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Confused and worried about minisplit sizing

user-5996667 | Posted in GBA Pro Help on

We’re building a house in Toronto, two stories (both about 1500 sqft each), first floor R26, second floor R32, second floor ceiling R70, with HRV, de-humidifying system, window-to-wall ratio 12%, windows mostly face east and west, windows are Fibertec high-efficiency, fiberglass frame, low e-coating. Whole house will have hydronic floors for heating. We’re planning Mitsubishi mini-splits for A/C, but we’re very confused about sizing and placing of heads (and outdoor units).

The HVAC load calculations say we need sensible cooling of 27300 BTU and latent cooling of 11700 (total 39000). This seems a little high to us, though still plausible. What’s the best layout and sizing of the minisplit heads? We have three bedrooms upstairs (each with ensuite bath). Main floor has great room (kitchen plus living), separate dining room, and library. Current HVAC design has second floor with three 9000 heads, one in each bedroom; main floor with two heads, 9000 in the great room and another 9000 in the library.

We’re worried this is oversized given our semi-tight envelope. Will the 9000 heads in each bedroom be too cold? Conversely, would a single bigger head in the upstairs hallway not provide enough coolness for bedrooms with doors closed at night? There seems to be a lot of advice out there that fewer bigger heads is more efficient than more but smaller heads. But wouldn’t the latter give us more control? This question seems particularly pressing for the second floor, which is not open-concept at all, and where rooms will generally have closed doors when they’re actually inhabited.

Any expert help and advice will be greatly appreciated. This is all new for us and this website has already helped us a lot.

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Replies

  1. ohioandy | | #1

    Others will post confirmation--with numbers, if you're willing to share more room-by-room details, floorplan, and variables used like design temps--that your load calcs probably aren't real-world helpful, and that you're in for cooling overkill. But never mind efficiency; purely from an aesthetic and mechanical point of view, putting minisplit heads in every room is so very inelegant. It gives individual control, sort of, but part of the point of building a tight, well-insulated house is that all the far flung corners are already uniformly comfortable. Comfort is designed in, which prompts me to suggest that it's also much better to incorporate a mini-ducted system at the design stage, rather than attempting to overlay all those individual heads on a conventional structure. It's analogous to the arguments for public transportation in Toronto--minisplits in every room is like the 401, and a mini-ducted is the TTC.

  2. user-1097046 | | #2

    The numbers sound high to me too. Allison Bailes (Energy Vanguard) uses a "rule of thumb" of one ton (12,000 Btu/hr) of cooling per 1000 square feet as a quick gut check for oversizing in high performance homes. Your layout as described gives you 50% more than that.

    I'd recommend going over the Manual J inputs line by line, checking for errors in both surface descriptions and areas. The cooling load is going to be most sensitive to window inputs. Make sure you are entering overhangs correctly; also that you are entering the actual U factors and SHGCs for the Fibertec windows, not using the software defaults. Also pay attention to the air leakage inputs, substituting in a safe air leakage target for the default airtightness.

    If your calculations hold up, revisit the equipment design. Right now it sounds like you're planning to install 45,000 Btu/hr in nominal capacity for a 39,000 Btu/hr peak load. With some careful equipment selection, you may be able to get a little closer to the correct total load while still meeting the room-by-room requirements. Mitsubishi has a great tool called Diamond System Builder that calculates the actual system output based on local climate conditions and design parameters (lineset lengths, particular combinations of indoor and outdoor units, etc.). I'd recommend working with your installer to get the final system sizing as close as possible to the calculated design load.

  3. GBA Editor
    Martin Holladay | | #3

    Thomas,
    You've gotten good advice from Andy and Jon. For a new, well-built house, it's almost impossible for heating and cooling loads to be so high that you need a ductless minisplit head in every bedroom.

    Jon's advice is good: you need to look at the inputs used for your Manual J calculation very carefully.

    Once you have a Manual J calculation that inspires confidence, you'll probably end up choosing between one of two approaches adopted by GBA readers:

    1. Put a single ductless minisplit head in the upstairs hallway, or

    2. Find a location (often in an upstairs closet) for a ducted minisplit, and figure out a way to get ducts to all of the bedrooms, or most of the bedrooms.

    If your house is well insulated, relatively airtight, and equipped with high-performance windows that aren't oversized, the "single ductless unit in the hallway" approach can work, although you need a somewhat flexible family, willing to leave bedroom doors open during the day.

    One final point: If your house has a good envelope, you won't need the hydronic heating system. Just use the minisplits. If you take my advice, you'll save between $10,000 and $15,000 (money you can use to buy better windows).

  4. GBA Editor
    Martin Holladay | | #4

    Thomas,
    If you haven't seen these articles yet, you might want to give them a read:

    Rules of Thumb for Ductless Minisplits

    How To Buy a Ductless Minisplit

    Who Can Perform My Load Calculations?

  5. Expert Member
    Dana Dorsett | | #5

    Oversizing the heads for cooling won't make the rooms too cold, every head it's it's own separately controlled zone, but the on/off cycling will result in lower latent cooling, lower comfort, and lower operating efficiency. When running a Manual-J it's important to be AGGRESSIVE, taking advantage of every possible factor that would reduce the load, be it shading from trees/hills/building, or even drapes / curtains / shutters. HVAC contractors typically do the opposite, and regularly oversize equipment by 2x or more.

