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Musings of an Energy Nerd

Will Minisplits Replace Forced-Air Heating and Cooling Systems?

Using ductless minisplits to cut the Gordion Knot of HVAC system commissioning

Two components connected by insulated copper tubing. A ductless minisplit system consists of an outdoor unit (below) that usually sits on a small slab, and an indoor unit (above) that is usually mounted high on a wall. These components are manufactured by Mitsubishi.
Image Credit: Mitsubishi

Because forced-air heating and cooling systems are assembled on site from a great many parts, there are many ways for installers to make mistakes. Researchers have repeatedly shown that a high percentage of residential forced-air systems have major problems, including duct systems that are poorly designed, poorly located, and leaky. Other problems include incorrect refrigerant charge and too much or too little airflow over the cooling coil.

The classic solution to these problems — in addition to the obvious step of better duct system design — is to insist on a more rigorous commissioning process. “Commissioning” refers to the process of testing and adjusting installed equipment to be sure that it performs in accordance with the manufacturer’s specs and the designer’s intent. Although commissioning is rare for residential HVAC systems, it is a routine step for commercial and institutional buildings.

Energy Star makes commissioning mandatory

As I noted in an earlier blog, The Energy Star Homes Program Raises the Bar with Version 3, the Energy Star program has decided to include requirements for the commissioning of forced-air HVAC systems, beginning January 1, 2012. The following commissioning steps will be mandated:

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  1. user-723121 | | #1

    A professionally installed standard HVAC does perform very well, it is all about the skill and commitment of the installer.

    Homes without basements that put ductwork in unconditioned spaces should look for alternatives like the minisplit system. Another method for southern homes if they are 2 story would be a main floor mechanical room with 2x4 web trusses for the 2nd floor. The ductwork can be run in the floor trusses and inside the building envelope.

  2. GBA Editor
    Martin Holladay | | #2

    Response to Doug
    You wrote, "A professionally installed standard HVAC does perform very well, it is all about the skill and commitment of the installer."

    Unfortunately, every researcher who has ever assessed existing residential installations has reached the opposite conclusion. Most forced-air HVAC systems don't perform very well -- especially in states where ductwork runs through unconditioned spaces.

  3. homedesign | | #3

    Privacy Expectations
    I think Ductless could "work" IF there were enough indoor units(heads) to provide the privacy zoning that most American Families expect.

    I think even a modest (1400 sf) 3 bedroom Home (with bedroom doors)would "need" 3 to 4 units or otherwise some method to exchange air between "zones" when doors are closed.

    Martin, or anyone,
    assuming a 1400 sf home (Energy Star or better) in a Mixed Climate.....
    Estimated installed price range for a 4 head system?

  4. user_963668 | | #4

    Zones and Leaks
    Mini splits are great for open floor plans or big rooms or big BTU amounts (cooling). But if have a small tight house 1300sqft with small rooms (7 vents) its virtually impossible to have balanced heating and cooling. Would need like 4 indoor units, plus would still need a backup heat source for the colder climates.

    What plumbers don't check to see if all faucets are getting cold and hot water? They don't have to check all pipes for leaks? Plenty of bad plumbers have leaks, just like a bad HVAC install will have leaks. The HVAC leaks are less noticeable, cause less damage, so most people don't care. Plus getting to a well designed HVAC ductwork takes a lot of planning, when some just want to throw it together with flex or 8 or 6" round duct everywhere.

  5. user-939142 | | #5

    not quite so fast
    but then we need to commission the ERV/HRV system.... so much for getting rid of the ducts!

  6. gusfhb | | #6

    I have lived with minisplits for a few years now, 2 different houses. I think in the case of AC, zoning in a well insulated[late 20th cen] house is overblown. in temperate climates, it is mostly about dehumidification.
    In a really insulated[21st century] house, zoning is next to pointless. CFL bulbs and doors open in daytime, worst case it may be slightly 'stuffy' in a closed bedroom. Someone mentioned transoms in a previous thread.

    I have 3 minis in my current house, and the one in the master is too large[smallest they make] and will not be needed when I replace the leaky slider, who knew?

    Seriously, a moderatedly tight, moderately insulated house makes so many problems go away.

    Sure, the desert is a harder job, Insulation, air tightness, shading.............

  7. GBA Editor
    Martin Holladay | | #7

    Marc Rosenbaum responds
    [Marc Rosenbaum sent this comment to me by e-mail and asked me to post it.]

    - Yes, we need to still commission the ERV.

    - There is research ongoing about using a point source heater in modest superinsulated housing. Our Eliakim's Way homes, and Anne Perkins' homes at Wisdom Way in Greenfield (Robb Aldrich at Steven Winters), and Carter Scott's homes, all have different approaches. We don't really want 4 heads in a 1400 ft2 house. Having said that, one approach is to use a ducted minisplit but then we have the same duct issues.

