GBA Logo horizontal Facebook LinkedIn Email Pinterest Twitter Instagram YouTube Icon Navigation Search Icon Main Search Icon Video Play Icon Plus Icon Minus Icon Picture icon Hamburger Icon Close Icon Sorted

Community and Q&A

IF Not …. Then Why Not ?

homedesign | Posted in Energy Efficiency and Durability on

How about an affordable not-so-big house?
3 Bedroom 2 bath ….1,400 sf…..
One DUCTLESS Mini-split
and a thoughtfully DESIGNED ventilation “system”….

With properly ‘tuned’ enclosure design and “tuned” windows……
Could it be made to “work” (satisfy 90% of occupants) ?
in San Diego?
in Seatle?
in Boston?
in Fargo?
in Dallas?

If Not ….. Why Not?

GBA Prime

Join the leading community of building science experts

Become a GBA Prime member and get instant access to the latest developments in green building, research, and reports from the field.


  1. GBA Editor
    Martin Holladay | | #1

    Q. "Could it be made to 'work'?"

    A. My guess is no. I vote for a minimum of two ductless minisplits. Here is my reasoning:

    1. The house will probably be either a one-story house or a two-story house.

    2. If it is a one-story 1,400 square foot house, it will be a stretched-out ranch, which is not a good layout for a single ductless minisplit.

    3. If it is a two-story house, a single ductless minisplit can be made to work for heating, but you will need two ductless minisplits (one on each floor) if the occupants want cooling.

  2. homedesign | | #2

    Let's limit the discussion to One Story
    Why does it have to be "stretched out"?

    and even if it is "Stretched out" ...
    Why would it be "not-so--possible " ?

  3. GBA Editor
    Martin Holladay | | #3

    If you have a single point-source heater, upstairs bedrooms stay warmer in winter than downstairs bedrooms.

  4. homedesign | | #4

    Let's start with a One Story .....
    and let's start with a Ranch House in San Diego....
    Why is it "not-so-possible" ?

  5. homedesign | | #5

    I will add a "what if" ....
    What if the occupants have learned to leave the doors open when privacy is not a concern....
    in order to "Charge" the room (surfaces and mass) with a "good condition"?

  6. GBA Editor
    Martin Holladay | | #6

    Yes, it would probably work for San Diego. Of all of the locations in the lower 48 states, San Diego is the one that needs almost no space heating and almost no cooling.

  7. homedesign | | #7

    We agree about San Diego
    Now what if we move North to LA? or perhaps San Francisco?
    and assume an HRV or ERV

    Why can't we just crank up the dial on the R-Values of the Exterior Surfaces?

  8. GBA Editor
    Martin Holladay | | #8

    Q. "Why can't we just crank up the dial on the R-Values of the exterior surfaces?"

    A. Ah, yes -- the Passivhaus approach. That approach is fine, up to a point -- the point at which the additional insulation is no longer cost-effective.

  9. homedesign | | #9

    One Mini-split vs. Two Mini-split .... (at $3,000 to $5,000 each)

    Should "Buy" a lot of "R-Value" ... eh?

    Especially with a double stud wall and a Vented Attic

  10. kevin_in_denver | | #10


    I have begun construction on a 1400 sq. ft. two story net zero energy house in Denver, to be heated with just one PTHP on the lower level.

    We can add another one to the upper level if needed for cooling in summer. Installed cost is under $1000:

    Interested energy nerds in Denver are welcome to visit this infill site at 2430 S Cherokee St., Denver, 80223

  11. Expert Member
    Dana Dorsett | | #11

    A 28 x 50 house where the bedrooms & utility rooms are on the ends with a common open kitchen/dining/living room in the middle comes in at 1400', and is amenable to point source heating. It's better that the bedrooms not be on corners (with 2x the exterior surface area).

