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Community and Q&A

3rd Party Duct Design

Psalvaggio | Posted in General Questions on


I’m looking into installing a Fujitsu ducted mini-split system in my home and trying to find out what the best way of going about this would be. I’m more than comfortable with the installation aspect of the system but I just want to get the design part right. I’ve run a room by room Manual J on and believe I have a good handle on the equipment that I will need to get.

My home is about 2,400 square feet single story ranch style home built in 1974. Climate zone 3B. According to my zip code the design temps are 80F/44F. 

I know the Fujitsu slim duct units have a static pressure of 0 to 0.36 so I’ll have to keep the runs short. But for calculating the best size of ducting and fittings and registers to use is a bit over my head. Is there a good source online that I could use to design this for me?


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  1. Expert Member
    Akos | | #1

    Getting the duct design sorted is not that hard even for DIY. You can get a copy of manual D and walk through it.

    You can also ballpark the design if you have simple runs (fewer than 4 90 deg bends). As long as you keep velocity bellow 400FPM (CFM through duct / area of duct in sqft) and and stay away from flex you won't have enough losses to matter. For complicated runs I like to use semi rigid aluminum ducting, this is slightly more restrictive than hard pipe but easy to fish through tight spaces.

    Since you have the load for each room, you can back calculate the CFM each need, most units run around 400cfm/ton. For example a room with 2000btu load would need 70CFM, works out to a 5.7" round duct at 400fpm, so use 6".

    As a ballpark, bedrooms need 6" duct, bathrooms 4", open concept living space 2 to 4 6" ducts.

    Generally you want to avoid blowing air at people, and if you are using it for heating, it is good to get some flow around the perimiter especially if you have larger windows.

    One item to watch is the air filter. You want to use about 2x the size the standard AC rules of thumb call for and always best to go with either a 2" or 4" filter if you can fit it. There are standard filter grills that take 2" filters.

  2. Psalvaggio | | #2

    Here is my manual J report from CoolCalc. It's a little skewed because I had it setup for zoning off the bedrooms and the living areas as separate so there's more people than need to be in there. Also I attached a floor plan with the square footage and how many CFM the report says the rooms need. I would like to stick with either ceiling cassettes or ducted units.

    I've included 3 options that I think would work and just wondering what you guys think would be best.

    The 1st option would be to do a Fujitsu AUU12RLF 12K BTU ceiling cassette in the center of the kitchen and a Fujitsu ARU18RLF 18K concealed ducted unit to supply the living room & family room, paired to a Fujitsu AOU36RLXFZ1 36K BTU condenser. Then have the bedrooms & bathrooms on one Fujitsu ARU18RLF 18K concealed ducted unit paired to it's own Fujitsu AOU18RLFC 18K condenser.

    2nd option would be 3 ducted units all Fujitsu ARU18RLF 18K concealed ducted. The kitchen & living/family room units would be paired to a 36K condenser and the bedroom unit would be paired to it's own 18K condenser.

    3rd option would be doing 4 ducted units. Kitchen and living/family room would be the same as before. The kids rooms would be run on either a concealed ducted 7K or 9K unit and the master bedroom and bathrooms would run on another either 7K or 9K concealed ducted.

    As far as all ducted goes, the 3rd option would be easiest for being able to keep the duct runs short and being able to have returns in every bedroom.

    The other thing I was thinking was instead of using ducted units for the kitchen, living room & family room, if I can get away with using 2 ceiling cassettes that would be much easier. I don't know if a single 18K ceiling cassette situated in the middle of the kitchen and living room would be enough to cool off that area with another 7K unit in the family room?

    Let me know what you guys think, I'm willing to look at other options also if you think something else may work better.


  3. Deleted | | #3


  4. Expert Member
    Akos | | #4

    A couple of pointers.

    You generally want to stay away from multi splits. They have much lower modulation range and lower efficinecy than one to one units. Also the indoor heads on multi splits don't modulate. If they are sized correctly, they can be made to work but one to one will always be better.