    - Cost - we're in a high cost area, but ballpark single zone system $3,000-4,000, 2 zone $6,000, 4 zone $8,000-9,000.

    -- Marc Rosenbaum

  8. user-963733 | | #8

    Appliance vs. system
    Really, home owners WANT to regard their AC as an APPLIANCE, not as a SYSTEM with many parts which has to be installed by some kind of engineer. A window unit is an appliance, a dehumidifier is an appliance, a mini-split is an appliance, you can install them and expect them to work as you thought they would.

    But central forced-air HVAC is a system. If you have any doubt of this, visit the board called HVAC-Talk and see the many recurring complaints. The men (and rarely women) who install this are not generally qualified to be engineers, but when you consider all that must be done to design and successfully install central AC, they really are doing an engineering job. And sometimes not living up to the requirements.

    Airflow among the various rooms is highly important (especially when it carries the heating or cooling BTUs), and you cannot escape this by using mini-splits. For example, how can you possibly guarantee proper heating, cooling and dehumidification (especially the latter) in a bathroom without mechanically driven air exchanges? Will you require a separate mini-split head in every room?

    But anything which moves toward appliances replacing custom systems, is a step in the right direction. Three cheers for discussing this subject.

  9. 5C8rvfuWev | | #9

    It sounds like a great option for this mixed climate. I've spoken with 3 dealers for Mitsubishi who insist it is "too expensive" and "won't work" in a house; they've each talked about garages or stores.

    So I'm thinking maybe I can use (1900 sf, one level) a minisplit for the ~700 sf open space and a much smaller AC for the bedrooms, office, etc.

    To a dealer, the question re: minisplits seems to create the impression you're made of money. One was trying to convince me it would be great if I installed radiant floor heat AND a AC/heat pump. Sheesh.


  10. user-659915 | | #10

    Bringing inspections up to speed
    Local inspections dept. recently failed a ductless installation in a three-room garage apt. with stud bay hi-lo circulating fans. Had to put in baseboard electric as a supplement which almost certainly will never be used. Hoping that with a few more installations we'll have the track record to show the supplementary heaters aren't needed.

  11. P_Murphy | | #11

    My brother in law just installed three mini-splits in two builders and was shocked to discover that he could not turn the thermostat below 60. That value is designed in.

    Has anyone addressed the CO2 issue of using coal fired electricity at 30% efficiency for generation and transmission versus a natural gas furnace.

  12. user-626934 | | #12

    Response to James Morgan
    James -

    Did you do a heating/cooling load calculation? Did you show the manufacturer's capacity data? On what grounds did the inspector fail you?

  13. user-723121 | | #13

    State of the art
    Modern forced air furnaces really are a great piece of equipment, my first choice for HVAC. 95% efficient, ECM fan, modulating flame, super quiet. Couple this with a 16 SEER or higher AC unit and you can sit back and enjoy very low energy bills. Add a 4" filter, preferably the 20" x 25" and you will have a great system. Virtually every new home in the Twin Cities uses a forced air furnace and separate AC unit, I do not see this changing anytime soon.

  14. user-626934 | | #14

    Response to John Brooks
    John -

    Since when do most American homes have room by room zoned control of heating and cooling? At best they might have floor by floor "control" with the usual hot spots and cold spots that come with poor construction and poor installation.

    I have a single mini-split (12k Mitsubishi) in my 1,450ft2, 3-bedroom super-insulated house in a mixed-humid climate (Charlottesville, VA). Peak heating and cooling load are both about 8,000Btu/hr. The lower level is an open plan, upstairs there are three bedrooms, one is a loft over a double-height space, the other two bedrooms are typical (that is, with doors). Bedroom doors are open during the day, closed at night. Temperature stratification between floors is typically 1-2F. Room to room variation between the loft bedroom and the closed bedrooms is typically 1-2F, even at design conditions (18F winter, 91F summer).

    If you want a tighter temperature tolerance you can pay extra for a 2-head system....wall mount or ceiling recessed mount for the 1st floor...the 2nd floor system can be ducted and share some ductwork with the ventilation system. For example, see the last few slides from Marc Rosenbaum's presentation from the 2010 Passive House conference:,_November_6_files/2010%20Conference-HVAC-Marc%20Rosenbaum.pdf

  15. homedesign | | #15

    Response to John Semmelhack
    My comment about "zoning for privacy" was about acoustical and visual privacy.
    This means rooms with doors that are often closed when the rooms are occupied.

    I think we need a method to add/remove heat from habitable rooms even when the doors are closed.
    We also need to provide fresh air and remove stale air from our habitable rooms...especially when they are inhabited.

    Thanks for the link to the Marc Rosenbaum slides.
    The system shown in the last few slides addresses my concerns...
    but it is not-exactly-prepackaged.
    I don't really see the advantage of a system like this vs. the type of systems Armando and Doug are talking about.