    This pretty much describes the layout of my brother's 12" timbered log home in climate zone 4c, a home that he heats entirely with a wood burning stove in the central common area, even without high performance windows, and sub-code R values, and air leakage well above 3ACH 50. The laundry room/pantry on the SW corner runs a bit on the cool side in winter with the door closed, but the bedrooms are fine except during cold snaps well below the 99% outside design temp. (And they keep those doors closed 99% of the time to keep the dogs out.) It could just as easily be heated with a 1.5 ton mini-split instead of the wood-burner, and would be more comfortable during the temperature extremes if it had better windows & air tightness, with higher-performance walls & ceilings (though the thermal mass of the 12" logs gives a reasonable thermal performance for what it is- probably a dynamic-R equivalent of about R15 in that climate.)

    A high-R house with that approximate floor plan could be made to work with a single mini-split in Boston. You might need to use vacuum insulated glass windows for the doored off rooms to get there in Fargo.

  12. homedesign | | #12

    I was thinking that such a house would not need to be at Passivhaus standards...
    Perhaps a pretty good house with extra focus on the Bedrooms.
    Specifications for Bedrooms could be upgraded compared to the "common areas"...
    Better Windows and higher R-value surfaces ...especially in "Corner Bedrooms".

  13. kevin_in_denver | | #13


    Where do you buy VIG windows?

  14. user-723121 | | #14

    It is OK if bedrooms are cooler. John, describe the thoughtfully designed ventilation system.

  15. homedesign | | #15

    Hi Doug,
    For a house with a single Ductless Mini-split ....
    And a Climate like Dallas.....
    I am thinking of a ventilation design similar to what Dana described recently on another thread:
    "HRV systems that only exhaust air from the doored off rooms, with the ventilation air to those rooms being provided by jump ducts or door cuts"

  16. oberon476 | | #16


    At the moment you probably don't, at least not for what most would consider reasonably affordable.

    Although NSG (Nippon Sheet Glass) has been offering vacuum glazing panels for a few years in Japan, they are not really readily available on this side of the big pond from NSG - sort of - because Pilkington does offer the NSG Spacia VIG in North America, primarily to commercial applications (at least that I am aware of), but since they are fully owned by NSG...

    Last that I heard, Guardian's VIG product is still "under development", but I haven't really been following that closely.

    I suspect that a combination of technical issues and expense has left VIG (at least in the near future) as what appears to be a potentially great idea that isn't quite ready for prime time yet , at least in our part of the world.

  17. user-723121 | | #17


    Your ventilation system sounds fine. With the emphasis on tight homes we really need to provide continuous fresh air to the sleeping areas. I think the MN code calls for 15 cfm per bedroom plus 15, a 3 bedroom house would be 60 cfm continuous. The point here is to make sure the bedrooms are getting the 15 cfm.

  18. homedesign | | #18

    I totally understand the need for ventilation air in the Bedrooms.
    The way to provide the air is up to the designer.
    Considering a house with a Single Ductless Mini-Split ...
    I think Dana's suggested ventilation system for "cooling" makes sense during the heating season as well.

    It seems to me that a Ventilation System Design that supplies "conditioned air" to the Bedrooms would provide more comfort than a system design that supplies the HRV air directly to the bedrooms.

  19. wjrobinson | | #19

    Kevin, nice price, drawbacks noise? COP? Low temperatures? I think hyper heat mini split would be worth the added cost.

  20. Expert Member
    Dana Dorsett | | #20

    Kevin: PlyGem claims they'll be delivering residential VIG windows in 2014. I just checked my watch- it's a quarter past 2014 already. If you're serious about using them in the near future get on their hot-prospect list:

  21. KeithH | | #21

    I'm just a DIYer homeowner.

    I love my Mr Slim mini split and am a fan of the ductless approach, especially after getting an up close look at my 1970s duct work.

    But I'm really surprised to continually hear of hvac system designs that will require the homeowner to leave the bedroom doors open (people have doors for a reason!) or have to use a lot of jump ducts (aka giant sound tube around the door that just got closed to keep sound out).

    What's sustainable? Making sure that the next person isn't going to gut the house to deal with lifestyle limiting hvac. If I was house shopping, I'd consider a home with a high end mini split heat pump and no other heat (zone 5b) but not if it had one head and solved air distribution via open doors or jump ducts everywhere.

