I purchased a 2-story, 2250sqft home built in 1986 which is located in Glendale (Phoenix) Arizona.

In 2016 the HVAC Split system was replaced with a 5-ton Lennox 13SEER (cheapest Lennox series still meeting code).

If manual d calculations we're performed, someone used them as toilet paper. The return grilles are:

•30x12 Under closet cavity return, exposed frame work, unsealed and leaky.

•20x10 necking to 8x16 ducting
Both grilles are non-filtered

The filter is a basic 18x24x1 under the AHU which only keeps large crap from the coils. I tested a MERV 8 and checked static pressure. My lord.... Back in goes the near MERV 3

In case you're wondering, the installer tapped the blower motor to LOW and with new filter and the return configuration it is within published spec.

Long winded, just wanted to provide a system overview because I'm systematically redo'ing every inch of this homes HVAC!

TL;DR

4 Bedrooms 2 baths upstairs totalling 1000 (almost exactly) living sqft. It will be getting 1200cfm from the unit.

I dislike undercutting door bottoms for returns and no viable walls exposed to common hallway for transfer grilles.
Also, getting a return from downstairs to the attic for direct room returns would be an act of a higher power (and sacrifice of a hall closet upstairs).

Jump duct is the best solution.

I already ductulated sizing. However, there is nothing I can locate for typical guidelines. I want to combine three jumps into one 16" trunk duct in the attic using a Wye dumping return air through a fixed curved blade register from the stairwell ceiling, directed towards where two 30x20 filter grilles are being installed. (MSTBDRM Is getting it's own jump).

Last HVAC tech I asked had never seem combined jumps. But made a good point to oversize the common 16" duct (0.02"@500cfm) so not to push return air to another room.

I refuse to do tacky DIY work, especially if it'll have an inspector in the future dinging items when I go to sell. I have well educated myself in HVAC theory. However my practical experience is limited duct fab replacement in a few homes.

Any prosumer or HVAC/EPA cert'd pros feedback and tips for combing the Jump Ducting would be appreciated!

THANKS