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Ducted VS. Ductless Minisplits

Jameson Hagenauer | Posted in Mechanicals on

Hello,

We are in the selection process to replace a 25-year-old 80% gas furnace (supplemented with an electric heater in the second floor) as part of a basement remodel. This forum has been immenseley helpful as I’ve researched HVAC upgrades. I’m hoping I can get some input on my particular situation.

We are in Tacoma, WA (Zone 4 per the GBA climate zone map). The house is a 1-and-half-story Craftsman bungalow built in 1910. Construction is typical for the area and period—full basement with half the foundation wall extending above ground level, 2 x 4 construction, cedar siding, plaster-and-lath walls. Previous owners have replaced most of the single-pane double hung windows with vinyl double-paned, but the front façade still features the original single-pane glass (no storms). Another older remodel lightly finished the upstairs ½ floor, adding R11 to the central roof span and knee walls and closing off one end of the space. We have no insulation in the first floor walls.

We are replacing a 3.5 ton gas furnace and are looking at multi-zone mini-splits with ducted (first floor and new basement room for ~1110 sq ft) and ductless (upstairs ~650 sq ft.) zones. Heating is our primary concern, though we do currently use a window AC to cool the main floor for a few weeks a year. We’ve eliminated gas-furnace/AC combo and ductless-only solutions.

·       Solution 1: Mitsubishi MXZ-4C36NAHZ outdoor unit (rated 36k BTUs cooling and 45k heating) with an SVZK24NA air handler (25K cooling / 25.7k heating) and two MFZ-KJ09NA (9k cooling / 11k heating) floor units for the second floor. The bid also includes replacing all duct work.

·       Solution 2: Daikin RXTQ60TAVJU outdoor unit (57K cooling / 57k heating) FXTQ36TAVJUA air handler (36k cooling, 40k heating) and one FXAQ12PVJU wall unit (12k cooling 13/5k heating) for the upstairs. Bid did not recommend replacing all ducts.

 

I was a bit surprised at the sizing of the Mitsubishi outdoor unit and air handler since they are replacing a 3.5 ton furnace and condition an additional 750 sq ft of space. The bids for ductless and furnace solutions ran from 4.5 to 5 tons. My attempts at load calcs came in at 55.4 k heating (via loadcalc.net) and  42k-54k heating via highseer.com, though I’m the first to admit I could be way off.

 

My questions, for those of you who’ve made it this far: does the sizing of the Mitsubishi solution seem reasonable? Any experience with either Mitsubishi or Daikin ducted/ductless combinations you can share?

Thanks

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Replies

  1. GBA Editor
    Brian Pontolilo | | #1

    Hi Jameson.

    Why not have load calcs and system design done by a professional and make sure you get this right? Though you didn't mention plans for future upgrades to the building envelope, if you do plan on insulating the first floor, for example, that should be considered. This may be helpful: A Beginner’s Guide to HVAC Design

    1. Brian Wiley | | #2

      I’d second what Brian said, Jameson. There’s a lot involved in the proper sizing of equipment and without a Manual J, S, and D it’s all guesswork.

      I’m running the same HVAC gauntlet as you are, and found that the above calculations were about $1000+ total for my 1,200 sq foot single-story. Not nominal, but relative to the cost of all the equipment it seems like a small price to pay to know that everything will function optimally.

      1. Wooba Goobaa | | #4

        Brian, who / what service did you use for your calcs?

        1. Brian Wiley | | #5

          Sorry for the delay in reply; hopefully it’s not too late to do some good.

          Paul at Tradewind Designs is in the process of redesigning our HVAC system. We’re not finished yet, but so far he’s been great to work with. He can be found at manualjdesign.com

    2. Jameson Hagenauer | | #3

      Brian,

      Thanks for the link. I'm actually trying to find someone to perform independent load calcs now, though that's proving harder than I would have thought. The bids I have are from reputable local installers, so I was surprised at the disparity in system sizing, thus my question.

  2. Jameson Hagenauer | | #6

    Following up on my own post: I was finally able to find someone to perform independent load calculations, and the Mitsubishi solution fits those calcs, and that 5-ton Daikin solution was way oversized.

  3. Walter Ahlgrim | | #7

    Consider having bids for 2 separate systems the one head one compressor system are able to operate at much lower speeds and you can shut down one system under light loads. You also get a redundancy bonus in that when something breaks the other system will still be working often for very little more cost.

    Walta

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