    Also, people have a physiological need to reach a low body temperature during sleep. Low is relative to the person of course but a design that makes bedrooms the warmest part of the house is likely to be another design failure. If the system results in a 70 degrees in the bedroom to make it 64 in the kitchen, most people are not going to be happy with the system. Without ducts, what does the next homeowner do? Use an electric dish heater? How 1970s. Install a 2nd mini split? There goes all those careful Manual D/J/S etc. calculations. Given the very small cost differential for a dual head system vs a single head system, it seems like even a 'modest' 1400 sf house designer should be looking at two heads.

    My question is why don't I hear more discussion of use of multi-head units to address this problem? Or limited use of ducting with concealed ducted mini split units to address two immediately adjacent spaces. Yes, those ducts are also noise transmitters but most people have accepted the level of noise transmission occurring from standard registers.

    Again, just a diy homeowner remodeler.


  22. Expert Member
    Dana Dorsett | | #22

    Keith- I have yet to hear anybody EVER advocate single-point heating a house with the heat source in a bedroom, only the converse.

    It's completely possible to manage the heat loads of the doored off rooms in new construction to be heated with a point source in an adajacent space. It only becomes a matter of cost when going to climates as cold as Fargo. On a DER in a US zone 5a climate I was involved with a couple of years ago the wall assemblies were ~R40 whole-wall, and the windows were all U0.18 Paradigm triple panes. It was a 3 story building divided into three separate apartments, with one ductless head per floor. The doored-off bedrooms are just fine even at -5F with the doors closed even WITHOUT cranking up the temp in the common living/dining/kitchen area. (But -5F might be considered a warm-spell in Fargo after this past winter. :-) ) The 99% outside design temp at that location is +5F, with a binned hourly January mean temperature of about +25F. Lows in negative double-digits are somewhat rare, but 0F isn't.

    Just 'cuz my brother's leaky sub-code house in zone 4c might run cooler than desirable in the bedrooms with the doors closed at 0F doesn't mean a newer tighter higher-R house in a zone 5B climate will have any comfort issues, if you plan for it.

    The size of a jump-duct capable of delivering adequate ventilation air cfm is smaller in cross section than typical tract-housing door cuts, and MUCH smaller than heating system return path requirements, and need not be designed/implemented as a sound conduit.

    What you lose by going to a multi-split is typically efficiency, and to some extent, low-temp capacity. If you install a smallest-available ~7000 BTU/hr head in a bedroom with a design temp load of 2000BTU/hr you've stolen defeat from the jaws of success- it would have cost less cash to lower the load to under 1000BTU/hr and thus self-heated when occupied, for even higher comfort & efficiency, and instead of modulating in a functional range most of the power used by that head will be standby mode power, since at the average loads the space will be heated passively by the adjacent space. You might as well heat the space with auxilliary radiant cove heaters for those fewer than 5% of all hours where it can't be passively heated via the adjacent space- the lower efficiency of those operational hours is more than made up by the high-efficiency of a single head ductless. A higher-R 1400' house can easily come in with a design heating load at say -2F within the output range of a pretty good 1-ton, or even a 3/4 ton. If they made cold-climate 1 ton multi-splits with heads options in the sub-3000BTU/hr range it would be one thing, but they don't.

    If you insist on a bedroom with a sweeping sunset-view would it be tough to design-out the (cooling) load- call it one of the exceptions that proves the rule. Yes, giving that up is a compromise- isn't everything?

  23. KeithH | | #23


    I wasn't actually suggesting a by-design installation in bedrooms, though a/c mini splits end up in bedrooms as retrofits. But many people talk about by design using only one unit centrally with 'natural' airflow into bedrooms and offices. After all the reading I've done about how our ducts must be sealed and our return air balanced via Manual D/J, it doesn't make sense to me that we revert to passive adjacency heating. Perhaps that works in a ranch. Add multiple floors or a split level design and I begin to wonder. Last, I wonder what soundproofing (such as with Roxul Safe N Sound, as I've done in parts of my house) would do to that adjacency heating.

Log in or create an account to post an answer.


Recent Questions and Replies

  • |
  • |
  • |
  • |