  16. Expert Member
    ARMANDO COBO | | #16

    State of the Art II
    I’m right there with you Doug; I also like high efficient furnaces and AC units running low. First thing is I make sure all manuals are done properly by an independent ME and that all systems are commissioned and balanced. Not long a go, an HVAC contractor in Dallas told me that in 46 years running his business, I was only the second person ever to ask him to commission and balance a residential system; I guess is something I take for granted in NM.
    I have used minisplits for small buildings, “casitas” or guest and pool houses, trying to box them in the walls so they are not so ugly, but I can imagine clients of nice homes wanting a whole bunch of those systems “popping” out in every room. Yikes!!!
    Besides, you can’t beat fresh and filtered make-up air and humidity controlled by an IAQ thermostat… it’s like worry free system to me... try that with your minisplits!

  17. gusfhb | | #17

    I am puzzled by the replies
    I am puzzled by the replies doubting efficacy of mini splits in modern houses, despite the multiple claims of their success.

    It should be noted that almost no house with hydronic heating has an air interchange system for each room. Individual room heat loss, or gain, is insignificant in any house being discussed here. By code,every bath has either a window or a fan, every house discussed here would no doubt have a fan. Is air conditioning in the bath reason to run ducts everywhere? The air does in fact get there. My house is not super insulated, and every room downstairs stays cool, dry and comfortable with one 9000btu minisplit centrally located. If it was relying on it for heat, with the higher Delta T and relatively poor insulation[for this crowd] it would not work effectively. Had I been able to design from scratch, I have no doubt it would be fine

  18. homedesign | | #18

    Response to Keith
    OK...Bathrooms are habitable...
    Perhaps I should not have said "habitable rooms"....
    I see no need to provide a supply duct to an average bathroom.

    For my Climate (Hot-Mixed-Humid) I can not imagine a bedroom without a dedicated supply and a path for return air(or otherwise it's own minisplit "head")
    If the minisplit option is used...I believe fresh air should be provided when the door is closed.

    My thinking is not so different from the "Premises for Homes" slide #25 in the Rosenbaum Presentation,_November_6_files/2010%20Conference-HVAC-Marc%20Rosenbaum.pdf

  19. gusfhb | | #19

    To repeat myself:

    To repeat myself:

    >>>>>It should be noted that almost no house with hydronic heating has an air interchange system for each room.<<<<<<<

    and, as the majority of the northeast is such, I am going to say it works.

    And despite what we may be accused of thinking, we stink no less than other parts of the country .

    So I am going to say it, people who insist on having their doors shut under design conditions [for that is what we are discussing, since intermediate conditions will never ever be a problem] are WIERDOS.

    Sorry had to be said

  20. user-723121 | | #20

    Consumer expectations
    When I started researching superinsulation in 1982 the thought occured to me people would buy into the concept if the superinsulated house looked and operated like a conventional home. History shows this idea to be reasonable with a few apparent differences like deep window and door openings. Customers actually preferred the deep window jambs as a place to sit or place a houseplant. We used conventional heating and cooling systems to keep costs in line and to keep the superinsulated homes as consistent as possible with the business as usual homes.

    The selling features were comfort, very low energy bills and quality workmanship. Comfort is hard to describe to a potential customer but is something they understand very well once they have purchased and lived in a low energy home.

    As a builder I am always looking for a better mousetrap when it comes to systems, with reliability, ease of operation and cost effectiveness being the guide. Building envelope breakthroughs were made in the late 1970's and 1980's with superinsulation, Passive House is what I would consider refined superinsulation and I strongly support PH. I am in favor of new systems for buildings if they are uncompromising to the homebuyer, I believe you are headed for confrontation by asking occupants to leave doors open to stay comfortable.

  21. user-939142 | | #21

    in that Premise for Homes Slide at the end, is it indicating to use the wall unit in some sort of ducted system?

    mits has indoor ducted units but they don't achieve the same effeciency numbers, nor are compatible with the new hyperheat models. i wonder can the wall mounted unit even push that much air

    its basically home building a normal heat/ac system on a mobile home scale. maybe if a similar heat pump existed in that market compared to the mits it would be the better option

  22. user-963733 | | #22

    Not all of us are Yankees
    Please people, do some thinking about humidity control too! Consider the Houston TX climate where summer dew point seems to average 75F. For decent indoor humidity the dew point must be in the 55-58F range. You MUST have air changes in any room with a humidity source or with leakage from outdoors. Do you think you can do that using mini-splits?

    I actually think it could be done, but don't see anybody paying attention to that problem. I think the key is thinking about heating as one system, cooling as another, and ventilation as a 3rd. There might be a dedicated dehumidifier as a 4th system. Design something that works and THEN see if you can consolidate into fewer physical systems, try not to let it suck at one need just because it is difficult -- for example humidity control.

    Best of luck -- Mark J.

