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Mini-Split Sizing and Layout Questions

Jason Dennis | Posted in General Questions on

Upcoming new construction in southeastern NC (CZ3).

52′ x 40′ rectangle. Insulated, elevated slab. Simple gable trussed roof. Wrap around porch on two sides.

2×6 @ 24″ OC framing with dense pack cellulose in the walls. Vented attic with blown cellulose to at least R-38. All windows and double outswing patio doors have a U-factor of 0.30. Air sealing goal is between 1 and 1.5 ACH50. Front and side door are estimated at ~R-5.

I would like to use mini-splits for the entire house. I would prefer all ductless, or, if necessary, a combination of ducted and ductless.

I’ve done load calculations with CoolCalc, BEopt, and by hand and all three results are extremely close. The total house block loads are…

14000 Btu/h Cooling (2 occupants and 2400 Btu for appliances)

18000 Btu/h Heating

I used CoolCalc for room by room and then did it by hand also. Again, results very close. The attached floorplan shows the room by room loads, which add up to 14000 and 18000 heating and cooling respectively. Do the total block loads and the room by room numbers sound reasonable for the given details?

Considering the layout, how could mini-splits be sized and placed most effectively?

The kitchen, dining, and living room is one large open room (40′ x 17.5′). The total heating load for the entire room is just over 6kBtu/h. Is it possible to use a single head unit for the entire room, or would two heads be needed due to the length of the room? The hallway opens directly into the large room. The dotted lines are some wood beam framing around the opening.

Can a single unit be used for the hallway and guest bedrooms? A single head unit for the master suite (bed, bath, and closet)? What to do about laundry room?

Thank you guys for the help and advice so far.

Hopefully Dana will weigh in… 🙂

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Replies

  1. Trevor Lambert | | #1

    Looks like too many rooms with too small of a load for ductless to be an efficient option. You could do a 6k ductless in the great room, but if you also do a ducted system for the rest, I'm not sure there's much point.

  2. Jason Dennis | | #2

    Trevor,

    Thank you for the reply. Do you think a single ductless head would handle heating and cooling the entire kitchen, living, and dining, even though it's 40' long? Or is that size not much of an issue since it's open?

    Also, would a single ductless handle the master bedroom, bath and closet since the doors would essentially always be open?

  3. Trevor Lambert | | #3

    Yes, single head should handle it no problem. A single handles the entire first floor of my house, which is almost 60' in the longest dimension and includes two doored rooms, 1350sf total.

    "would a single ductless handle the master bedroom, bath and closet"
    Maybe, depending your tolerance for temperature variation. But then what are you going to do for the rest of the rooms? A single ducted unit should be able to handle all those rooms, including the master suite.

  4. Expert Member
    Akos | | #4

    Your loads are low enough that unless there is big difference in cooling/heating load during the day (ie large west facing windows) necessitating extra zoning, you can use a single ducted mini split.

    Looks like you can mount one in the ceiling of either in the hallway in the bedrooms or the pantry and have pretty simple runs to all your rooms.

    I would run at least one duct to the windows in the living space (or a long throw diffuser on the wall across from it), getting a little bit of extra heat and cooling there makes a big difference in comfort.

    If you are careful with the layout and up size the ducts a bit, you can probably go with a low static unit, but a mid static might be a safer bet.

    Since the blower on the mini splits operates most of the time, you can also use it to distribute fresh air for your ventilation system. Not as good as dedicated ducting, but easier/cheaper. I would still run individual stale air pickups for your ERV/HRV to the kitchen/bath/laundry rooms.

  5. Jason Dennis | | #5

    Trevor and Akos,

    Thank you both for the input. My initial thought (see attached pic) was that I could use...

    - a single ductless unit for the kitchen, dining, and living

    - a slim ducted unit mounted in the hallway ceiling near the guest bedrooms for the hallway, office, bath, and both guest rooms (ducts could be super short this way)

    - a single ductless unit for the master suite (bed, bath, and closet)

    But this would leave me figuring out how to heat and cool the laundry room. A single ductless unit there would be major overkill I think.

    Is there a way to use a ducted unit for the master suite AND the laundry room? (similar to what I'm proposing for the guest bedrooms and office?

    Akos, were you suggesting that a single ducted mini could be used for the entire house? How long can the duct runs be with a compact ducted unit that I could mount in a drop soffit?

    1. Expert Member
      Akos | | #6

      The smallest 3 zone multi is 30kbtu, already way too big. You can go with individual outdoor units for each head, but even these will be oversized, you'll end up either cycling them or running them on min modulation most of the time.

      Since you have a single story strucutre, I don't think there is too much benefit for two zones but that is doable.

      You can definately do a single unit for the whole house. You'll have to run a trunk to both sides of the house already to supply the bedrooms, to do the whole house with a single unit all you would have to do is upsize the ones towards two bedrooms.

      Laundry is something that can see a fair bit of humidity and it is in the corner of the house, I would definitely run a supply duct there. You can T it off the supply to the kitchen.

      There are no length limits when it comes to ducting, there are only pressure limits. You can always extend the length of the duct as long as it is sized correctly.

      If you keep the layout simple and minimize bends and upsize the air filter, you'll have no problem supplying the entire house even with a low static ducted unit.

