re: Adding another return air vent

I just came across this return air duct sizing chart which might be helpful on adding additional return air ducts to your house. LINK

This charts shows you’d need 16 inch flex duct to provide at least an additional 600 cfm or return air or 2, 12 inch flex ducts. I’d be interested in knowing what size return air ducts the HVAC firm is quoting you on.

I may just take fish’s advice and add my own extra return air duct - I’m about 400 cfm return air short on my 4 ton system. I need 1600 cfm but have a little short 1200 cfm of return air capacity on a 24 x 24 return air grill.