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Community and Q&A

Alternative options to 6k ductless for second floor of 1.5-story house

Chris_in_NC | Posted in Mechanicals on

We have a 1.5 story house with a 440 sq ft second floor bed/bath.
The 1400 sqft first floor has cathedral ceilings on the south side, with the staircase along the cathedral area (heat rises right up to the second floor).

There is always a temperature delta between floors and between the north/south ends of the house, so a separate upstairs zone would be great.

The manual J and other HVAC design was done professionally a few years ago, by a trusted company well known on this site. This is a full system, whole-house replacement. Zone 3A, North Carolina.
Design calls for:
9k ducted for first floor north (9.2k heat/5.6k cool) in crawlspace
18k ducted for first floor south (16k heat/12.7k cool) in crawlspace
6k ductless upstairs (4.6k heat/ 2.7k cool)

The ductless would end up directly above the master bed, because of wall and ceiling geometry.  Not my favorite option, as I’m worried about gurgling linesets and having to deal with the messy logistics of cleaning a ductless head above the bed.  Please let me know if I’m overly concerned about those things; I don’t have any experience with ductless yet.

Not sure a multisplit with a 6k slim duct would be a good solution, as it would have to share a bigger outdoor unit with a 9k or 18k head, and have turndown issues.

I’m struggling for other options that would work with the low loads upstairs.  I’m not super concerned about squeezing out the greatest efficiency, I’m focused on avoiding comfort issues.
Motorized zoning damper on the first floor (south) ducted unit to supply the 2nd floor, and deleting the 6k ductless head?  There is an existing duct chase between the crawlspace and unconditioned attic, but it’s a tortured path.
The existing 2nd floor ductwork and registers are above the collar ties, in unconditioned space.

I wish someone would make smaller-than-9k slim duct heads for 1:1 systems, or that I could afford a baby VRF system for residential (Airstage 4k miniduct!).

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  1. fomur | | #1

    Mitsubishi offers 6K single zone units and they are quite efficient (33 SEER & 13.5 HSPF) not recalling their updated version 2 ratings. They also modulate well I think it can modulate down to as low as 1.5K on heating. I bought one but yet to install. Also, unless you want to stick with Mitsubishi, actually now there are other 6K single zone units in the market. Haven't tried it out but you can consider Blueridge 6K single zone minisplit, or any other equivalent unit that was made by Midea like Cooper & Hunter etc - as they are simply rebranded units (sometimes Bosch or even Carrier offer similar options). It seems impressive on the paper as they have 12.6 HSPF2 rating 28 SEER2, considering these are version 2 ratings, which tend to be lower than version 1. Not sure about the quality but at least you can get them under $1K. Also, they offer slim cassette option despite at a lower efficiency but may be a good compromise as they can even fit between 16 OC joists. The only issue is I think these will modulate down about 3K vs 1.5K Mitsubishi - which may or may not be an issue based required load on that space. But worth to consider. In any case single zone separate unit will be probably more efficient than a multizone unit IMHO.

  2. Chris_in_NC | | #2

    Bumping this back up. Is a multisplit the worst possible option for two zones of rather different sizes and loads, or would this be better served by reducing to two 1:1 ducted units, adding zoning dampers to create the small 3rd zone?

    There is also the option of a single Mitsu air handler and zoning dampers, but sounds like the only advantage over a multisplit is the overall equipment cost.

    This is a total HVAC replacement, so the only thing convenient to carry over is the floor registers (the 2nd floor bedroom may get a soffit). Sure not going to replace the flex duct above the collar ties in that 2nd floor bedroom.

    The more I learn about cleaning and maintenance of ductless heads, the less I want one above our bed.

  3. Chris_in_NC | | #3

    Also, would using a 9k ducted for the 2nd floor be an issue, if the excess capacity is ducted to the first floor? The first floor unit would likely be reduced down to a 15k instead of an 18k, etc.
    Thermostat would be on the 2nd floor.

    Small zone issues... always the small zone issues.

  4. kyle_r | | #4

    I would do two 1:1 ducted systems, one for the first floor one for the second. With the right duct design, I don't see why you would need two systems for the first floor. For the second floor a 9k system should be fine. There is typically very little difference in turndown or maximum capacity between a 6k and 9k ductless system anyways. You should only need one register for the upstairs bedroom and one for the upstairs bath. See if you can steal some closet space between the two rooms to put the air handler and have two short duct runs perhaps in a soffit.

    1. Expert Member
      Akos | | #7

      +1 on this setup.

      Some of the 9k ducted units can be mounted vertically and have have pretty good turndown ie:!/product/47329/7/25000/95/7500/0///0

      This can easily fit in a back of a closet with short duct runs to your bed/bath. It would take up only about 1'x3' of floor space.

    2. Chris_in_NC | | #8

      A single first floor unit is definitely an option, especially if split into two zones to solve the South solar gain problem. Wouldn't break my heart to be able to use flex duct either, if using a multiposition air handler instead of a low/mid static ducted unit.

      I can probably create a soffit on the shared wall with the bathroom, because we've got the ceiling height and can drop registers on the sides for both rooms. Closet space is already challenging. Those 9k units are wiiiiide though, which is most of the packaging challenge with a soffit.
      I honestly looked at the Mitsu SEZ, Fujitsu, etc., and really didn't see anything with much of a turndown on the 9k 1:1 units, unless I'm really misreading the min/max specs.

