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Mitsubishi Ceiling Cassette Issues

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Good Afternoon ... After much googling and little response from my contractor, I've decided to reach out to someone online for a little advice on the issues I've been having. Hopefully this is the correct place.

TLDR: I don't think the ceiling cassette is properly sealed, or properly installed, and causes a couple issues. The contractor initially tried to address them, and say that I'm the only case he's seen of this issue, but nothing further has been done. Is there a way to properly install/seal these units? Had I known this was how this would go, I would have just opted for a wall unit.

Full Story, and apologies for the length: Moved into a brand new house, but the upstairs just can't get cool enough. There is a constant differential of at least 5*F and sometimes 10*F to where the thermostat is on the first floor. To add to the complications, my builder, in an effort to save money, completely skipped adding a return air register upstairs, saying that because the supply vents are numerous it's not needed. IMHO the builder just went as cheap as possible and doesn't care.

I decided to install a mini-split in the bedroom since 95% of the time it's just me here. Why cool the whole house when at night I really only want my bedroom cold. The well-reviewed contractor I selected sold me on the ceiling cassette option and it seemed nice because it would be out of the way, not taking up wall space. Unit finally gets installed and I don't need to crank down the central AC to get my bedroom to a comfortable temperature.

The next afternoon I walk into my bedroom and it has that hot attic smell. Temps outside were in the high 90's. I place my hand next to the plastic grill and can feel hot air coming out from around the sides. The smell was really strong. Contractor comes back, removes a few panels, puts some high temperature duct tape over the openings, and says that should fix it. It doesn't. As the attic heats up, the air expands, and it enters the living space. Contractor comes back, says he's going to put down a plastic sheet and use an adhesive to create a vapor barrier. Instead he puts down duct tape and spray foam. It's better, but it's still there. A slight improvement at most. At this point I'm no longer getting returned calls as they consider it fixed.

I started looking at the unit, removed the cover, and see that basically it's just an open hole into the attic once you get passed the blown in insulation. So yeah, it makes sense that air is coming in. The insulation in the attic is covering the unit. As the unit, and the surrounding drywall, expand and contract due to the temperature changes some popping can be heard. A few of the pops are almost startling at night. I fixed those with some extra drywall screws. Still have some light noises.

Move forward into winter when it comes time to use the heat pump option. Heat pump works fine and it warms the warm pretty well. Until the evening temps start getting well below freezing. When the temps are in the teens the heat function will run you out of the room. I have a set point of 68 on the remote. I woke up several nights to find the room was 76. I was told the remote is the thermostat and contains the temperature sensor. I've since come to believe that is incorrect. I think the temperature sensor is in the unit, and because of all the cold air in the attic, it is incorrectly sensing the room temp and continues to run at full blast. You can place your hand next to the grill and feel cool air.

My conclusion is that I need to do what my contractor wouldn't. First, use some plastic and create that vapor barrier around the unit. Second, build a box with foam insulation to surround the unit. Third, fill the box with some batted insulation, then close the box in, fill the holes for the lines with some expanding foam and call it a day. I'm hoping that at a minimum this slows down the air movement so that the hot attic smell in the summer is gone, and in the winter the unit does not heat to Hadien levels. And if needed, is still accessible for service below.

For reference the cassette I have is: MLZ-KP09NA-U1

Thanks for the help and advice!