    Even in Florida (and even with an ensuite per bedroom) 9000 BTU/hr head would usually be extreme overkill for most new code minimum bedrooms , and headed toward LUDRICROUSLY oversized for bedrooms in a high-R house in Toronto (which 10F cooler lower 1% outside design temperatures and lower latent loads.)

    Even a half-ton head per bedroom is likely to have a ridiculous oversizing factor too, but they don't make 'em any smaller. At Toronto's 84F 1% outside design temp and 75F indoors a typical no-name 6000 BTU/hr head would have at least 8000 BTU/hr of maximum capacity, and a Mitsubishi FH06 head
    would have over 9000 BTU/hr of capacity. (It's able to deliver 9000 BTU/hr even at 95F outdoors, 80F indoors.) Yes, it gets warmer than that in Toronto sometimes, but the solar gain through windows is what's driving your peak cooling loads, and that doesn't change much with outdoor temp, and it's unlikely your peak cooling loads are higher than 5000 BTU/hr in any single bedroom/ensuite.

    An IRC 2015 code minimum house in Toronto might have a heating load of 45,000 BTU/hr @ Toronto's 99% outside design temp of 1F, but your house would likely be much lower than that, almost certainly less than 30K, could be in the 20KBTU/hr range. Cold climate ductless systems typically have a maximum capacity @ 0F of about 1.25x their "rated" cooling size (varies with model & manufacturer- consult the extended temperature capacity tables.) It's definitely worth looking at using the ductless (or ducted mini-split) for both heating & cooling.

    Without knowing what is exactly meant by "...first floor R26, second floor R32..." it's hard to estimate or calculate U-factors. A wall insulated with 5" / R32 of closed cell foam between 2x6 studs 16" on center is a much lossier wall than a 2x4/R14 wall with 3" of continuous R6 foam on the exterior, but both are R32 at center cavity. The difference will be very small in the cooling load numbers, but it'll make a real difference in the heating load numbers.

    A pair of ducted mini-splits (one per floor) may prove to be a better solution than a head per doored-off room.

  6. Jon_R | | #6

    If you are going to do hydronic radiant, then consider a Chiltrix heat pump and some fan coils for AC. Avoids issues with oversized ductless heads or leaving interior doors open and still not getting the desired temperature.

  7. Expert Member
    Dana Dorsett | | #7

    Jon R: That's a bigger design (and support) issue to make it work in a Toronto climate, but it can probably be done with just one compressor if the heating water temp requirements are sufficiently low.

  8. user-5996667 | | #8

    Thank you everyone for your responses. Dana, thank you for such a detailed response. We will take a closer look at our heating & cooling calculations. Martin, we will look up the articles that you recommended.

    Dana, the walls are 2x6 studs with Roxul insulation, R30 for first floor and R36 for the 2nd one. We used Excel II for sheathing/air barrier. The roof will be R70 with dense packed cellulose.

    We originally talked to our HVAC contractor about using the mini-splits for both heating and cooling but were told that in Ontario the code requires a heating source for each room, including bathrooms and walk-in closets. Also, we were told that mini-splits alone are not enough for heating during cold winter months. Since we wanted to eliminate ducts we decided on hydronic floors for heating and mini-splits for cooling.

  9. GBA Editor
    Martin Holladay | | #9

    Thomas,
    Lots of homes in climates colder than Toronto are heated exclusively with ductless minsiplits. Your heating contractor is apparently providing advice based on obsolete information. Mitsubishi and Fujistu minisplits are available that produce useful amounts of heat when outdoor temperatures drop to -17 degrees F or -20 degrees F.

    If code authorities insist on heat in every bedroom, you can install an inexpensive electric resistance heater in each bedroom. Once you house has been approved by the code authority, the heaters can be removed (or rarely used).

  10. Expert Member
    Dana Dorsett | | #10

    There is no R30 or R36 insulation that fits in a 2x6 stud bay. Roxul that fit's in a 2x6 is ~R23.

    Excel-II is at most R4, from what little information I can find on it.

    So with a 2x6/R23 + R4 stackup it would be roughly IRC code-minimum for zone 6, but slightly better than zone 5. (Toronto is on the warm edge of zone 6 or cool edge of zone 5, depending on which weather station averages you use for calculating the 25 year annual heating degree day average.)

    This isn't really much of a challenge for current cold-climate mini-splits, at Toronto's 99% outside design temperature of -17C/+1F. Any cold-climate mini-split sized to cover the load at -17C would usually have you covered down to -20C, or even -25C.

    I'm skeptical about the assertion that "...code requires a heating source for each room...". I would assume most codes simply require that the heating system be capable of maintaining a minimum room temperature of 20C at the outside design temperature condition, but doesn't specify how that is accomplished. I haven't found it in the NBC, or Ontario codes (but I haven't looked for it exhaustively.)

    Mini-ducted mini-split cassettes can cover 1-4 (sometimes more) nearby rooms without much duct work, and is one way to both avoid oversizing and to meet a "...heating source for each room..." requirement. Room by room load calculations need to be performed to figure out which would need their own head, which could share a mini-duct cassette.

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