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Community and Q&A

Branch box location question

PatriciaCranberry | Posted in Mechanicals on

Branch Box for Mini-split system serving upstairs living quarters. The manufacturer says: don’t install in a hot and humid location for long periods of time. They also say: Branch Box is indoor use only. I’m in CZ 3, central Oklahoma. Can I put this in an attic in my location? I would say no. Should I try to get it inside a mechanical room location down on the first floor(near 1st floor ceiling/2nd floor floor)? I would still have to run all the branch lines through the floor and up the walls to the attic and then to the air handler locations. Just looking for the voice of experience here…I’m taking my equipment recommendations to my contractor today..but I’m not certain of his level of experience with this type of install.

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  1. GBA Editor
    Martin Holladay | | #1

    Good question. I would certainly say that an unconditioned attic in Oklahoma qualifies as a "hot and humid" location during the summer. That said, here is a link to a video showing someone who installed a branch box in an unconditioned attic:

    The usual caveat applies: Just because you saw it on a YouTube video, doesn't mean it's a good idea.

  2. PatriciaCranberry | | #2

    The Mitsu install book shows an above ceiling install ( I think) The illustrations are poor. Guess I will call Mitsubishi and get into the details. Maybe I could have it installed within in an insulated "box" built over/around the unit (in the attic) with a return air type vent cover as the access port. The return vent would essentially put the branch box in the conditioned space. Yes...the insulated box would need to be air sealed well at all seams and where it meets the ceiling.

  3. user-2310254 | | #3


    Just out of curiosity, how large is your home? This is a new-build, correct? Too often these systems seem to be oversized.

  4. PatriciaCranberry | | #4

    This is new build (36x60). I've got a good/comprehensive third party design from a knowledgeable company (in your neck of the woods, actually). But they couldn't weigh in on "where do branch boxes get installed in your area?"

  5. WLSchebaum | | #5

    I have a similar question about branch box location. My installer who worked with Mitsubishi to get the details fine tuned says that a branch box can be installed outdoors as long as its protected from the elements. The manual itself says that its possible as long as you use an accessory box, which is essentially a sheet metal box. Another contractor I've spoken with says that if installed outside it needs to be insulated. As of now, my branch box is outside, uninsulated but weather proof and my system works well. I'm in central Vermont where it tends to get pretty cold outside.

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