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Minisplit in Hallway or Ducted Cassette

michaelp_391 | Posted in General Questions on
New construction, climate zone 5 (NY), 1500 square feet, two stories, 2×6 walls with dense pack cellulose, 1 1/2” exterior foam, R49 blown on the attic floor, double pane Marvin windows, full basement. Drawing attached.
 
I used coolcalc to do a room by room manual J and got a heating load of about 24K BTU and cooling load of around 16K. For the upstairs, the total heating load is about 9K BTU, with no room individually above about 2K.
 
I’m debating whether to go with a one mini split head in the hallway approach upstairs, or a cassette that is ducted into the bedrooms. When I add the cassette size, clearances needed, supply/return ducting, and access, I don’t think it can fit in the hallway laundry closet, which is about 3’ deep, or any other closet. So I would need to get it into the attic, which is unconditioned, and then deal with HVAC contractors who may be less familiar with a ducted approach, not overly concerned about insulating/air sealing the stuff in the attic, etc. All told it will add significant cost, likely, and possibly introduce confusion and conflict depending on the exact contractor. So it’s really tempting to try to get away with a single head approach. Any thoughts here are appreciated.
 

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Replies

  1. Jon R | | #1

    Important factors include if you can keep bedroom doors open and how much temperature variation you can tolerate.

    Not inexpensive, but hydronic systems are easier to route internally and less of an issue when run through the attic (no leakage, less insulation for equivalent loss). And they allow zoning with no temperature differential (ie, superior comfort).

  2. Trevor Lambert | | #2

    I would increase the ceiling height a few inches to accommodate the ducting. If you don't decide to go that route, you'll want electric supplemental heat in the upstairs rooms. In the cooling season, you'll end up with rooms slightly warmer than hallway. If you're ok with that limitation, the non ducted option would probably be acceptable to you. I wouldn't even consider putting any part of the hvac in the attic.

  3. Expert Member
    Akos | | #3

    An inbetween option is going with one of these:
    https://ashp.neep.org/#!/product/29606
    The unit has connections on the side for branch ducts (square cutouts) that can divert 1/2 the rated flow of the unit. It is not meant for feeding individual rooms, but if you size your ducts and transfer grill/door undercut right it should still work.

    You could mount it above the stairs where the dropped ceiling won't feel like it is taking away height and run a couple of bulkheads around the perimeter for the ducts.

  4. michaelp_391 | | #4

    I do feel OK with slight temperature variations between the hallway and bedrooms. My major concern with a non-ducted approach is closing doors at night and being really freezing in winter. Or being hot/humid in summer. But within 5 - 10 degrees or so seems fine.

  5. Jon R | | #5

    You can achieve < ~5F with an open door. I doubt you will get < 10F with closed doors without moving some air (say 100 CFM) or using a heater.

  6. michaelp_391 | | #6

    Thanks @Akos that's an interesting option that might balance the low loads with the need to circulate the air to several rooms.

  7. Expert Member
    Peter Engle | | #7

    You can also build a tunnel in the attic that is part of the conditioned space, with insulation all around. If you're using trusses, you can specify HVAC space within the truss for this purpose. With your room layout, a cassette sitting right above the laundry equipment could have simple straight-line duct run to each of the bedrooms and even the baths if you wanted. Undercut the doors for a central return. You'd have to move the attic access to the hallway, but you're going to want pulldown stairs to service the unit anyhow. There are some well-sealed pulldown stairs available and you can add an insulated hatch above to improve performance. Trim it properly on the inside and it can look pretty good.
    This approach is still at least a few thousand $$ more than a single head in the hallway. You have to balance that against comfort. Think about how long and how often you will close the bedroom doors. If only for 8 hours at night, you won't see much temperature change. If they stay closed all the time, it'll be bad. Open doors during heating season will probably be OK since warm air rises up into the bedrooms. Cooling will be more variable. Cool air will want to sink down the stairwell. Ceiling fans can help redistribute the air some.

    1. Jon R | | #11

      > for 8 hours at night, you won't see much temperature change.

