps though. But in a high-R/low-U houses you can undershoot by quite a bit for hours and never feel the chill, due to the longer thermal time constant.

  71. user-1091878 | | #71

    Problems with mini-splits
    You mentioned that Carter Scott has satisfied customers to this point. when I spoke with my HVAC contractor about mini splits he mentioned that they have had a terrible time with sensors going out on the units at the rate of about 1+ per year. Efficiency and cost savings are nice but on going maintenance issues would quickly eliminate any gains. Anyone else experienced these issues?

  72. GBA Editor
    Martin Holladay | | #72

    Response to Chris Baddorf
    Anecdotal reports in New England (chiefly for Mitsubishi and Fujitsu units) are that installed units are performing well, with very few (if any) glitches or problems.

  73. user-1053392 | | #73

    Late to the party

    This has been a long and fruitful conversation! I don't know that I can add much, except that Coldham Hartman architects and myself,, did a deep energy retrofit last year in Amherst, MA and also only installed one Fujitsu on the first floor. Temperature differentials in the bedrooms were negligible, according to Hobo recorders. So, DERs are viable also. Our clients were OK in the summer as well, but I can imagine some folks wouldn't go for it.

    I'm in Vietnam now, and people deal with the heat a whole lot better than in the US. A lesson in human behavior for myself.

    Principal, DS Greenbuild LLC

  74. GBA Editor
    Martin Holladay | | #74

    Response to Doug Snyder
    Thanks very much for sharing your experience.

    Another satisfied minisplit user without any bedroom heat -- the list of satisfied customers is growing.

  75. sdsubob81 | | #75

    Mini Split installers
    I had a heck of a time getting realistic bids for doing Mini-Splits.
    I finally used my neighbor but he only did it since I was a friend. Otherwise he won’t do mini-splits. When I contacted the Mitsubishi Diamond dealer, he gave me, IMHO, a ludicrously high quote and it wasn’t even what I asked for him to bid. He “knew better” than what I wanted and gave me a quote for a less efficient MXZ2A20NA outdoor unit and two MSZA12NA heads for 8,546.66 which included a discount since it was “off season” for an open stud, can’t get much easier install.
    I asked for a quote for two individual 12,000 BTU Hyperheats. Not a HVAC guy but I work in the data center world where I deal with HVAC issues day in and day out. I knew what I wanted, had researched the heck out of the topic. Got the units I want and they work great.....

  76. GBA Editor
    Martin Holladay | | #76

    Response to Bob Lobsiger
    In some areas of the country, the installation of ductless minisplit units is still rare, and in these areas, there isn't enough competition yet for contractors to quote reasonable prices. Hopefully, as the installation of ductless minisplits becomes more common, this situation will change.

  77. FStephenMasek | | #77

    Robin Hood?
    If you use somebody to take something from others who do not want to give it, what does that make you? Fine Homebuilding makes Scott out to be some sort of hero, but he is most certainly not to those of us who do not want our tax money taken and given to him in the form of subsidies, all in the name of the "global warming" hoax. Note that Robin Hood took ill-gotten money from the government and give it back to the people, just the opposite of what those receiving subsidies are doing. Shame on them!

  78. user-1087436 | | #78

    Wow, I don't know where that last guy came from,
    but I know that it's a strain obeying the "Be Nice" directive.

    It's New Year's Eve 2013, we signed our contract yesterday, and we'll be breaking ground by the end of January. (You can do that in Zone 4C.) I've spent the last hour rereading all the posts on this thread, and it reminds me what a terrific asset GBA is. A big Thank You to all, and have the most prosperous and happiest of New Years!

    Gordon Taylor
    Willamette Valley

  79. GBA Editor
    Martin Holladay | | #79

    Response to Gordon Taylor
    Thanks, Gordon -- and Happy New Year. Good luck with your construction project.

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