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Manual S & Fujitsu Equipment Selection Zone 3A

maks8489 | Posted in Mechanicals on

I’m replacing an absolutely clapped out old 2 ton package heat pump & leaking ducts with a ducted mini split & moving duct work from a vented crawl space to a vented attic. I’ll be adding a little square footage in the future & of course improving insulation & air sealing as I can(plenty of room for improvement). I had about settled on a Fujitsu AOU24RGLX outdoor unit paired with a AMUG24LMAS air handler. The best I could do for a manual S is the one from & I was surprised to find that the system I’ve selected is not sufficient.

Obviously, I don’t want to oversize the system & latent capacity is a big concern here. Can someone get me pointed in the right direction? I could also use some guidance in understanding how the CFM numbers from my manual J(attached) play in to equipment selection & duct design.

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  1. b_coplin | | #1

    I'll respond to give your post a bump.

    Pay a professional to perform a manual J. If you didn't have capacity problems with your old equipment, it's evidence your calculations are incorrect. For comparison, your calculated heat load (~31k btuh) is similar to my slightly larger, basically uninsulated triple wythe brick house @ 8°F (~33k btuh).

    Consider keeping the ducts where they are and spending the money conditioning your crawlspace.

    1. maks8489 | | #3

      Professional doesn't seem to mean much around here I'm afraid. The current equipment does struggle on the most extreme days or extended hot or cold is 37 yrs old & falling apart though. I figured even with the added square footage, new equipment of the same size & ducting with close attention to details would do the job. I would love to condition the crawl space, but it just ain't gonna happen right now. I have other reasons for moving the ducts anyway. Thanks for responding, I will look over my Manual J again.

  2. kyle_r | | #2

    Do you really have no insulation in your floor?

    1. maks8489 | | #4

      I do not. Pretty common around here.

  3. kyle_r | | #5

    You have a few options. I think your uninsulated floor is really driving your heat load.

    1) Insulate your floor, etc to reduce your loads to within the 2 ton unit.
    2) Hire Energy Vanguard to do your Manual J, S, and D
    3) Put in a 3 ton unit

    Depending on your floor plan and ceiling heights, I would also consider running short duct runs in a central hallway or in soffits to keep them in the conditioned space.

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