      I took a brief look at some numbers and they suggested otherwise. I'd be interested in more details, especially at design temp.

  8. michaelp_391 | | #8

    Thanks Peter, yeah, it's looking like a plenum truss or other tunnel in the attic could be one of the go-to options. This would have loose fill insulation on top of it. The attic option somewhat complicates air sealing, insulating, and service access. But clearly there is no free lunch here - ducts move air to places you want it, but take space and have to go somewhere.

  9. michaelp_391 | | #9

    Perhaps a stupid final followup question - if the overall air handler/cassette thing was on the first floor, couldn't this be run between 2nd floor floor joists to floor registers? Is this approach common?

    1. PBP1 | | #12

      That seems doable (air handler on first floor), it was what I would have suggested. If you read about unconditioned attics, it's not good for a ducted air handler (I have 3 Mitsubishi SEZ 15K, 12K and 9K with a single 30K heat pump), you will need to insulate it, it will loose heat winter and gain heat summer. Also, I'm in western MT, with 2,100 sq ft and a heat load of around 27K (largest room facing North), thus your heat load seems high. My second floor is isolated with a separate entrance while you may have a stairway that will act as a stack. I remember working on stairway stacks for NREL, natural convection should be one way to heat the upper floor. As to doorways, consider louvered vents above each door, which may be controllable (open in winter, close in summer) - that's pretty old school - along with adequate space (or dampers) at the bottom of the door (again, another way to control air movement). Additionally, in-wall fans are pretty common/inexpensive (and quiet). If you have a supply to one room, you could install an in-wall fan (temp controlled even) for moving air in one direction or another. If you go with in-wall fan(s), be sure to have them wired in by the electrician before drywall goes up. I have a wire in one wall, terminated, in the case I would have needed to move air into a small workroom, separated by a pocket door. But I have not had to do that. My 9K SEZ is for the master, master bath, utility room and small bathroom, it does fine and unit is in the master closet ceiling with ducts (small runs) running between joists. You may also want to design/build in the filter/filters to a single strategically placed return, which will be a sink that will impact air flow from supplies (might also impact air flow from first floor via leaks or stairway).

      Taking a closer look, it seems like you have plumbing on exterior walls, I'd pay close attention to that - as heat in those bathrooms may be a necessity. My plumber would not install any plumbing in an exterior wall - except for outdoor spigots (set up with hot and cold, a decision I'll never regret). I consolidated all plumbing for on-demand hot water, home run manifold, bathrooms and laundry to within a 12' x 12' area. The second floor seems like plumbing is really distributed over a large area.

      You may look for a location that can gravity drain condensate. The SEZ condensate pumps annoy me and I'm looking to revamp. They have a buzz, but only during cooling. The SEZ are quite quiet in heating.

  10. Expert Member
    Peter Engle | | #10

    Yes, that would work. Of course you still need space in the floor joists to run the ducts. In most houses, there's a lot more utilities running through the floor/ceiling assembly than through the second floor ceiling.

  11. Expert Member
    Akos | | #13

    The branch ducting on the LG unit is meant for distributing air within the same room. I couldn't find any specs on how to size it and what is the pressure behind it but I doubt it is much. This will make any real sizing a guess. Also without a return on the ceiling of the 2nd floor, AC performance is not the best.

    Combined that with unknown flow from these branch ducts, I doubt it will work all that well. It will definitely be better than no cooling on the 2nd floor but not comfortable.

    Having the unit on the 2nd floor means that it can handle most of the cooling load directly and the branch ducts are mostly used for a bit of air transfer/cooling from the bedrooms.

  12. Keith Gustafson | | #14

    I had a hallway nounting min split for several years in my last house and it worked well[doors open] but as ducted mini splits have come down in price so much, today I would almost certainly use a ducted one.

    That unit was mounted on a large beam that crossed the hall, pointed directly at the master br door at the far end of the hall, and closer to the two other br doors, on either side of the hall. If I were choosing a location for a ducted mini split I would have chosen the master end of the hallway[or inside the master br door] and had short runs of duct to either side of the hall

    I don't think ducted minisplits are primarily intended for long lengths of duct

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