  23. GBA Editor
    Martin Holladay | | #23

    Response to Mark Johnson
    In a hot humid climate like Houston, increased ventilation generally RAISES the indoor humidity level rather than reducing it -- at least during the summer. So air exchange is not really a good solution to high indoor humidity.

    The best strategy: use an air conditioner -- either a conventional forced-air system or a minisplit, either of which works just fine. If the house is very tight and has a very small cooling load (because of the use of low-SHGC windows, for example), it may be necessary to also install a small stand-alone dehumidifier. These only cost about $250, and are an easy solution to high indoor humidity.

  24. user-626934 | | #24

    Response to John Brooks

    Regarding the advantages of the mini-ducted systems compared to a modulating furnace + standard AC - here are the advantages I see:

    1) Right-sized: standard split AC units are only available down to 18,000 Btu/hr. The mini-split units have nominal capacities as low as 9,000 Btu/hr.

    2) Variable capacity: the 9,000 Btu/hr mini-split units can ramp down to about 3,000 Btu/hr. 2-stage AC units are available, but more expensive...and they tend to be available only in 12,000 Btu/hr increments starting at 24,000 Btu/hr

    3) Modulating gas furnace...requires gas or propane...not always available or desirable.

  25. kevin_in_denver | | #25

    Don't forget PTACs & PTHPs
    Packaged Terminal Heat Pumps are one-piece appliances that take the no-commissioning premise to the extreme...
    For rough-in, they are like a window, after that, they are as easy as a refrigerator.

    Although they are considered noisy and are less efficient compared to minisplits, the cost difference is compelling: $700-$1100 vs. $2600-$4000.

    They aren't much noisier than central forced air systems, and remember, in a superinsulated house, they almost never come on. That means it takes a long time to recoup the difference in first cost.

    Here's one example:

    And more info:

  26. Chad Ludeman | | #26

    One mini-split per floor
    We have been using mini-splits in our tight and well insulated new homes in Philly, PA for two years now. We learned that putting one per floor (we build 2-3 story townhomes) works very well.

    There is no need to put them in separate bedrooms and it would be complete overkill in the size of the overall system. You can undercut the doors and/or use economical transfer grills above the doors if you are very nervous about it.

    The beauty of these systems is that it instantly creates multiple zones and allows greater user involvement to lower utility bills. Most of our clients use only the ground floor unit in the winter and the upstairs unit in the summer. We have only one unit located upstairs (not at all centrally) in our house with radiant heat and it keeps the entire house cool and dehumidified.

    I agree with the comments about generally poor design and installation practices by HVAC subs in the US. Some markets or very high end homes may have better performing subs, but the rest of us are stuck with the status quo. Mini-Splits have been hugely popular in Japan and parts of Europe for years. The US is simply resisting the change, but I think we will all see the demand and specs grow for mini-splits over the next decade in the US...

    Great article BTW.

  27. BsDwTKPfmJ | | #27

    Mini Splits
    Hi, Background: I live in Romsey, Victoria State, Australia about 50 miles inland from Melbourne in the Great Dividing Range. Our latitude is about the same as San Francisco and our height above sea level is about 400m (1300 feet). We get frosts in Winter, occasionally snow and we can be very hot in summer (up to 46 C/115F) except this past summer where drowning from too much rain was a more likely result than slowly cooking at 115F! About Split Systems: We have 3. A large one in the living/dining/family/kitchen--it's open plan with 9ft ceilings with two more in two bedrooms. They are Fujitsu Reverse Cycle Inverter A/C. This means they are heaters as well as coolers and use exterior Heat Pumps to do the work. They are extremely efficient, quiet and the inverter action means that they iron out the up and down of temperature change so that you don't have great highs and lows. We have only insulation in the ceilings, with single glazing on 25 Sq.ft. windows, and our exterior walls are plasterboard (drywall) stud with a cavity and exterior single brick veneer. Not exactly air tight and we lose significant heat thru the glass and drywall. However, I would highly recommend Reverse Cycle Split Systems, even in our leaky sieves of homes and much more so in N. America (I'm a transplanted Canuck from Vancouver) where homes are much better insulated. If Mini Splits work well for us at +46 (115F) they will work equally well when its cold. I have a Heat Pump Hot Water System which works to -10C drawing heat from the air. All heat pump systems in Oz are air based but maybe in N. America ground based heat pumps are required in places. I have included links to the to types of Heat Pump systems I use. and or on Facebook . Regards, Stan

  28. Z6MEHqYgNy | | #28

    Mini split -- How about Multi splits
    Here in northern Mexico (that's old Mexico) temps vary from -3c to +43c and humidity can be as high as 95% without a drop of rain (at night in the summer). Most buildings are built with a single wall of 6 inch concrete block and have a flat concrete roof. the roof can reach 70c under the midday sun from may to Sept.
    Mini splits and Multi splits ( one outdoor unit and two indoor units) are pushing installed HVAC out of the market for houses up to 3500 sq ft.. They began selling here 10 to 15 years ago and many people have had them installed and simply turned off the HVAC.
    Now I see multi splits with 3 indoor units to one exterior -- don't know much about the compressor efficiency on these new 3 to 1 units but I see that one indoor unit is downstairs and two are upstairs or vica versa. So three units rarely run at the same time.
    Got Mini splits in my house and I sold the central HVAC unit.