      I might add in a "dumb zone" for the master bedrooms. What this is that you oversupply that area (say 120% of the expected BTU) but install a zone damper+thermostat on it. You than adjust the zone damper that on closed setting it delivers say 75% of the BTU to the rooms. Ducted units would have no issue with this small change in press/flow and no additional controls are needed. You can now use that thermostat to give you a bit of extra temperature control in the bedroom.

      1. Jason Dennis | | #7

        Akos,

        It looks like Mitsubishi has a 25kbtu 3-zone outdoor unit and a 28.6kbtu 3-zone, but I'm not overly concerned about zoning. Same temp for the entire house is totally fine. I'm just searching for the most effective and efficient way to make that happen. If I end up with 2 or 3 zones, then so be it.

        I guess I was under the impression that I needed to keep the ducts as short as possible so I was thinking that a single ducted mini split would not work for the entire house. I know that load wise it would be fine. I'm planning on 9' ceilings throughout, so I can pretty easily create a a soffit for the ducted unit with plenum and duct runs.

        If you were going to do it with a single indoor ducted unit for the entire house, where would you be looking to place it?

        What about using two ducted units, one for each "half" of the house? One for the kitchen, laundry, master bath, master bed, and master closet, AND another for the guest rooms, bath, hallway, office and living room...any issue with the large open room (kitchen, dining, living) actually being supplied by two different units? If this was possible, that would be two 10.9kbtu ducted units and a 22kbtu outdoor unit. Would be slightly oversized, but not much. Thoughts?

        1. Expert Member
          Akos | | #8

          Extra units cost more money, this only makes sense if you need zoning like with a multi story house. In your case, I would go for a single unit.

          Where you are showing the ducted unit in your sketch is a good spot. I would not drop the entire entrance hallway ceiling down through, just where the unit is and where the trunk needs to cross the hallway.

          If that is a sliding door between your living room and the hallway, besides a return in the hallway, you should also run one to the living space. Looks to be a a straight shot in the ceiling of the bathroom.

          Length of ducts is not too much of an issue, bends/fittings/takeoff/boots is where you have to watch for pressure drop. You can go with a mid static unit, it has more than enough blower power to handle your layout.

          Also keep in mind that if the ducts are inside your conditioned space, they don't need to be insulated and except for the return not well sealed. This takes up much less space and quicker to do.

          Always install a large filter grill (2" or 4" filter) on your returns. Along with well sealed returns, means spotless blower and coil for many years.

          P.S. You generally want to avoid multi splits as they have much less modulation range and the indoor units themselves don't modulate. Since the cost of a multi is 2x the cost of a one-to-one, you are generally better off with individual outdoor units. Much better efficiency and significantly more modulation range.

          1. Jason Dennis | | #9

            Akos,

            Thank you for the detailed response. There is not a door between the hallway and living/dining/kitchen area. The dotted lines between the hallway and that area are some wood beams that frame the entrance into the main room (aesthetic only). It's completely open, so probably no need for a second return in living room?

            My intention, regardless of number of units, is to install the unit and any necessary ducts in a soffit that would be inside the conditioned envelope.

            I very much like the idea of a single indoor unit and a single outdoor unit. Some folks that I have spoken with lead me to believe that the ducted mini split units like the ones from Mitsubishi, would not easily allow me to run ducts far enough to reach all rooms from a single unit near the center. So you don't see an issue running ducts from the ducted unit location (shown in the drawing above in red) all the way to master bath and laundry room?

            I understand your point about the benefits of one-to-one. That being said, if I were going to use two indoor units as I suggested above (one for each half of the house), it would be better to use two outdoor units as well and maintain the one-to-one?

            The biggest issue with a single unit is probably going to be a drop soffit ACROSS the main hallway which I would like to avoid if possible, but if it turns out to be the best option, then so be it.

          2. Trevor Lambert | | #10

            The only real downside to a second unit is cost. It's up to you what it's worth to not have the dropped soffit. A minor advantage is you'd have some quasi redundancy. If one or the other unit fails, the remaining functioning unit will make the house much more pleasant to reside in while waiting for the failed unit to get fixed.

          3. Expert Member
            Akos | | #11

            Your master bedroom side has a heat load of around 7000btu, which translates to around 270CFM of airflow. You can flow that through two 3.25x10 ducts. You'll barely notice these running across the ceiling in the hallway.

            All manufacturers make low static, mid static and high static units. Mitsu PEAD mid static heads are good 0.6 in wg, their high static SUZ units are good for 0.8 in wg. For low static I would look at other manufactures as the Mitsu units have very weak blowers.

            Sometimes it is simpler to go with a high static unit as it looks very close to a standard air handler and you won't get charge the mini-split premium.

            Something like this would be a good fit in your case:

            https://www.mitsubishitechinfo.ca/sites/default/files/SB_SVZ-KP18NA%20%26%20SUZ%E2%80%90KA18NAHZ-TH_202001.pdf

            Along with the ERV/HRV, you could install the unit in your laundry room and run ducting from there to the whole house. Slightly bigger bulkhead needed in the hallway to feed the other side of the house, but not much bigger than above.

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