      Any other brands that have high turndown like that Midea? I can look up individual models, but brands would help for a starting point.
      From the brands I've filtered through at NEEP, so far looks like Gree, Midea, and the associated private-label brands from those companies, like Bosch and Carrier.
      I know at least one of my local Mitsu/Fujitsu installers also uses Carrier.

      I can't believe Fujitsu, Daikin, Mitsu, Hitachi, etc., don't seem to have anything close to that for 1:1 unit turndown.
      The Daikin VRV Life products look really cool, and they do have a 2-ton outdoor unit, but likely very expensive overkill for our sub-2000 sqft house.

      1. Expert Member
        Akos | | #9

        You can go with a single unit provided you can get a large return near the ceiling to the 2nd floor. This large return is the key to good cooling as stack effect works in reverse in the summer and all the air leaks are drawn in through the ceiling from the attic. It is these air leaks that make the 2nd floor hot. Without nice snorkel to move this hot air and condition it, the 2nd floor will always be hot.

        I have installed a couple of Midea units and have no issues with them. For the price and performance you can't go wrong, they are after all the world's largest HVAC manufacturer. This is one of the 12k slim ducted units install in a hallway ceiling. The units are wide but you can usually make it work.

        1. rondeaunotrondo | | #13


          Sorry to hijack here a bit I want to switch out my 6k ductless in our master and envi 500w in a small nursery next room over between a small set of stairs. . I know you’ve done a lot of the Fujitsu vertical concealed ducted before but didn’t know you had installed the Midea mini ducted. I have a very tight 1.5 level and want to squeeze it in the top of our stairs with a dropped ceiling. Trying to decide between a vertical placement Fujitsu vs. Midea/senville with return coming from the bottom. EV spec’d an 343cfm with 8” duct into the master and 6” into the nursery with ideally returns in each room. I love the plug and play idea of the Mideas. Appreciate your advice. $17k quoted from a reputable hvac company just too expensive to not do myself.

  5. walta100 | | #5

    Seems to you are putting the cart before the horse.

    Before you throw a ton of money at equipment, I think you need to have a look at your air sealing and insulation levels.

    Let me start your home in my opinion is the worst possible case and calls for all the desperate solutions I hate.

    Old half story building leaks more air than any other style so what ever conditioned air your equipment makes simple leaks out almost as fast as you make it.

    Keeping the conditioned air in the living space and out of the vented devil’s triangle behind the knee walls of an old building is next to impossible.

    The sloped ceiling upstairs are so shallow that meeting modern insulation requirements make foam insulation the only option.

    Consider removing all the old insulation and spray foaming your roof deck making the devil’s triangle part of you conditioned space with a blower door test before and after and keep air sealing until you get under 3 ACH50.

    After the insulation is in you may decide you are happy enough with the old equipment.

    Note any sloped ceiling is a cathedral ceiling.

    You may find these articles interesting.


  6. Chris_in_NC | | #6

    Although I agree with the premise of what you're saying, the old gaspack sitting in the yard is on its last legs, and I'm not interested in replacing it with another packaged unit. All the ductwork is....bad. I don't think the gaspack would last until after the work is done, so I don't think I have the luxury of ideal work sequencing, no.

    The house is from 1989, and although it does have a small section of kneewall, it's where a hipped/shed roof attaches with a ledger at the gable end of the room, with 2x10 rafters (so would need a hip venting solution if rafters are insulated). That area does need work, as it fully contains the ceilings of both downstairs bedrooms. Currently is insulation under subfloor, poorly insulated kneewall, etc.
    There aren't any kneewalls related to the sloped bedroom ceilings, just a wedge of collar tie space where the old flex ducting runs.
    I think the current insulated ceilings are 2x10 furred out with a 2x2, so that's not a bad starting point.
    It's not an old Cape, no.
    Is it in need of air sealing like any poorly executed house from the '80s? Yes.

    Windows and doors are the top of the list, along with exterior air sealing, because everyone likes leaky aluminum single hung windows without thermal breaks, right...?
    I've found fist-sized holes cut in the Thermoply sheathing behind the exterior light fixtures, etc., so there are multiple things to address. Not sure if this is our "forever" house because I'm still 20 years (or more?) out from retirement, so it's all a balancing act.
    I modeled the entire interior volume of the house for my own 3D visualization aid, which works well in transparent mode (attachment). The blue/teal are the unconditioned volumes. Purple is the 2nd floor master bed/bath.

  7. user-723121 | | #10

    Thermoply sheathing, sounds like a Centex home. They used it here in MSP for a while but I think the code took it out at some time. Sounds like a big project to make this home efficient.

    1. Chris_in_NC | | #11

      Thermoply was fairly widespread in areas of the Southeast in the '80s from what I understand, not sure the specifics. Apparently very common in Texas too.

      The original 4 houses in our neighborhood were built at that time, as spec houses by an architect and some related family members, so I don't think any production builder like Centex had a hand in it.

      1. user-723121 | | #12

        I saw Thermoply on new houses in Houston as recently as 2010. All of the ductwork was in the attic, not much attention there to energy efficiency.

        1. Chris_in_NC | | #14

          You'd be hard pressed to find a new production-build house in our area that didn't have ductwork in the attic, honestly. Most people don't know any better, it's cheaper and easier for the builder, and you don't lose any of that lucrative square footage.
          Until the code catches up, or people get wise to the fact that the only benefit is to the builder, nothing will change.

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