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Mitsubishi Slim Duct (SEZ) in unconditioned attic?

Inverter0815 | Posted in Mechanicals on

I live in a two story colonial with an unfinished attic (insulation on attic floor) in NJ. The house has a single zone 2.5 ton A/C that does not sufficiently cool the 2nd floor. All 3 bedrooms are upstairs. We typically set the thermostat to 67F in July/August. That’s the only way to get the upstairs somewhat tolerable (<75F). Had a HVAC contractor over and he suggested two Mitsubishi slim duct (SEZ-KD12NA4R1.TH) installed in the attic. One to serve the master BR and the other one to serve both kids rooms. It would be connected to a multi outdoor unit (MXZ-2C20NAHZ2 or MXZ-3C24NAHZ2). This would allow us to separately control master BR and kids BR’s temperatures. What got me thinking is that Mitsubishi recommends installing the SEZ units in a dropped ceiling or closet. Can these be installed in an unconditioned attic as well? The gets up to 130F on a sunny day in the summer, so I am a little worries. Would a traditional air handler be a better idea?

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  1. Chris_King | | #1

    20k btu unit seems incredibly over powered for three bedrooms. Did the contractor do any calculations or did he look at the room and just suggest it? I also live in NJ and use a 18k btu unit for 1,500 sq ft of 1960s poorly insulated home. Seems like your biggest issue is that your current system is single zone.

  2. Inverter0815 | | #2

    Each of the proposed units can dial down to 3800 Btu and the outdoor condenser can dial down to 6000 Btu. So if both units are on, each can operate between 3800 and 12000.

    Another alternative would be an 18000 SEZ unit for all three rooms together but of course then I'd lose the individual control between master BR and kids BR.

    But my questions was really more about the SEZ units in unconditioned attic.

  3. Yupster | | #3

    It sounds like you have an existing forced air system in the basement? If this is the case, the likely cause of your uncomfortable rooms is a combination of things. Poor insulation, and poor air sealing will make any room uncomfortable regardless of how much energy you pump into it. I would look at the air sealing in particular. Fairly easy to retrofit and may help mitigate the problem. Then have an hvac contractor with a flow hood come measure the air flow upstairs. You are likely getting improper airflow through a poorly balanced system. There are a variety of ways that this can be fixed. The key is getting someone who is willing to do measurements, static pressure, flow at the register, temperature change through the system etc. I design these systems everyday and measure and commission as many systems as I can.
    You should definitely NOT install any ductwork of any kind in an unconditioned attic. This is a terrible idea. If you are stuck on the idea of patching the system with more equipment, find a contractor who will perform an aggressive manual J heat gain calculation. A minisplit will work, if you can't find space for it without putting it in the attic, consider installing a ductless minisplit, wall mounted or cassette style.

  4. dsmcn | | #4

    A ducted mini split will require return air and access. Return air for about 400 CFM means about 400 sq. in. of grille. If you also want an MERV filter, figure about 20" x 25" for the return air grille. The unit itself requires service access for the full width plus about 12" on the side where the controls are. You will need probably 12" between your drywall lid and unit for the makeup air plenum or duct. So that's a lot of square footage out of a ceiling, and don't think you can stick it in a closet or bathroom ceiling without cutting a major hole in the closet door or wall, because a 400 CFM draw requires it to come from the whole upstairs.

    The unit being exposed to the super-heated attic air will be a burden, but you could wrap insulation around the unit and the ductwork to reduce the loss. The makeup air can't come from the attic for a variety of reasons, delta T being just one of them.

    Sounds like a wall wart would be the best solution for the master bedroom. People live with these on the wall all over Europe and Asia. I look at them and think how lucky the occupant is. Building a soffit or bookcase around them reduces the obviousness.

    Is there a closet with shared wall space between the other two bedrooms? The SEZ can be installed vertically. If you can steal or create space between the rooms, the duct could go up a few feet and branch into the upper wall of each bedroom.

    Agree that your BTU choice sounds like overkill, but as you rightly point out, the units can kick down to only 3,000 BTU or so, and will operate more efficiently at that level. Also, the price difference between 9k and 12k is not significant.

  5. Inverter0815 | | #5

    Yupster: The existing AC air handler that serves the whole house is in the basement and because this is an old house, the ductwork was originally designed for forced air HEAT only. It works well for heat (because that rises anyway) but hardly any cold makes it upstairs, even if I close most downstairs registers. Therfore, zoning is not an option.

    David: the three rooms are all close together, so even one SEZ unit connecting all three rooms could be an option. I would want return air from each room as well and not central, so the filter would probably have to be in the attic. If I go with one unit, I'd probably go with a 15 or 18.

    Again, my concern was not that I'd lose a couple of SEER due to the system being exposed in the attic. My concern was more that the system breaks down if I try to operate in 130F.

  6. Expert Member
    Dana Dorsett | | #6

    Why mini-duct units, rather than wall /floor coils, or ceiling cassettes? Three half-ton heads, (one per room) or one half-ton or 3/4 ton for the master bedroom and a 3/4 ton SEZ in the closet between the other bedrooms is probably a better fit.