  29. user-626934 | | #29

    Question for Kevin Dickson

    Do you know of a PTHP that doesn't switch off the heat pump (and revert to electric resistance heat) at around 35F and below? I suspect that in super-insulated residences in most climates, the vast majority of heating energy use is when the outside temperature is below 35F, which would make most PTHP's about as inefficient as electric resistance heat.

  30. seDUbRtWbM | | #30

    Commissioning residential hvac systems is really not that difficult although it does take an educated tech. I think I could teach an installer faster how to design, install and commission a regular split system then I could repair a malfunctioning variable refrigerant flow multi head mini-split system. Mini split technology is foreign to most technicians using ac - dc inverters powering variable speed DC compressors, etc. I have watched technicians systematically (with the help of the manufacturer rep.) replace every single part in a mini-split and not fix the problem.

    Codes and EStar can and hopefully will go a long way in getting better systems installed. Letting people put ducts outside the house or using building cavities (paned bays) for ducts is ridiculous and should have been removed long ago. Architects design buildings without regard to how these systems will be installed or what it will take to heat or cool the space. I have been to houses that have large rooms with solid glass walls on slabs with 20' vaulted ceilings and they expect you to heat and cool it with ducts, where? Many have unrealistic expectations of what can be done. Integration in the design phase of the building would also go a long way in getting better systems installed.

    Most high efficiency mini-splits max out around 12 EER, 20 SEER and 11 HSPF. They also have fairly low capacities in the 8-12k range but perform in a well insulated house. Prices tend to be even higher then Marc has suggested in NJ mostly do to a lack of experienced mechanical contractors. Prices for electricity tend to be high ~ $ .18 per kWh and NG is pretty cheap ~ $ 1.33 per therm. You would need a COP of ~ 3.8 to beat the cost of a 95% NG furnace/boiler. Carrier is supposed to be coming out with a split system air sourced heat pump using mini-split compressor technology with the highest specs I have seen. 16 EER, 20 SEER and 13 HSPF. Rumors claim a COP 5 with full capacity at 17 degrees. We'll see but it's encouraging anyway. Our electricity is pretty dirty so unless your power your HP with a solar system your probably doing more harm then good.

  31. kevin_in_denver | | #31

    Response to John Semmelhack
    We have simulated the yearly performance of a home in Denver, and the resistance heat comes on for only 4 hrs per year. The PTHP setpoint is typically 25F, not 35F. Couple that with the winter average daily temperature of 32F+, and a reasonable nighttime setback, and it works out pretty good.

    COPs are still a bit low, but you're getting heat, AC, and ventilation for under $1000. Replace it (yourself) in 10 years with a more efficient unit when they become available.

  32. homedesign | | #32

    Thermal Bridge
    A few years ago I had a GE zonline PTAC in my office.
    maybe there are better examples available from other companies.
    It looks like it would be a thermal bridge 24 hours a day.

  33. smalld | | #33

    Stan Smith, you are right on the mark! I have experienced over a number of years living in a quite a few locations in Australia and parts of Asia where they use reverse cycle systems. The comfort was amazing, the zonal application was so efficient, and they worked in varied climatic conditions identical to North America from the Gulf to the 54-55th Parallel, from the Atlantic to the Pacific.
    Please------ Let's not be so myopic as to assume our building, heating/cooling, or sustainable water and energy concepts are the end all --be all. We in North America as a whole are in fact several decades behind such countries as Australia, Israel, and many European Countries.
    Really glad that the subject of 'SPLITS' was brought up and I hope more people from other areas of the planet will join in and introduce/verify the systems that they have in many cases been using very efficiently for years.

  34. Z3bcoLVRwf | | #34

    more advantages
    Stan Smith and Rhaud MacDonald are absolutely correct.

    Take it from a just a regular consumer who's moved and traveled a lot and who's watched a lot of trends take off over the years. I (and my friends) have been checking out the mini-split systems for some years now. A minisplit is in our future when we remodel our house.

    As for our region, Louisiana and Mississippi are slow to change, but many folks were inspired to research and buy minisplits after the levees broke and storm surge devastated so many homes through the region.

    People wanted options other than another huge HVAC system that was far too expensive to run and maintain, and too susceptible to damage. Enter the mini-split!! HVAC is on the way out, no matter what the HVAC suppliers and installers would prefer. Mini-splits have only been limited by cost (coming down significantly) and the fact that information and suppliers weren't readily available in the US, particularly down here, although they're the standard in Europe, Asia, Australia, etc. The tipping point in the US has been reached.

