I recently had a section of my return duct replaced/rerouted (in order to install support posts to raise a sagging living room floor). I am concerned that the way the duct was laid out is causing some airflow problems and am hoping to get some advice about how to proceed. I've hopefully attached pdfs with sketches of the original layout (original.pdf), the duct as it was reconfigured (as_built.pdf), and an alternative layout (alternative.pdf).
Background: The house was built in the early 1950s. There is a return in the dining room, 2 in the living room, and 1 in each of the 3 bedrooms. The DR is closest to the furnace followed by the LR and then the 3 BR. All of these are 14"x4". The ducts are steel and have been sealed. I recently had a 20"x20" return (feeding 12" flex) installed in an addition built in the 60s. (Yes, someone built an addition and added supplys but no returns.)
The section of duct that was removed was between the DR and the LR returns. This section of duct was 20"x8" and split into a 9"x"8 to the 2 LR returns and a 12"x8" going to the 3 bedrooms.
As rebuilt, each LR return is connected to a 6" flex duct which is connected to a new section of 12"x8" return. I am concerned that the 12x8 duct is undersized to handle the 2 LR returns in addition to the 3 bedrooms.
Following this change, the DR return is pulling more air and is whistling to the point that I leave the grille off. Additionally the BRs at the far end of the house don't seem to cool quite as well as they did. The company that did the work has done good work for me in the past (they did air balancing; added the return in the addition to take pressure off of the DR and LR returns; sealed the ducts, etc), and I am meeting with them later this week to discuss the situation. I wanted to get some advice before talking to them. I'm wondering if the configuration shown in alternative.pdf would improve things. I'm also wondering if putting dampers on the LR and DR returns would help regulate the airflow and reduce the whistling through the grilles.
Sorry for the long post. Let me know if more information would help, and thanks for any help.