Homes in my 1980 subdivision are all 1600 - 2000 sf and builder installed 3.5 ton units with 20 x 20 return air filter grills as standard. Filter grill is just above the baseboard and feeds directly through the wall to the air handler sitting on a stand in the laundry room on the other side of the wall. The space under the a/h stand is sealed with duct board and there is no additional filter under the a/h.

I've been getting quotes on replacing my system, and have read varying "standards" related to return air sizing. A number of the contractors told me adding another return air filter grill is mandatory, others didn't address RA at all, and one said the 20 x 20 was adequate, but adding another would be "better". BTW, adding another filter grill to directly feed the same space under the a/h would be easy... through the wall around the corner.

My question is this... How does one properly compute RA sizing and does the location of the RA filter grill (i.e., distance from the a/h) make a difference in this computation? I've learned not to always base my decision on the opinions of folks who want my money... I'd rather double check for myself before I shell out the dough. After all, they don't have to live with the decision... I do!