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!
So the RA size has to be computed based on CFM, regardless of how far it is from the a/h? Please excuse my ignorance, but what is the CFM output for a 3 ton, 3.5 ton, 4 ton? I've got the BTU conversion figured out, but I don't know how to convert tonnage/BTU to CFM.
So... two 20 x 20 return air filter grills would be proper for a 4 ton XL20i (1600 cfm), for a total of 800 square inches.
Would it be problematic if the return was oversized.. for example, if I had a total of 800 square inches (two 20 x 20 filter grills) for a 3 ton?
No over-sizing the return does no harm. What does need to be considered is the the size of the duct behind the return grill that it is attached too. if it is only a wall cavity 4 inches deep by 14 wide then you only have 56 square inches for air to flow.
This is where the 144 square inches per ton misconception is used when contractors choose a return grill. If you have a section of duct with the dimension of 10 X 10 equaling 100 square inches it will deliver 400 cfm at 0.07. Now look at a 4 inch wall cavity between studs if it is boxed out to 20 inches wide by 4 inches deep you have 80 square inches for air to travel. Now place a grill over that opening and consider the louvers in the grill taking up 30% of the opening and now you have 70% free unobstructed space for the air to enter the duct.