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Community and Q&A

Minisplit System for a Passive House

eust2023 | Posted in General Questions on

I am on the building committee working on a Habitat For Humanity project in southeastern Mass. The project is a three bedroom 1400 sf passive house construction. 900 sf on the first floor mostly open space with one bedroom and a bathroom and 500 sf upstairs with two bedrooms and a bathroom. I ran my own manual J with some online software that come up with a design load of 11,348 BTU/HR.

The first contractor mentioned that he would do a manual J load calculation and quoted a 24K Fujitsu outdoor unit with a 12K wall mount unit downstairs and 2 7K units for the upstairs bedrooms.  When I asked why the second floor bedrooms each had a unit but not the first floor. He acknowledged that he had missed the first floor bedroom and he would redesign using a 36K unit with an additional 7K downstairs. When I asked about the manual J he said he couldn’t find the paperwork. I asked for a redesign using one head on each floor. Final design is an 18K outside unit with a 12K downstairs in the open area and a 7K upstairs in the hall.

This made sense until I read the specs, the 18K unit has a max heat output 22K at 5 degrees and a minimum of 6,200 at 47 degrees. This seams way over designed assuming my manual J is correct. My thought is to recommend two single zone wall mounts a 9K downstairs and a 9K upstairs as this is the smallest size, I can find a spec on. Hopefully I will get a second quote next week. Any thoughts on this are greatly appreciated.

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  1. Expert Member
    Akos | | #1

    The last thing you want in a low load home is an oversized multi split with a bunch of wall mounts. This will not run well, consume a lot of power and have comfort issues.

    Such low load you can handle the whole house with a single slim ducted unit. This can be mounted into the ceiling of a closet on the main floor or if you are doing floor truss it can be inset with a bit of careful truss design.

    All manufacturers offer different versions of these with various pressure ratings. You can generally use a mid static unit (ie Mitsubishi PEAD or Fujitsu 12LUAS1R) but these don't look like normal air handlers so your HVAC tech might charge magical price for installing it. A simpler option is to go for a multi position air handler such as the Mitsubishi SVZ or Carrier 40MUAA which looks like a standard furnace.

    Either of these is probably a good starting point:!/product/64682/7/25000///0

    Since these are modulating units, they run the blower 24/7 at some speed so you can share the ducting and tie the fresh air supply from the ERV into the return and use it to distribute the fresh air through the house.

    I've attached a picture of a slim ducted unit mounted in a drop ceiling feeding a place with 2 bedrooms + den. The air handler is the box in the far right corner.

  2. walta100 | | #2

    I agree that ductless system is a no go for Habitat house as the rules require 3 or more bedrooms and limit the total square feet of floor requiring lots of small rooms.

    Last I looked the rules did not limit ceiling height. You could use deep open trusses to support the second floor and keep the HVAC on the gap between the first-floor ceiling and the upstairs floor. The down side is the stair case will be longer so you lose about 6 sf of floor space.

    Akos do you have a photo of that room after it was finished? How serviceable is the unit?


    1. Expert Member
      Akos | | #4

      Not the best picture but you can see the outline of the access panel in the corner. The unit has about a 3'x3' access panel underneath it which is large enough to reach all the electronics, drains, AC connections and be able to remove blowers if ever needed. The whole unit can also be taken out but disconnecting the ducting might be a challenge.

  3. paul_wiedefeld | | #3

    Agreed with the above- installers go to great lengths to avoid installing the simple, best system: a tiny ducted system. More efficient, more comfortable, easier to replace and service, easier to conceal, better filtering, etc.

    1. eust2023 | | #5

      Thanks for all the replies. I originally had the same thought of using one ducted mini-split. My concern is that it will be difficult to balance the system with the two upstairs bedrooms facing South and the downstair bedroom facing North. Also, the first quote I got was for a ducted system and it was way way over budget. My thought is that the downstair unit will heat the entire house with slight temperature variations in the bedrooms. The upstairs unit is only required to provide cooling in the summer. According to the energy rater the cooling load is very small, less then 10% of the heating load. No matter what size unit I install it will be oversized for cooling.

      1. maine_tyler | | #6

        I'm no expert on heat pumps, but it seems like there is mostly a consensus that 1:1 splits will give you the most versatility and leeway in your type of situation (if not going ducted). For example you could turn off the downstairs unit in summer and just use upstairs for cooling, or visa-versa in winter. And you will have better turn-down.

        I find it hard to believe one ductless unit downstairs can heat upstairs, but I don't live in a super insulted house.

        "max heat output 22K at 5 degrees"

        What's your design temp, is it 5 F?

        Mitsubishi makes 6k units.

        1. eust2023 | | #10

          The design temp is 12 degrees. The contractors that carry Mitsubishi never got back to me.

      2. Expert Member
        Akos | | #7

        The place with the ducted unit in the picture is two stories. There is a bit of temperature difference between the two floors but it is usually within 1F to 2F.

        Instead of trying to get fancy HVAC to deal with solar gain, the better solution is to get windows with low SHGC for anything south facing, even better install some exterior shading.

