I installed a central boosted return duct in the 2nd floor hallway of a 100 year old house with a new attached basement, single floor addition, and a new 105K BTU/3.5 ton central air system. The 2nd floor had no functional returns, and temps were 10F higher than the first floor. I ran about 25ft of 8" galvanized including 5 elbows down through existing available cavities into a crawl space located 650CFM 8" Centrax. There's about 15ft from the Centrax to an I-joist duct cavity including 3 elbows and an angled takeoff. This cavity dumps into the air handler return manifold about 8ft away. The takeoff actually mounts to a 1/16" aluminum plate screwed to two I-joists. Some photos may help:
It has reduced the hot daytime temp delta between the two floors from 10F to 5F, and dropped 2nd floor humidity substantially. The new attic ventilator I installed reduced it another 2F, for a current 3F delta between floors. I think this isn't too bad for a 100 year old 2 story home with 30+ year old blow in insulation, and not enough of it in the attic at that--only about 4 inches.
However, both runs of the 8" piping are oscillating at somewhere around 30Hz, enough to be felt through the floor and heard in the hallway and small laundry room directly above the ducts and blower. As you can see in the photos, existing obstacles didn't allow securing the 8" duct to the floor joists with standard methods, and required installing the longest straight section at an odd angle. Note that I didn't add the plumbing strap until _after_ the duct was fully assembled. Did that for good measure. With just the motor and takeoff attachment points I could yank on the mid section and it didn't move more than 1/8". So it's structurally solid, just not sonically solid at ultra low frequencies.
What's the best way to lock down or add mass to this long 8" duct so it doesn't vibrate the living space above it like the trunk of some kid's car? It's not that severe, but enough the homeowner is complaining and wants it fixed. Does anyone sell something like 5/10 lbs two piece round mass dampers that you can wrap around the pipe and screw/clamp together? A few of something like that would likely solve this problem. I've Google's around and haven't found such a thing. Any/all ideas appreciated. Thanks.