Static measurement would be by a Pro,to determne the duct systems overall resistance to air flow,and the ability to move air thru the ducts you added.
Testing would likely show that if you block the 2nd floor return ,to force the added returns to pull some air,the static woyld increase to the point that the fan can not move the required air flow,for the equipment to work properly and safely.
Simple test:Feel the air flow at supplies in Bedrooms,now block the 2nd floor return,feel the decrase in flow in the bedrooms,due to increased static.Same is tru to see if returns are needed,feel the supply with the door open,then closed the diference ,if any ,is the flow you should get ,if you add returns.
To get the returns you aded to work properly,they would need to be larger,and the duct going to the 2nd floor return,may need to be larger (may not have the space or access) as well,plus better fittings(no tees).
Your best bet is the passthru or jumper,returns unless you get a Pro to design the returns to be ducted .
Many thanks - makes sense
I appreciate your input dash.
I plan to add a 12" jumper from each bedroom into a 12" ceiling grate directly above the 2nd floor return grate in the wall. I plan to use a T to join the two jumpers into one grate. With the 12" dia., a T should not be restrictive, you think?
I am willing to add a 12" grate for each jumper, I'm just trying to minimize the visual disruption in the ceiling.
I'm not going to rework the 6" returns I installed as the money is already spent and the work is done. Besides, I'm restricted to the 4x12 boot through the header to the join the 2nd floor return from the attic. This is the bottleneck I cannot overcome.
As a last resort, I may add pass thru's above each bedroom door but I'd rather not have that potential substantial compromise in (sound) privacy.
12x12 in each room,10" from bedroom,12" from Master,ducted into a 12X24 in ceiling of hall.Based on you have double the supply in the master.
If you jion the two into one similar sized duct it will reduce air flow ,Y fitting is always better then a T fitting for air flow,if need one.