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Multi/mini-split cassette buried under blown in insulation?

Nplus | Posted in Mechanicals on

I am in the planning phase of a house renovation (San Francisco Bay Area, zone 3C) which includes a new HVAC system and am considering a multi-split system with ceiling cassettes and ducted units. The unconditioned attic insulation is blown in fiberglass on the attic floor. Although not as efficient, the ducted units may have to go into the attic due to space constraints. The ducting would also be buried under insulation. None of the manuals and online references I have found seem to specify whether ceiling cassettes or ducted units can be buried under blown in insulation. Can these units be buried under insulation? If not, is one solution to build an airtight cover out of rigid foam insulation with spray foam (e.g. Great Stuff) around the penetrations?

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  1. Expert Member
    Dana Dorsett | | #1

    A foam-board box around it wouldn't meet code, since an electrical fault could cause a fire.

    If you go with Fujitsu's dedicated mini-duct units, they (unlike the competition can be mounted vertically, which makes it pretty easy to mount them where they are easily serviced, say in the back or side of a closet, or it's on ultra-slim "utility closet". Check out the 1.5 ton Fujitsu mounted vertically in this house in Berkeley:

    That takes up maybe five square feet of floor area(?), and it's very accessible. Note that the soffited duct run below ceiling level too. If you run the ducts in the attic to be buried under insulation, use hard-piped duct and seal every seam & joint with duct mastic. Maybe you can even hire the outfit that did that installation to assess your situation, since they're in you're neighborhood and seem competent to figure out how to mounted it in the attic while keeping it within the insulation & pressure boundary of the house. The contact person would be

    Larry Waters: 707-342-1981

    That house was covered about a month ago in this GreenTechMedia blog piece:

  2. GBA Editor
    Martin Holladay | | #2

    Ideally, HVAC personnel would have access to the air handler for future service, so burying the air handler in insulation isn't ideal. Here's a link to an article that recommends, "Don't do it."

    That said, some experienced consultants, including Bruce Harley, say that you can do it. In a recent article, "Bruce Harley’s Minisplit Tips," Harley was quoted as saying, "You’ll need one [a ductless minisplit] upstairs if you want air conditioning upstairs, or you can put in a little air handler to serve each bedroom. The air handler can be installed in the attic, covered with insulation. If you have several bedrooms upstairs, ducts help."

    It seems to me that thick fiberglass batts on top of the air handler would be better than blown-in insulation, because the batts can be lifted more easily if the unit ever needs servicing.

  3. Expert Member
    Peter Engle | | #3

    It's more a question of when they need servicing, not if. I would be most concerned about the condensate drains. Those fill up with biocrud and often need cleaning once a year or so. Burying the air handler and drains in insulation will make this quite a chore, not to mention dangerous if there's no flooring.

    FWIW, most codes require an unobstructed catwalk to access air handlers, and a work platform on the service side. Covering them with insulation would be an "obstruction." Also, air handlers above living spaces require drain pans with secondary drains leading to daylight. These would also not be conducive to covering with insulation, especially blown-in that would fill the pans.

    All more reasons not to put air handlers in attics, or to use conditioned attics if you really need to do it.

  4. Expert Member
    Peter Engle | | #4


    I've got similar concerns about the Berkeley project you posted. When that system requires servicing, they will have to tear out the sheetrock closet to get to the connections and drains. None of these should be obstructed by permanent construction. I also question the need/wisdom of covering the air handler with bubble wrap when everything is inside the conditioned space.

    I would rather see the ductwork closed inside a finished soffit as shown, but the air handler enclosed with a removable cowling of some sort. It would be relatively easy to fashion an enclosure using louvered panels, door slabs, or something similar that could easily be unlatched and removed from around the equipment.

  5. Expert Member
    Dana Dorsett | | #5

    Service access looks pretty easy on that Berkeley installation. The return grille below the cassette is removable, and the (not yet installed in the picture) door gives ready access to both the cassette and the short plenum above the cassette. I suspect (but don't know for sure) it was designed with sufficient clearance on the right side of the door for disconnecting the plumbing that the entire cassette could be replaced without having to demo any of the wallboard.

    But alternative, more accessible solutions would not be hard to figure out. Perhaps John Semmelhack will weigh in on how he dealt with access on this vertically mounted Fujitsu in a shallow utility closet, which looks like it might be framed for a full sized door:

    The point is, even with framing & wallboard a vertical mount only takes up about five square feet of floor area if space is tight, and it can be worked on while standing firmly on the floor. The 3/4 & 1 ton cassettes are slightly smaller and can probably be squeezed into about 4 square feet, with full service access.

  6. Expert Member
    Peter Engle | | #6

    I agree, and I like the installation you show. The unit is clearly centrally locate in the house for short duct runs, and potentially the unit is accessible from both sides. It's also got a plumbed drain pan under it. Certainly this is a better alternative than burying it in insulation an an unconditioned attic.

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