Comments

  • HVACNUT
    HVACNUT Member Posts: 5,864
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    coloflyer said:
    I started looking at the unit, removed the cover, and see that basically it's just an open hole into the attic once you get passed the blown in insulation. 
    Can you post a pic of that? The cassette, above the ceiling should be basically sealed. Return air through the middle of the grill, air flow through the coil, and then out the supply sides. If you can see into the attic from inside the cassette, then something is very very wrong. 
    And all indoor sensors are in the cassette. There's nothing in the controller. 
    GGross
  • coloflyer
    coloflyer Member Posts: 5
    edited January 2023
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    At this moment it is covered by the duct tape that the contractor placed over it. So while it's covered, I wouldn't call it sealed. The areas I am speaking of are what I have circled in this picture of the cover. Behind those covers is the duct tape the contractor slapped up there. Otherwise, unless you moved the blown in insulation out of the way, it would just fall right through like tribbles on a space station.
  • GW
    GW Member Posts: 4,702
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    How much of a gap, might you say? I would think a 1/4 would be ok. It’s hard to get much tighter than that 
    Gary Wilson
    Wilson Services, Inc
    Northampton, MA
    gary@wilsonph.com
  • coloflyer
    coloflyer Member Posts: 5
    edited January 2023
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    There might be a small gap around the unit itself, but where I circled those panels, those are the service ports. Once removed (and you remove the duct tape left by the contractor) you can stick your whole hand into the attic to access the refrigerant and electrical lines. If work were needing to be performed you would need to move the blown-in insulation out of the way first.
  • HVACNUT
    HVACNUT Member Posts: 5,864
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    I'm not familiar with that model but it almost seems like it can be ducted, but they didn't cap it off. Let's see pics.
  • coloflyer
    coloflyer Member Posts: 5
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    I spent a couple hours this weekend building a box around the unit with some 2" foam. I now see why they opted to not put a plastic vapor barrier around it, as the way it is installed, nearly flush with the joist, there is no way you could reach it. Had it been installed, centered between the two, it would have been easy. Even sealing it as was done with the front side would have made a difference. They simply put metal tape and mastic (my guess on what they used) on the joist and against the unit, which left several inches between the tape and the drywall for air to come in. I put a couple bats of R38 over the top of the unit. I ran out of foam board to create a top for the box, but can do that another day. Because I had to place it on the other side of the joist, I'm not sure how air tight I can truly make it. But the added insulation seemed to help with the popping I was hearing. It did not, however, help with the escalating temperatures.

    As you can see from the picture, they had simply placed duct tape from above, and below, where those service panels are. This area I highlighted in yellow. In green I highlighted the area that sits against the joist. This is the area where I felt the majority of the cold air coming in. Some of the pieces of blown in insulation would move against the breeze. There is probably around 1/4" of space between the unit and the truss. I shot some expanding foam in this cavity and the amount of air coming out of the crack in the green area nearly stopped. There is still some from the yellow, but nowhere near what was coming out of the green. If you were to remove the duct tape you see in the highlighted picture you can reach right into the attic. I always felt this was a bizarre way it to be installed. I always envisioned a self-contained square box over a slightly smaller square hole.

    I suppose my next question is going to be around additional technology to keep the comfort of this room consistent. It looks like I would need the MHK2 thermostat and a Wireless Interface 2 in order to control the device from a wall thermostat, which would provide more accurate temperature readings, and also allow control via the Internet. Those installs seem pretty straightforward, unless I'm missing something.
  • bburd
    bburd Member Posts: 923
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    I see those units mostly in commercial occupancies, with conditioned space above rather than an attic. Do the installation instructions say to do anything different in this situation?

    Bburd
  • coloflyer
    coloflyer Member Posts: 5
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    bburd said:

    Do the installation instructions say to do anything different in this situation?

    After the install when all this started, I tried to find something to this effect to use a silver bullet against the contractor. There didn't seem to be anything in Mitsubishi's documents to that effect. I tried to contact someone at Mitsubishi and was unable to get a direct answer, other than it will work. There are plenty of posts online about it being installed in attics, not really stating a preference for or against, but I found a few other posts similar to mine about sealing it. Had it been on my first floor, between the first and second floor, I'm sure I'd not have had these issues. I'm questioning how many of these the contractor has installed. I think they've installed plenty of wall units, but never a ceiling unit in an attic. I think they know they screwed up and rather than address it have just decided to blow me off. Several other owners in my neighborhood have asked about getting a mini-split upstairs as they had the same issue as me with overly warm second floors. I took back the recommendations I gave them and suggested those folks go with a wall-unit.