I’ve been conducting a manual J, S and D by myself for my house and I’ve hit a wall. (I went with a firm for an initial design, but wanted to continue iterating on the designs and they’ve been difficult to partner with in that way).

My problem: I cannot tell if my design would be delivering too few CFMs for my needed loads, and, if so, why. I can’t even tell if this is a valid question.

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I designed a ducted, concealed heat pump system—with trunks, supply runs and all—that satisfies TEL (total equivalent length needs) and pressure drop requirements.

I used the http://hvac.betterbuiltnw.com Manual J + S software recommended on this site and also the ACCA manual D speedsheet for calculating duct sizes: https://www.acca.org/viewdocument/acca-speed-sheet-for-manual-d. This was also a very helpful resource: https://hvacrschool.com/manual-d-speedsheet-walkthrough/

The loads for this zone are:
– Heating load: 21,500 BTU
– Total cooling load = 15,900 BTU (sensible load = 12,100 BTU; latent load = 3,800 BTU)
(Note: BetterBuiltNW does not output total CFM needs in its Manual J calcs. I’m not sure other software does)

I’m designing around this Fujitsu concealed ducted heat pump unit:
Max CFM: 554
Outdoor Unit Model #⁺: AOU18RLFC
Indoor Model #⁺: ARU18RLF
Rated Heating Capacity (Btu/h) @47℉⁺: 21,600
Rated Cooling Capacity (Btu/h) @95℉⁺: 18,000
Full ASHP specs: https://ashp.neep.org/#!/product/32101/10/21500/85/15900/0/11940/744865/1

This unit should work for these load demands as discussed here: https://www.greenbuildingadvisor.com/question/how-to-size-heat-pumps-for-cold-winters-and-hot-and-humid-summers

But, and I can’t tell if this is a bug in its software, BetterBuiltNW is telling me that this ducted configuration and its CFM output per supply register in aggregate will output heating and cooling loads that are much lower than my design needs (see the attached images).

Why is that? Does this actually suggest that the system won’t satisfy my design loads or is this a bug? Is there an equation that calculates heating loads from outputted CFM (so I can reverse engineer what BetterBuiltNW is outputting in that table)?

I’m confused by this because a Manual D seems to determine the CFM output at each register by taking the total CFM output of the blower and distributing it by room according to the same distribution of the total heating load across all rooms. And if a unit’s heating capacity is large enough to satisfy the total heating load, then a ducting system with sound TEL, duct sizing and pressure drop will move that capacity proportionally just fine. The core unknown for me is whether a system has “enough total CFM”, but I have not figured out how one calculates that. And it seems that CFM tends to positively scale with total capacity (for example: you won’t find a 800 CFM unit with the same capacity as the unit above; in other words, if I want more CFM, I would get a bigger capacity unit, but then I’d wind up with an oversized unit).

What am I missing here?

-Luke