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Community and Q&A

Contract Specs for Ducted Mini-Split Install

cody_fischer | Posted in Mechanicals on

I live in climate zone 6A and am putting in a mini split system.  For a variety of reasons, I have no choice but to go with a ducted system, with the ducts located in an unconditioned attic for a portion of the system. 

As part of the project, I’ll be removing the old insulation, air sealing and then blowing in R-45 loose fill cellulose. The mini-split ducts will be buried in the new insulation. 

I am looking to the GBA community to help me complete the list of specifications below for my mechanical contractor to ensure the install is done well to minimize condensation risk. I’d love folks to fill in the blanks and suggest other things to add to my contract / agree with my installer / inspect. I’m also happy to spec specific products if folks recommend. 

  • Ductwork
    • R-12+ insulated ducts will be used
    • All duct seams will be sealed with mastic
    • Duct Blaster test will be conducted, must achieve total duct leakage < 3 cfm25 per 100 ft2 of conditioned space
    • Any leaks identified will be sealed with mastic (not tape)
    • Plastic jacket on the exterior of insulated flex duct will be inspected carefully before and after installation (prior to insulating). Flaws in the plastic jacket must be repaired.
    • Galvanized register boots will be covered with duct insulation that includes a plastic jacket and air sealed to gypsum ceiling with canned spray foam
  • Commissioning / Refrigerant Checks
    • Standing pressure test at 450 PSI for minimum of 1 hour
    • Bubble test all connections with leak-detection solution like Refrigeration Technologies Big Blu 
    • Use Nylog or equivalent on all refrigerant connections
    • Vacuum decay test (min 500 microns) – system must remain below the decay target for at least 10 minutes to pass
    • Final check with an electronic refrigerant leak detector

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  1. Expert Member
    Akos | | #1

    Looks pretty good, it is important to set expectations, at the last HVAC install I had to actually ask the installer to connect a micro gauge.

    I would add taping the duct to the register boot and checking flow rates part of the commissioning.

    The only sure fire way to keep ceiling registers from sweating is to cover it in spray foam. This is a good application for one of the smaller two part kits. You can do this as part of your air sealing.

  2. Patrick_OSullivan | | #2

    > Any leaks identified will be sealed with mastic (not tape)

    There's really nothing wrong with quality tapes for sealing surfaces they're suited for. Mastic is useful for curves and complex intersections, but I don't think it's necessary to completely forbid tape.

  3. Expert Member
    Dana Dorsett | | #3

    I'd increase the micron test time to an hour as well. Ten minutes is just too short (even if it has already passed the 450 psi positive pressure test), especially if there happened to be any water left in the system.

    When dealing with a pro you shouldn't HAVE to specify, but the 45oPSI test should be done with dry nitrogen, not air. (There are plenty of idiots in the HVAC biz...)

    Another line item under "Refrigerant Checks" would be to insist that the line set lengths be measured the refrigerant charge calculated and adjusted if needed. While the pre-charge on the compressors can do reasonably well over a decent range of line set lengths without adjustment, very short line sets can require weighing-out some portion of the refrigerant charge, and longer line sets may need to have additional refrigerant weighed-in for it to operate at spec. It is easy for mini-split installers get sloppy, and some won't bother to check if they are within the manufacturer's specified range just assuming that it will work (and it usually will, at least sort of.) Pointing it out just keeps them dotting those "i"s, crossing them "t"s, in case they've gotten out of the habit.

  4. cody_fischer | | #4

    Thank you all for your responses. Sincerely appreciated!

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