    Minisplits are a tremendous boon for anyone living in a flood prone area! The pipes (narrow copper pipes, not huge ductwork runs) can be installed high, under ceilings, behind drywall, attics, floors--the lines are minimally invasive so they are a great alternative when remodeling or rebuilding - just pipes and you can hang the unit high on the side of your house. Saves it from not only drowning, but also from thieves looking for copper.

    Quiet, efficient (talk about a huge savings on your bill switching from HVAC to minisplit). And the air is not as "hard" on sinuses. Every place we've been in that has a minisplit installed, folks comment on the comfort of the temperature and that it doesn't feel as drying or uncomfortably frigid. The heat is the same way--comfortable. All the concern about humidity has not been a problem and we're in humidity central here in Louisiana. No ductwork, no mold, dirt, other particles being spread in the air?

    Biggest problem here has been HVAC companies and installers who don't want to have any part of a minisplit install, so folks have been doing it themselves and begging someone from an HVAC service to come handle the refrigerant.

    I'm thrilled to see this discussion, because I know it's another sign that my mini-split costs and options will improve rapidly over the next few years.

  35. user-828441 | | #35

    Has anyone looked into the cost structure for the installer as a reason they are resisting the change. I'm just thinking that because these systems are so much easier to install compared to conventional forced air then the cost must be coming from the unit itself. This greatly reduces the opportunity for the distributor / installer to make its money. Any real world experience to backup this theory?

  36. mheslep | | #36

    Replacing the unconditioned attic furnace/AC for 2nd story
    I have new, efficient hybrid gas/heat pump carrier in the basement of my two story, and am now considering what to do about the upstairs second zone, served my aging gas furnace/AC unit with ducts which is unfortunately in the unconditioned attic. I'm in the midst of air sealing the 1300 sq dt 2nd floor ceiling/attic, and was about to proceed on to tightening up the old insulated duct work system with mastic or hiring out for an aeroseal job. The aeroseal would apparently run some ~$600,and a replacement one - two ton heat pump in the attic sometime in the next 2-4 years another $1500-$3000 installed - all of which must still operate in the hot/cold attic, forever bleeding unnecessary energy.

    Now comes the ductless mini split, which seems (?) to offer an economic away out out attic based HVAC, that is one with out re-running ducts in renovated dropped ceilings. However, the second story has four bedrooms and two baths (with ten supply registers), distributed among teenagers that, in the steady state, keep doors closed. While I'll soon have the attic at R50 and sealed, the walls will have stay ~R13, old double glazed windows. So I'm concerned about even distribution of heat and cooling. Will some transfer grills do the job? Does mini split make sense? How many zones? Will 3-4 zones mean pipe running all over the back side of the house?

  37. GBA Editor
    Martin Holladay | | #37

    Response to Mark Heslep
    1. You didn't tell us your climate.

    2. Conditioning a house from a single point source -- or at least from a single point source on each floor -- works best in a small, compact home with very low levels of air leakage and very thick insulation. You're in experimental territory if you're talking about an older home with 1,300 s.f. per floor.

    Remember, though, people lived without central heating and air conditioning for hundreds of years. Your teenage children can always open the doors if they are uncomfortable.

  38. B_Carr | | #38

    Further mini-split questions
    I'm looking at the mini-split as a possible option to replace an aging HVAC system in a one-story, 1,800 sf home in hot/dry climate of Phoenix. Are there limitations to the distances between the outside unit and the indoor head unit? Can an outdoor unit be placed, say, on one side of the house with the head unit placed on the other side? Besides the long run of copper tubing needed, is there an issue with this installation? Also, can insulating the copper tubing increase the efficiency of the unit?

  39. GBA Editor
    Martin Holladay | | #39

    Response to Brad Carr
    The copper tubing that carries refrigerant from the outdoor unit to the indoor unit is routinely insulated.

    There shouldn't be any problem running the tubing from one side of a house to the other, assuming it's not an enormous mansion. If any minispli installers have further information on the maximum distance for such tubing runs, please let me know.

  40. mheslep | | #40

    Response to Martin
    Martin, thanks for the response:
    1. Mixed climate, Virginia.
    2. I'm only interested in applying the mini split to a single floor, as I said the other floor is already otherwise serviced. As we know mini splits can have multiple heads; they need not be a single point source system. My question then is how many heads are reasonable given a very tight ceiling, or, should a mini split be avoided altogether without doing a deep energy retrofit to the walls as well, as I think you suggest?

    Yes people survived for hundreds and thousands of years without permanent structures of any kind, and many other things that we enjoy and enrich our lives today, including the internet.

    Apologies for my post placement here - apparently the Q&A section more appropriate.

