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Sizing Ductless Heat-Pump for ADU

Christian Wagner | Posted in General Questions on

Im in the process of selecting a ductless heat pump for a small, open floor plan ADU I am building. Im trying to interpret the analysis done by my Title 24 Green Energy consultant (calcs attached). ┬áMy consultant assured me that while the “system output” shown is grossly over estimated the actual “system load” calcs (highlighted in red) are much closer to actual size and she recommends using these to size my minisplit unit. However these calcs are not based on “manual J” parameters. She says that these calcs are actually conservative and she would size up. The numbers do seem reasonable (17kbtu for 1000sqft).

Can anyone help me understand what these calcs on the Title 24 analysis are based on and are they good enough to size my system or should I do a full manual J (not sure where to start with that). thanks!
-Christian Wagner
Petaluma CA, climate zone 3C

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Replies

  1. Expert Member
    Dana Dorsett | | #1

    The 99% outside design temp for Petaluma is 29F, a temp where the max-capacity of a modulating heat pump is higher that the "rated" capacity. (see page 6, page 12 in PDF pagination: https://higherlogicdownload.s3.amazonaws.com/ACCA/c6b38bda-2e04-4f93-bd51-7a80525ad936/UploadedImages/Outdoor-Design-Conditions-1.pdf )

    A heating load of 17K for 990 square feet of conditioned space in NEW construction is super-conservative, overly padded for the 99% load, even when using a design temp of 24F (which is indicated in your attached sheet.) Rather than "reasonable" a load ratio of 17BTU/foot even at 24F is quite high for tight new construction. That would be a typical heat load ratio at ZERO F outdoors for a tight 2x4 framed house with clear glass (not low-E). Your real heating load is probably closer to half that if it's 2x6/R20 type of construction with U0.35 or lower windows.

    Almost any 1.5 ton and several 1.25 ton and even some 1-ton modulating heat pumps would have more than adequate spare capacity ever for the calculated loads indicated. eg. A 1-ton Fujitsu -12RLFC slim-ducted unit delivers over 17K even at +17F:

    https://ashp.neep.org/#!/product/25349

    The 1.5 ton version would deliver over 22K @ +24F:

    https://ashp.neep.org/#!/product/32101

    There are "mid static" versions with beefier blowers that can use skinnier ducts if need be:

    https://ashp.neep.org/#!/product/25348

    There are others that would fill the bill. With a design cooling load of 11K (which could be right, if there is a fair amount of west facing window) it's probably not worth going for a 3/4 tonner, even though a 3/4 ton cold climate mini-split would likely cover your actual loads with margin.

    1. Christian Wagner | | #3

      Also curious why all the models you suggest are ducted?

  2. Christian Wagner | | #2

    Dana - Thank you. I really like how much thought you put into your responses. As such I have been made aware of the inefficiencies of oversized heat pumps.
    Yes this is 2x6 R21 wall/R38 ceiling U0.29 avg. fenestration. However uninsulated slab finished floor (maybe not ideal). Its funny the output is quoted at 30k for the sake of the title 24! I think a lot of people see small numbers like 12k btu and can't believe it will keep up in any climate.

    1. mgee219 | | #4

      Christian what type of unit did you end up going with? I am on this journey right now with our "Pretty Good House" 2 bed ADU in Oakland (40 degree F) CZ3. 25'x25' and I have done the online Manual J/Manual S calcs and come up with needing a 9000 btu system max. 2x6 framing for walls floors and roof R20 in cavity and R8 to R10 ext continunous, zip system and tape everywhere... and when I call HVAC subs they push for 12000 or 18000 BTU systems. Our T24 ballpark came back at 12,000 - but the calc was wrong - called it an uninsulated slab on grade when in fact it's 2x6 wood framing with cavity and exterior rockwool. We are considering doing a PerfectAire DIY minisplit heat pump, all ductless single zone for us.

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