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Community and Q&A

Sizing Daikin Minisplits

jaccen | Posted in Mechanicals on

I was wondering if the good people of GBA could provide their opinion on a couple of mini splits set for installation.  We had a local engineering firm use Wrightsoft to size our equipment.  They spec’d Fujisu, but those options are off the table for our current situation.  We spec’d out some equivalent Daikin models and decided to go with them:

We have faith in our contractors and our designers.  There just seems to be a bit of a disconnect on this.  Our Daikin supplier has come back w/ alternative models as they feel the above models are not large enough.

In short, it goes from the design combined 2.5 ton to a combined 4.0 ton.

For background on the site,  the brief details are the following:

Location: between London and Kitchener, Ontario, Canada (ie. Zone 6)
1rst floor: ~1500 sqft
2nd floor: ~1300 sqft
1rst & 2nd floor insulation: R-23 continuous exterior, R-22 batt interior
Attic: R-100
Windows: U value 0f 0.20 (they are large, though)
Anticipated leak test: ACH50 between 1.0 & 2.5 (hopefully better)

Backup heat: Panasonic heated exhaust fans for bathrooms, electric fireplaces in basement & 1rst floor, and an inline heater for ERV supply runs for emergencies (property has access to multiple, routinely fuel stocked, large generators as it is agriculturally based so the house is all electric)
The future occupants did not have AC at their past house and only envision using it about 2 to 3 weeks out of the year (the wife strongly prefers the open air of the country as opposed to being “couped up”).
Plans are attached for reference.

The cost difference is an extra $5000 compared to previous units.  So not chump change, but doesn’t doom the project either.

A few questions:
1. While the supplier’s motivation for recommending a beefier unit may be in good faith, do the models calc’d numbers seem reasonable?  My gut is telling me that the supplier is used to spec’ing houses not built to code or very leaky factories (they have told us they specialize in residential and large factories).
2. Does anyone know what the turndown ratio is for Daikin’s?
3.  I know rules of thumb are not to be consulted.  However, reading through most of the GBA articles on sizing it seems that most Pretty Good Houses in New England need the following:
8 to 10 btu/sq ft for heating
9 to 11 but/sq ft for cooling

I realize one needs the calcs (which we have), but with a similar climate that would put this 2800 sq ft project at:
22,400 btu to 28,000 btu for heating
25,200 btu to 30,800 btu for cooling

The spec’d units exceed that.  My initial gut, before talking to the supplier, was that there was the potential for short cycling–not that things were undersized.

Basically, any opinions on the numbers spec’d are appreciated.  It’s a new build, so I can’t use Dana’s fuel bill method.


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  1. PBP1 | | #1

    Quite an amazing build, double my R values in Zone 6 (2100+sq ft), which is heated with a single 28.6 kBTU/h rated heat pump and three ducted air handlers (hence reduced efficiency compared to one-to-one and non-ducted).

    4 ton combined does seem quite high. However, your plans show some "future" spaces (to be conditioned?) and the sq ft does not appear to include the unfinished basement (another future conditioned space?). Would future spaces add another 1,000+ sq ft?

    With R-23 continuous exterior, I guess plumbing on exterior walls is no problem (e.g., mud room). And, the (future?) stairs in the garage, will they eat into one bay?

    1. Expert Member
      MALCOLM TAYLOR | | #2


      Mudroom plumbing can go in wall between it and the kitchen. Waterlines through the floor of the cabinets, and if necessary an air admittance value under the kitchen sink = no plumbing in the exterior walls.

      1. PBP1 | | #3

        Thanks, I've got floating vanities on my mind ;-)

  2. Expert Member
    Akos | | #4

    This is a great resource for cold climate heat pumps:!/product_list/

    Always best to confirm with the engineering data from the manufacturer as sometimes the data is off a bit.

    Installing a 5 ton system where a 2.5 ton was speced makes no sense. There is a turndown but that is still limited. With heat pumps it even makes sense to slightly undersize and make up for the those polar vortex days with a backup strip heater.

    The equipment cost more, ducting is bigger, louder units, run into cycling issues during the shoulder seasons and humidity removal.

    Install the equipment that was designed for your place, you'll be happier and so will your wallet. People are not used to high efficiency buildings thus the silly sizing they are suggesting.

  3. ktim | | #5

    I have been trying to find a supplier north of you for our build. I get the same response everywhere, 'you need a 4 ton unit'. No calculations. No questions about construction (we are double wall). Just their '30 years of experience'. I also had one supplier spec a 12k head for a 509 sq ft apartment!

    Ignore the supplier and trust the designer that actually did the calculations.

  4. jaccen | | #6

    Thanks for all the replies.

    Thanks. We do have some plumbing in the exterior wall (kitchen sink and mudroom sink). We did try to minimize exterior plumbing, but we were also bound by the amount our plumbers would "bend" in regards to their standard practice. The continuous exterior insulation does alleviate many of the usual 2x6 wall concerns. The future space, if ever installed, would have its own ductless mini-split. The stairs would slightly eat into one bay; however, we have a minivan for the "whole fam" and a small hatchback as a commuter that will easily fit underneath it. We always try and get away with the smallest vehicles possible for a number of reasons I won't bore everybody with.

    Thanks. That was my thought on the matter, but it always is better to hear someone else rationalize it back to you.

    Agreed. It has been *extremely* difficult sourcing subs/suppliers. We spent all the time and effort to get the plans and calcs right (and, quite frankly, we needed more time as we were learning as we went). Unfortunately, all of our replies seem to be "well, we've always done it this way." Which is disappointing because it seems like much of "always done it this way" is not.........being diplomatic.........right.

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