Low static compact duct: supply register relative location and through velocity

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Am a mechanical engineer and to-be-homeowner of an almost ” Pretty Good House” in Hartford, VT (CL 6A). After BEOpt modeling and Manual J calculation: cooling load for the two story 1500 sq.ft. space is 16 kBTUH. I am planning to use low-static, compact ducted units (a 9 kBTUH for 2nd floor bedrooms, and another 12 kBTUH for the 1st floor living+kitchen+dining+guest bed) from preferably Fujitsu, if not Mitsubishi. I am planning to have dropped ceiling in the 1st story and plenum truss on the 2nd story to accommodate the air-handlers and duct-work.

I have studied articles here in GBA, and Energy Vanguard and Manual S, T and D calculations, and ACCA Manual D quick start tutorials. I am following direction from _John Semmelhack and Benjamin Knopp_ provided in their NAHC 2018 presentation:

Duct System Design “Rules
• All ducts in conditioned space
• Central returns
• Smooth turns for all trunks
• No turns with flex duct
• 450 fpm in-duct velocity
• Standard/mediocre takeoffs and boots
• Size supply registers for throw (but don’t hit people)
• High-efficiency, “over-sized” filter
• Balance every system, learn from mistakes

To avoid Garbage-in_garbage-out with Manual D,
• What should be the a good  thorough velocity from supply registers? Does curved blade registers archive good mixing while not hitting people in the room?
• What to keep in mind about relative location if supply register in reference to return grill, ERV/HRV diffuser?
• Does home run type distribution is easier to balance than trunk-branch system?
• In case of central return, is the over-the-door transfer grill recommended?

Thank you for your kind attention!

Note: Ductwork seems more costly than the outdoor and indoor unit combined! This lead me to use a split ( 9 + 12 k system instead of a 18 k combined system). I will also have a Rheem type indoor Heat-pump water heater (HPWH); With reduced system capacity in frigid winter coupled with a indoor HPWH, the over-sizing might be justified!

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