  41. GBA Editor
    Martin Holladay | | #41

    Second response to Mark
    My answer hasn't changed -- you're in experimental territory here. Of course you can put an indoor unit in every bedroom if you want; that's expensive, though, and the units would be oversized. You can also used ducted minisplits, which have a lower efficiency.

    My guess is that, if the downstairs is conditioned, a single upstairs unit (in a hallway) will make the bedrooms fairly comfortable -- except for west-facing bedrooms with unshaded windows, which might overheat during the summer. But I wouldn't guarantee the truth of that guess.

  42. GBA Editor
    Martin Holladay | | #42

    Response to Mark Johnson
    The fact of the matter is that many New England builders of superinsulated homes now have experience using one ductless minisplit unit per floor -- in the living room downstairs, and in the hallway upstairs -- and such systems can work very well. Success depends on a compact house design and attention to envelope airtightness.

    We all need to be willing to open our minds to the possibility that there are new ways to design, heat, and cool homes. If our minds our closed, we won't learn.

  43. user-963733 | | #43

    HVAC, remember all 4 initials please
    Rather than guessing, you could be better informed by a Manual J heat gain/loss model which illustrates heat gain room by room. That would show you the room(s) with minisplit heads will have a far negative heat gain, while other rooms have significant positive heat gain. In the absence of a ventilation system to exchange air at a good rate between the heat source and the cooling source, it is risky to predict there would not be uncomfortable temperature differences.

    I like the technology of minisplits and would like to see them succeed as much as possible, but here are some reasons why significant parts of HVAC are omitted, and new applications must be proven to work, not assumed to. Remember part of HVAC is VENTILATION and a forced air system delivers typically 3-10 ACH (air changes per hour) when AC is running and often STILL has trouble maintaining even temperatures room to room. In the absence of this ACH what mechanism will convey the cooling BTUs to each room? If an open door were the answer, then an open door would solve most room-to-room temperature problems.

    Remember humidity also, an important part of HVAC where outdoor dew points exceed a certain level (about 60F). Would you expect one stand-alone dehumidifier to evenly control humidity in all rooms? We can show from experience it does not. It is my thesis that some ACH value is required to homogenize the house air qualities, temperature AND humidity AND other qualities.

  44. user-963733 | | #44

    Narrow or not?
    Many of us are not in New England. Many do not have a modern superinsulated home. To focus on just that is turning into a very elitist discussion.

  45. GBA Editor
    Martin Holladay | | #45

    Response to Mark Johnson
    Really, I tried my best to provide advice to Mark Heslep. I did not tell him that he needed a superinsulated house, nor did I tell him that he had to move to New England.

    Your advice was also helpful. However, your statements are too absolute. I mentioned New England examples because they are examples of successful implementation of ductless minisplit systems with only one or two indoor units.

    Finally, in response to your question, "Would you expect one stand-alone dehumidifier to evenly control humidity in all rooms?" the answer is yes (assuming the dehumidifier is properly sized). Armin Rudd has shown this method to work well in Houston.

  46. c3hdQkcong | | #46

    Looking for homeowners who have used mini-splits
    I'd really like to interview a homeowner who opted for a mini-split in their home. If anyone can refer me to one who would be willing to discuss their experience for an article I'd like to write for, I'd really appreciate it.

  47. B_Carr | | #47

    Calculating heating and cooling btu
    Can Martin or somebody else direct us to a website that provides a calculator for determining heating and cooling btu/h to a reasonbly close estimate? Great stuff as always Martin.

  48. GBA Editor
    Martin Holladay | | #48

    Response to Brad Carr
    I'm not sure I understand your question. Are you interested in performing heating load and cooling load calculations for a specific house, to determine the size of required equipment? Or are you interested in estimating annual heating and cooling costs for a specific house? Or something else?

  49. B_Carr | | #49

    Martin - I was looking for a
    Martin - I was looking for a calculator that gave a close estimate of heating and cooling loads for a specific house to determine the size of required equipment, knowing of course that specific calculations are probably left to an expert with specific software

  50. GBA Editor
    Martin Holladay | | #50

    Response to Brad Carr
    There are lots of free online calculators for figuring heat load and cooling load. Here's one:

    Here's a comprehensive list of load calculation software, compiled by the Dept. of Energy:

    Note that one of the columns lets you know whether the software is free.

  51. user-969321 | | #51

    My experience with ductless mini-split heat pump (DHP)
    I live in a 130 year old 30' x 30' farm house with rock wool blown-in insulation in the rough-cut 2x5" walls, located in east central Ohio. It is not very tight.

    Last December I decided to install a Fujitsu 9-RLS DHP (rated at 26 SEER and 12 HSPF) to supplement or replace the old boiler. The 9-RLS is rated to operate from 3,000 to 22,000 BTU.

    I am admittedly a fanatic about saving energy, keeping the temperature set at at the minimum 60 degree setting and I don't try to heat the second floor. (I hope to live more comfortably in the future when I get the house super insulated.)

