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Layout for ducted minisplit?

Jerry Liebler | Posted in Plans Review on

My house’s floor plan has too many partitions to depend on comfortable heat distribution with ductless so I’ll take the slight efficiency hit to use duct ed distribution. The plan is to keep the ducts inside the “conditioned” envelope and very simple and all in the ceiling service cavity, both supply and return for the air handlers. My house is a rectangle with the long side running east to west. A central hallway divides the north and south areas. My proposed layout uses two air handlers, two zones, one “bedroom” zone on the east and a “daytime” zone on the west. Both air handlers are located in a “dropped ceiling” over the hallway and get return air through a grill at the east end and supply air goes to a “trunk” , also located in the dropped ceiling, flowing toward west. The air handlers and supply trunk ducts are mounted to the framing that supports the ceiling drywall. Supply branches, serve individual rooms though ceiling mounted” registers and run either from north to south or south to north . The top of the trunk ducts is the bottom of the branches and the top of the drywall. The trunk ducts are 6″x24″, the branches are 3″x14″ (could be 3″ x 20″). I plan 4 branches from each air handler. The ERV has independent inlet ducts and returns it’s processed air to the return air “box” of the east air handler. A 20″x20″x1″ disposable filter will be above each return air grill. All rooms that have supply ducts will have “jump ducts” for return air over their doors. I’m wondering, should I locate the supply register(s) closer to or further from the supply? Further, i think, results in more even temperature but clearly results in more pressure loss. My air handlers are capable of 0.4″wc @ 360CFM and can be “dialed back” to accommodate lower back pressure duct systems. I believe there is no need to add balancing dampers and there by additional losses. Will I be disappointed?

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Replies

  1. User avatar GBA Editor
    Martin Holladay | | #1

    Jerry,
    Q. "I'm wondering, should I locate the supply register(s) closer to or further from the supply? Further, I think, results in more even temperature but clearly results in more pressure loss."

    A. I'm not sure what you mean by "further from the supply," but I'm going to guess you mean "further from the air handler."

    The answer is that the branch ducts leading to each register should come off the trunk duct as close to the air handler as possible, as long as the supply register is properly located in the room it is serving (so that the register or diffuser has good "throw").

  2. Jerry Liebler | | #2

    Martin,
    I think I didn't ask the right question. "as long as the supply register is properly located in the room"
    The question should be how does one choose the location of ceiling supply registers? Closer to inner wall or closer to outer wall? Where is "properly located in the room"? for a ceiling mounted supply register..

  3. User avatar GBA Editor
    Martin Holladay | | #3

    Jerry,
    Check out these two online design guides:

    1. Engineering Guide: Air Distribution

    2. Building with Ducts in Conditioned Spaces.

    This latter document notes: "Supply registers can be located high on interior walls, rather than at the perimeter. Remember, these houses have much improved thermal shells. Careful attention to the face velocity at the supply boots and proper selection of registers or diffusers can throw air to the exterior and achieve good mixing. Use ACCA Manual T to select grilles. It’s not likely that the typical stamped metal grille will be the best choice. Again, ASHRAE agrees, “Many new buildings have well-insulated envelopes or sufficient thermal integrity so that supply registers do not have to be located next to exterior walls. Placing registers in interior walls can reduce duct surface area by 50% or more, with similar reductions in leakage and conductive losses.” This approach may not be best for every house, and if it is used grilles must be carefully selected for proper throw and mixing."

  4. Jerry Liebler | | #4

    Martin,
    Thank you! I had read what you Quoted and found long throw supply "commercial fittings" are , while available, sort of, are expensive and not easily "sourced". If I can get a duct "design" that allows supply "registers" to be located at the perimeter,even in the ceiling itself, then the registers can be the cheap residential junk commonly sold and still have great air mixing and distribution. The top surface of these ducts is the ceiling with r60+ above it's the best place to incur conduction losses, conduction "losses" out the bottom and sides aren't losses at all they are simply part of getting the heat into "conditioned space" The same goes for leakage, if it's not out the top.. So what is wrong with using chases, appropriately lined "building cavities" to make these "ducts". Then they can be large enough to allow "proper air distribution" with registers located along the building perimeter forcing even heat distribution with lowest possible energy loss.
    Edit: What I'm proposing is to construct airflow channels in a ceiling service cavity that serve as duct work for HVAC distribution. The air handlers are slim duct mini split heads. The top of the ceiling air barrier is part of the building's primary air barrier. the bottom is the top surface of the ceiling drywall.
    Each duct will terminate with register mounted in the ceiling directing the flow primarily downward.
    If needed or desirable the ducts can be lined with some form of "duct liner". My first cut duct layout has 4 "branch" ducts per air handler, the "branch" ducts are 3"x20.5 " about 20' long and support a flow of 90 CFM. The supply "extended plenum" is 6"x24" and supports 4 branches the layout is an H with the air handler at one end of the crossbar. the top of the extended plenum is the top of the branches. the air velocity in the 20 feet of plenum is 360 ft/min, in the branches it's 211 ft/min.

  5. User avatar
    John Semmelhack | | #5

    Jerry - your idea would be a code violation everywhere that I know of.

  6. Jerry Liebler | | #6

    John,
    Thank you, perhaps you can help.
    How does the code define a duct? and what makes it a duct? the code certainly permits airflow in ducts,within "building cavities" exactly what must be inside a cavity to call it a duct? if all the sides are drywall,? if all the sides are an approved duct liner? Edit: Or a cavity lined with sheet metal on all sides? What defines a duct? Exactly what is the code violation? There simply must must be a code compliant way to have a duct ed system above a dropped ceiling (the space between the actual and dropped ceiling is a "building cavity').

  7. User avatar GBA Editor
    Martin Holladay | | #7

    Jerry,
    The fact is that a properly designed duct system -- with a good trunk and long-sweep ells (take-offs) coming off the trunk -- will do a better job of air delivery with less static pressure than using building cavities, even if you try to make the building cavities airtight.

    There are excellent reasons why code officials insist on real duct with sealed fittings, rather than site-built attempts to create a "duct" with rigid foam or drywall.

  8. Jerry Liebler | | #8

    Martin,
    Thank you. BUT a simple duct system with an H pattern with 4 branches @ 22"x 3" and a trunk @ 26" 6" using less than best " design" where the branches intersect the trunk by simple penetration of it's sides. will allow 25' trunk ( the cross of the H) and 20'+ branches. to deliver 100 CFM per branch with a static loss of less than 0.2"wc. I can "fabricate" the clearly non standard rectangular duct sections by "re purposeing" 8" and 9" round sheet metal duct segments. I can also fit the branches in my service cavity and the trunk in a hallway dropped an additional 6", in that hallway I can run my ERV duct made entirely out of 6" round sheet metal duct. My question to John needs a complete answer! Edit It may be easier to "fabricate" the trunk using duct board but the very low profile branches scream metal duct. I can do all the " repurposing" on site with tools most everyone has, a hammer and tin snips + some angle iron and "c " clamps. What is a "code compliant" sheet metal duct?

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