A combination of factors all came together - in the worst way - which left my underside ducting in shambles. The main piece of ducting in each half of this double wide looked like swiss cheese, and the flex duct got ripped and torn by the re-level contractor. It's easy enough to replace the flex duct - but the main plenum ducts were completely ruined.
Based on the way the two halves of this place fit together, I was able to plot a straight line at the ceiling that would touch every room. From there, I consulted a couple contractors on how to lightly frame a "drop soffit" that would hang from the ceiling and enclose new HVAC ducting. Then all the calculations began with sizes and fpm's and length and all kinds of HVAC parameters. It all came out at 16" round duct or 15" square duct. I opted to use both to make some aspects of this easier.
For the long run at the ceiling, I went with 15 square ducting. I contacted a local sheet metal shop and they made them in very short order. They used galvanized 26 gauge steel, and provided all the interconnecting hardware. I also had them custom make a 15" square to 16" round transition piece.
Next I needed to transition from the 12" round exhaust port on the bottom of my blower/exchange unit to 16" round pipe. As I found out - much easier said than done.
My planned route was coming out of the bottom of the blower/exchange unit, expand to 14" diameter, make a 180° U-turn to come up through the bedroom floor at 16" diameter. It took some doing and arranging parts and pieces....but by cracky - we got it done. From there it was straight 16" diameter pipe up to the ceiling and then a long slow 90° to the above mentioned transition piece.
Man oh man - what a difference!!! When my AC comes on now I can actually feel the air blowing through the rooms. On the first full month of use - my bill dropped from ~$400 to $120. Most of this drop is due simply to almost all of the ducting being inside the insulated envelope of the floor and walls inside the house. Only 52 inches of duct is outside of this!!! Hence even leaks are inside the house!!
From there, it was finish out the framing/drywall - and use the new dropped soffit to make a bit of a more modern statement. All-in-all it all came out incredibly good!