    The main floor has doors open during the day, but I assumed I would need to install a low wattage circulation fan. Within days I decided to decommission the boiler and I see no need for a circulation fan. The temperature ranges from 3 to 5 degrees colder in the opposite corner of the house from the HP, depending on severity of outdoor temperature.

    The coldest weather since installation were 3 consecutive days in January with nights at -3, +8 and -1 degrees F and day temp at 16, 20 and 19. The 9RLS was not quite able to keep up, letting the room in which it is installed drop to 54 degrees on the coldest nights. I now have the HP wired through a separate meter, but didn't have that installed in time for that cold spell. My total electrical consumption for each of those days was 20 to 22 Kwh per day. I am guessing about 17 per day was the HP.

    The unit and installation cost me about $2300. Lower than some have reported, probably due to this being an economically depressed area.

    John Semmelhack asked
    "Do you know of a PTHP that doesn't switch off the heat pump (and revert to electric resistance heat) at around 35F and below?"

    The Fujitsu dos not have any resistance heat built in, and I think that is true of the other modern efficient units as well.

    Brad Carr asked
    "Are there limitations to the distances between the outside unit and the indoor head unit?"

    The installation instructions say "Refrigerant suitable for a piping length of 48 ft is charged in the outdoor unit at the factory. When the piping is longer than 49 ft additional charging is necessary."

    The dealer told me that if I ever need to relocate the unit or remove it during remodeling, the unit can pump the coolant back into the compressor to reverse the installation process.

  52. GBA Editor
    Martin Holladay | | #52

    Thanks, John
    Thanks -- all very useful information.

  53. user-939142 | | #53

    each minisplit system has
    each minisplit system has different vertical and horizontal runs that it can accommodate with the copper piping (note there is also a vertical limit) check the specs. larger units obviously allow more length.

    as far as trying to use a minisplit for 4 bedrooms upstairs, there are models that connect to a single concealed vent device - basically your building a small split version of a typical HVAC system. if you can locate the indoor concealed ducted vent device centrally, you can duct it to the rooms and duct the return intake to the hallway. the rated effeciency values go down for the concealed ducted models, but that may be more to do with how they are measured. the same outdoor unit connected to a different indoor model is more effecient, so clearly it can't be that much worse.

  54. user-954346 | | #54

    recent report
    NREL has a report titled
    Field Monitoring Protocol: Mini-Split Heat Pumps
    D. Christensen, X. Fang, J. Tomerlin, and J. Winkler
    National Renewable Energy Laboratory
    E. Hancock
    Mountain Energy Partnership
    March 2011

  55. servant74 | | #55

    old days
    When I was a kid we didn't have HVAC until my dad installed it in a 1956 house. Put in 5 tons for 1400 sqft. He cut vents in the bottom of doors where we had hard floors and put vent covers on both sides of the doors, or cut about 1" off the bottom of the master bed room door where there was carpet. That worked pretty well for the return air, and it all returned air to the hallway where the air return to the HVAC inside unit was in a former hall closet. He also put all the ducts inside the insulated envelope of the house.

    This worked well in the super hot summer of 1958 (over 30 days over 100F if I remember right). ... I remember going with him down to the Lennox factory in Ft Worth to get the both the inside and external units. By todays numbers they were not efficient. We used NatGas for heat, as heat pumps were not around back then.

    The climate was hot and normally dry, but for some months of the year it was humid enough to be uncomfortable so refrigerated air versus evaporative coolers were appreciated.

    He also retrofitted a 5T ac into a black Buick the next year, and we took a driving vacation through Marfa TX and the Big Bend area where it got pretty hot. ... That worked well too! ... I won't say any of it was efficient, but it worked. ...

    Oh yes, he was a MechEngr with specialty of HVAC and vibrations, worked for Harley Davidson and later Bell Helicopter Co. ... Doing HVAC isn't rocket science, but it does take some understanding of heat and balancing flow. ...

  56. user-1005777 | | #56

    Installed a Mitsubishi Mini split
    We had a 12000 btu minisplit installed last October.(2011). Our home is a manufactured mini home 16 feet by 70 feet. The unit is installed in the living room, about mid way down the trailer. Walls are 6 inch fiberglas, and standard double pane windows from 1992. We are located just inland from Saint John, NB, Canada. The unit performed well throughout the winter. Baseboard heaters were set to come on at 16C, but never did. They came close when the outside temperature dropped to -26C. Bedrooms are at either end of the building and are a little cooler than the open plan LR and Kitchen/DR.. Just perfect for us. We had a Vanee 60H HRV installed at the same time. The system has solved many of my wife's asthma problems, and the comfort level is much better than the baseboard heat. Equalised power bill dropped from $140 to $110 a month. $31.80 of that amount is basic charge and taxes.

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