    In your case I would figure out how to get everything on 2 ducted units. You can mount one into the loundry room to handle the living space and one into the ceiling of the hallway or hallway bathroom for the rest.

    With ducted units, generally best to have only a single simple and well sealed return with a filter grill. With a good filter, this will keep your ducts spotless for many many years. For return form the rooms, go with either transfer grills or door undercuts.

    Your Man J shows duct losses (also the heat gain from appliances seems high). Putting your hvac and your ducting inside the conditioned space probably cuts 1/2 to 3/4 ton load off your hvac size and will make a big difference in operating cost.

    A couple of bulkheads are easy to hide. For example you can do most of the main trunk in a bulkhead above your kitchen cabinets and fair bit of the rest in a dropped ceiling in the hallway between the bedrooms. Interior ducting doesn't need insulation, so it is not as bulky as one would think.

    Also check the data sheet for the specific unit. For example the 18K Futjitsu delivers 21k of cooling.

    P.S. If your ceiling is actually R19, bumping that up a bit is an easy cost save on equipment size and operating costs.

  5. Psalvaggio | | #5

    Thanks for taking the time to answer my questions.

    Your reply got me thinking of what I was originally thinking of doing. There already is a drop ceiling in the hallway. It's not exactly a conditioned space though since it's open to the attic but it should be easy enough to close off the top of it in the attic and insulate over it. There is a extra closet that is in the master bedroom that shares a wall with the hallway. Originally I was thinking of using this space to put the ducted unit in and have the ducts run in the drop ceiling but the couple of companies I had out to give me quotes didn't seem to like that idea so I scrapped it. But here is basically what I was thinking.

    Have the Fujitsu ARU18RLF 18K concealed ducted unit vertical in the closet with a return at the bottom in the hallway. Have a 8" trunk line that is in blue with 6" branches going to each of the bedrooms & master bath (green) and a 4" branch going to the hall bath (orange).

    The reason I would use a 18K instead of 12K unit is because the amount of CFM I need to cover all the rooms and bathrooms is about 530. The 18K has a maximum airflow of 554 CFM where the 12K only has 383 CFM. I'm not sure how much that matters but if it isn't a big deal I would prefer going with the cheaper 12K unit since it would be plenty of BTUs to cover the areas according to the Manual J.

    As far as the kitchen, living room and family room go, how good are the ceiling cassettes at cooling off a large open area? Because if I could get away with a 18K cassette between the kitchen and living room and then have a 7K cassette in the family room I wouldn't mind running each of them off their own outside condenser.

    In the example I have attached the blue cassette would be the 18K unit and the green one is the 7K unit.

    It is about 54 feet from the farthest kitchen wall to the farthest living room wall.

    1. Expert Member
      Akos | | #6

      If you already have a drop ceiling there aready, it is a much better way to go.

      Looks like there is plenty of space in the hallway ceiling for the unit itself. Makes the ducting much simpler as it is all just straight shot. You can also go with the low satic unit (cheaper).

      You can get a slim unit into a 12" space but if you use furring channel for the drywall, that can be reduced to around 10", which is not much more than you need to run the 8" duct anyways. I think an 8x12 or 8x14 is a better size duct. Make sure to include dampers at the takeoffs to be able to fine tune the flow.

      Don't worry about the CFM. What matters is the BTU each room needs, so if the unit provides sufficient BTU for your loads there, than you can use that unit. Use the CFM/BTU ratio of the actual unit to figure out your flow needs.

      I cool a 43' long space with a single wall mount. It works well except when the late afternoon sun hits on some larger west facing windows. There you start to notice temperature difference between the two ends.

      The bigger issue is heating with a single central unit. The perimeter areas will be quite cold if old construction with minimal insulation. You'll probably need some supplemental electric heat for comfort.

      4 way units also need a large space to install, which might mean re-framing the ceiling. You are probably better of going with a one way or two way units, these fit between standard joist spacing. I also think the 4 way units are quite ugly.

      Figuring out how to get a ducted unit in there as well would work much better. You'll get better efficiency and air distribution plus less cost than a two zone multi.

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