I had a 5 ton Lennox XC16 installed today. I'm a little concerned about my return air capacity. They installed one 18" flex (short run) with a 24"x20" grille and a 10" flex (25' run) with a 12"x12" grille. I am concerned that the grille size for the 18" run is too small and that I probably need to up the size and grille of that 10" run. Both grills are centrally located on opposite ends of my house and I want to stick with two for now. Would I be ok in saying they need to upsize the grille for the 18" run and upsize the diameter on the 10" run? If so, how big for each? I'd appreciate any advice you guys could lend to me so I can hit the installer up tomorrow before he hits me up for the bill. Thanks!
Added: Sorry forgot details, I live in Houston and the returns are both in the ceiling of an older 2300sf home with 8' ceilings. All work was within scope, just not positive if done correctly. I'm pretty sure a Manual D was not performed or if he would even know what static pressure is. Do I need to persuade him to make changes to the return air and/or grill sizes before I release funds?