  4. user-1112693606 | | #8

    If you are going for actual PH certification, 11,000 Btu/h looks pretty high for that size building in your location. The manual j protocols don’t accurately model extremely low loads so that overestimation wouldn’t be surprising.

    One small ductless head downstairs and one upstairs should work fine in that case. We’ve used that scenario successfully in a number of projects. The stack effect means you need two units to maintain comfort— the heat load is mostly on the downstairs unit and in cooling you use the upstairs. And you also get some redundancy with the two units.

    Two factors make it possible to avoid ducting and heat a super insulated home with what is normally a small point-source heating/cooling appliance. First heat losses and gains in any given room are small, so no part of the building needs a lot of Btus. And then a balanced mechanical ventilation system is enough to even out temperature differences between spaces.

    If this is a PH certified project I’d plan on a couple of the smallest cold climate models. In Mitsubishi’s product line two fs06 one-to-one units will likely do—you’ll know for sure when you get the wufi model.

    If it’s a pretty good house and your heat load is in line with your estimate, then I’d probably do an fs12 downstairs and either an fs09 or an fs12 upstsirs. Not a lot of difference between those when you look at the specs. Because the turndown is important with low load houses, I think you definitely want two one-to-one units rather than a multihead.

    Hope that’s helpful. Good luck with your project.

    1. user-723121 | | #9


      Can you expound on this a bit?

      "Two factors make it possible to avoid ducting and heat a super insulated home with what is normally a small point-source heating/cooling appliance. First heat losses and gains in any given room are small, so no part of the building needs a lot of Btus. And then a balanced mechanical ventilation system is enough to even out temperature differences between spaces."

      I agree with the heat loss being low in a superinsulated home, define how the mechanical ventilation works and evens out the temperatures.

      Thank you,

      1. user-1112693606 | | #15

        With exhaust ducts from kitchen and bath and fresh air to bedrooms and living space, even at low cfms for these registers, there is enough air movement to distribute heating/cooling throughout the space —even if the floor plan is not open.

        But this doesn’t work well between floor levels because of the stack effect. Hence the need for separate units upstairs and down.

        1. mjezzi | | #18

          Does this work even if the bedroom doors are closed at night?

    2. eust2023 | | #11

      Norm thanks for your response. The house is not PH certified but built to PH specifications. I am waiting for the energy rater to provide a load calculation which should be more accurate than mine. I realized that the software doesn't allow me to put in a separate outside temp for the basement walls so my load from the basement is way too high. we are going to use aero-barrier to air seal so I know our infiltration will be to PH specs. Our attic insulation is R-60 and the walls are R-49.
      I will have to try to chase down the Mitsubishi dealers again.

    3. Expert Member
      Akos | | #12

      The issue with the design Norm proposes is that you still will need ducting for the fresh air supply. If you look at the heat loss in a low load place, that ducting is about the right sized (maybe a bit too small) for also supplying heat and cooling.

      A slim ducted unit is about the same BOM cost as a wall mount, so you are adding the cost of a 2nd wall mount. Wall mounts are also not zero maintenance, the filters on them don't do much filtering and eventual blower and coil cleaning is pretty difficult, messy and expensive.

      Doesn't sound like you are in a place with lot of cooling but the single wall mount in the hallway can work for cooling as long as the doors are left open. If you close doors, the bedrooms will start to overheat.

      If you are already installing ducting, why spend more on equipment to get a setup that doesn't work as well? With a ducted unit you can also install a decent filter for better IAQ.

      1. eust2023 | | #13

        Akos Thanks for the information. We do have trusses for the first floor so running the ductwork should not be a problem. The difficult part will be finding a contractor.

        1. Expert Member
          Akos | | #17

          What I find has worked well in terms of cost is to have the HVAC contractor only price and install of the ducted unit and have the ductwork done by others. Not all will be up for this as warranty issues might be complicated.

          This works well if you have an HVAC engineer design your system and ducting and somebody competent can do the commissioning of the system. Your average HVAC techs are not the best at doing commissioning.

          Since you have the space with trusses, the simplest is to design a home run setup where each room has its own dedicated supply line. These can connect to a duct board or plenum on the air handler. The runs can be flex pipe as long as they are oversized to account for the extra restriction a sub standard install will have.

  5. user-1112693606 | | #14

    Sorry, this is a bit complicated. So I think there are two scenarios here for handling heating/cooling and ventilation.

    You can do ductless heat pumps and a completely separate ducted ventilation system—either erv or hrv. In that case air filtration happens in the hrv or erv. This is what I’ve typically done.

    Or you can combine ducted heating/cooling and ventilation can share those same ducts, which is what Akos is describing.

    Both will work. There are some advantages and disadvantages both ways. If I had to guess Akos’ system probably has the edge in install cost. But there’s something to be said for keeping complicated mechanical systems separate. And I think you can get greater efficiency with the separate systems.

    Hope that clarifies things.

    1. Expert Member
      Akos | | #16

      Can't argue with the efficiency of a wall mount. A slim ducted unit gets close but they are still the highest SEER and HSPF.

      Having owned it for a while, I can tell you if my wall mounts fail, they will be replaced with a ducted unit.

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