    David: Are you sure that the SEZ units can be installed vertically? (I thought they were horizontal-only.) The manual only shows horizontal installations:

    So far as I know only the Fujitsu xxRLF mini-duct units have a vertical installation option.

  7. Yupster | | #7

    Does it work well for heat because hot air rises or because you have lots of air leaks accelerating stack effect, drawing warm air from downstairs where the ducts are working well upstairs? That's common around here. Air sealing reduces that stratifying effect, which would help keep your upstairs cooler in the summer.
    As far as your ductwork goes, often you can't get appropriate air flow upstairs because something in the system is too restrictive. Sometimes it's a high efficiency filter, sometimes it's a cooling coil with a high pressure drop, sometimes it's undersized ductwork. All of these can be remedied at significantly less cost (usually) than a whole new system. Undersized ductwork can become right sized if you can reduce the airflow in your equipment. You can do that by reducing load on your system by air sealing and insulating. Or you can add a couple of ducts or maybe a return. No way of knowing for sure without measurements. And as a bonus, you won't have additional maintenance on other equipment or energy use.

  8. Expert Member
    Dana Dorsett | | #8

    YUPSTER: Using the same ducts for both heating and cooling is inherently problematic. The cooling load and heating loads are very rarely proportional to one another. Tweaking the ducts to be ideal for cooling puts off balance for heating, and conversely. Cooling loads for upper floors are often disproportionately higher than lower floors, due to lower shading factors, higher direct solar gain.

  9. Inverter0815 | | #9

    Dana: the walls that could be used for wall units are interior walls and then there is the argument with aesthetics. I am aware that nothing performs as well and as efficient as a wall unit, but it really is not an option. There is no closet between the bedrooms that can be used so that's not an option either.

  10. Expert Member
    Dana Dorsett | | #10

    OK, no suitable wall area got it.

    And the reason for passing on ceiling cassettes? (SLZ-KA09, or similar)

    The KA09 will throttle back to ~3K out at low speed, very quiet, no duct leakage or duct design issues, can be sealed well to the ceiling gypsum, etc.

  11. Inverter0815 | | #11

    Not a fan of the look of ceiling cassettes (2'x2' is massive) and one of the rooms is rather small (120sqft) so I think that could cause issues with cycling.

    Also, probably cost prohibitive. I doubt I can get 3 ceiling cassettes and a matching outdoor unit (24k btu) installed for under $20k. NJ is expensive.

  12. Expert Member
    Dana Dorsett | | #12

    I'm positive you can get a 3 zone compressor and 3 ceiling cassette installed for WELL under $20K, maybe even for less than $10K if put out to competitive bid.

    At internet store pricing the hardware is about $6.5K for a 2.5 ton MXZ-3C30NA2 + 3 SLZ-KA09s. In competitive bidding it shouldn't be more than about 2x that, even in expensive markets. But it's overkill.

    Fujitsu has some 7K ceiling cassettes. At internet pricing a 2 ton AOU24RLXFZ compressor with a pair of AUU7RLF and a AUU9RLF is $4.5-5K.

    As a point of reference, I've seen multiple bids for a PAIR of AOU24RLXFZ and 6 wall coil heads total (diverse sizes) come in for ~$15 in a gold-plated community in MA (Martha's Vineyard) in competitive bidding.

    It's not as if a 2 or 3 zone mini-duct cassette solution would cycle less than the ceiling cassettes. It's all about the minimum modulation range of the multi-split compressor.

    If you want to do it all as a single mini-ducted zone, the Fujitsu xxRLFCD series has advantages over the Mitsubishi SEZ units, including higher efficiency and more blower power, and the ability to mount it vertically, say in the back of a cabinet or closet, where it's more easily serviced.

  13. Inverter0815 | | #13

    I know the Mitsubishi allows me to set it up in a way that the fan will turn off in auto when the compressor cycles off (when using an external MEK thermostat, it's an option in the hidden settings). I don't think the Fujitsu allows that so during your typical summer night when the compressor turns off around 3am, the Fujitsu would continue to run the fan and reintroduce humidity. Unless Fujitsu has some hidden options that I don't know about.

    Just saw that Mitsubishi is coming out with a new MLZ one-way ceiling cassette that would fit within standard 16" floor joists, so that may be an option. But not sure when they will be available in the US market.

  14. walta100 | | #14

    Yes if you throw enough equipment in the attic you can keep the upstairs cool.

    In my opinion the contractor’s solution is the one that puts the most cash in the contractor’s pocket. It is also the solution with the highest operating costs. Putting equipment and or ductwork in any unconditioned space is not just a bad idea it is stupid, dumb and lazy idea.

    The real solution starts with an energy audit including blower door testing with IR photography. This will help you identify and fix the air leaks and get you get the correct insulation installed. If you still have a problem, get a manual J calculation to tell you if need more equipment or better ductwork.

    My guess is the audit, air sealing and insulation will cost less than equipment proposed for your attic. The real payoff will come from savings on your electric bills for years to come. Adding more equipment will only raise your monthly bills.


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