sizes for exhaust and air return
I need to get a replacement a/c and furnace system for my 2 story 2000 sq ft home built in 1980. I had two local contractors (both well recommended) come out and got somewhat different inputs but both suggest a 4 ton system which equals the original capacity. Main items I have questions on:
1) flow area for return air: Original equip had a 30 x 10 inch register. We placed a built in cabinet (it's a long story) that has a 3.5 x 42 inch area for return air. It is positioned about 3 inches in front and 5 inches below that 30 x 10 register. Our existing system did work with this, but first contractor thinks it's probably OK, the second says it needs to be opened up. It would require $500 of rework to that cabinet to install some kind of decorative grill in place of the solid facade panel currently there. I was thinking that I could take a wait and see on this one as it can be modified after the new unit is installed. Is there a definite way to determine what's adequate?
2) The exhaust stack size. Currently have original thing in place which is a heavy wall "pipe" 4.2 inch inside diameter, I'm told it likely contains asbestos. Goes from the ceiling in the garage straight up thru the roof, about 20 ft long. Current arrangement has both the furnace and 50 gallon gas water heater exhausts Y connected just below and sharing that passage. First contractor doesn't bring it up, second guy says it's not adequate for both. I think he said that current code requires a 4" metal vent for the furnace alone. Any way to know if my built in "pipe" will be adequate for both the furnace and water heater?
Thanks for your assistance.