Sizing the Air Return - Does Location of the Return Make a Difference?

[Florida Joy]

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!

[second opinion]

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!

Well your one dealer that told you the 20x 20 was adequate would be correct if you where using a 2 ton system.

2 CFM per square inch is what is required. The location of the supplies is far more critical than the return location.

[Florida Joy]

Well your one dealer that told you the 20x 20 was adequate would be correct if you where using a 2 ton system.

2 CFM per square inch is what is required. The location of the supplies is far more critical than the return location.

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.

[Florida Joy]

Well your one dealer that told you the 20x 20 was adequate would be correct if you where using a 2 ton system.

2 CFM per square inch is what is required. The location of the supplies is far more critical than the return location.

The other dealers told me the requirement was 144 square inches per ton. Does that sound right?

[second opinion]

The other dealers told me the requirement was 144 square inches per ton. Does that sound right?

Only if you want it to be undersized by at least 30%

[second opinion]

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.

400 cfm per ton is standard times 3 tons equals 1200 cfm

[Florida Joy]

400 cfm per ton is standard times 3 tons equals 1200 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?

[second opinion]

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.

[Florida Joy]

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.

The space behind the return air filter grill is the open cavity under the air handler stand, covered by duct board. This cavity measures approximately 30 inch high by 36 inches wide by 24 inches deep. Big enough?

[second opinion]

The space behind the return air filter grill is the open cavity under the air handler stand, covered by duct board. This cavity measures approximately 30 inch high by 36 inches wide by 24 inches deep. Big enough?

The cavity would be large enough, just increase the return grill size to accommodate.

[Florida Joy]

The cavity would be large enough, just increase the return grill size to accommodate.

By gosh by golly, I think I've got it! Thanks everyone!