Is 20x20 grill large enough for 18 inch return?

13 years ago

I have replaced a 5 ton Goodman AC with a Carrier Infinity 5 ton 2 stage AC. But I am having problem with static pressure on high stage(0.85 with brand new 4" filter. 0.92 with 4" filter from HomeDepot).

The installer put in an additional 18" return with a 20x20 grill. Does anybody know if a 20x20 grill is large enough for a 18 inch return?

My installer told me that it is. But I have a piece of paper showing me that a 20x20 grill could only move 831 CFM air while a 18 inch return could move 1000 CFM, so a 20x20 grill is too small.

Is my installer right?

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Comments (4)

zippyhvac
13 years ago
You don't get the full benefit of the size of a filter grille because of the vanes - the vanes are a slight blockage to airflow.

Of course, there are "low restriction" filter-grilles that cost twice as much, but 20x20 won't cut it at all with a 5 ton.

bigger is always better with filter grilles, there's almost no cost difference between a 20x20 and a 20x30.

You need 2000 cfm total for 5 tons...

A 20x20 by itself is waay too small for a 5-ton a/c.

I run two 20x30's, each with their own short (3 foot) 16" duct for my four-ton unit.

Even so, on stage two (1600 cfm) you can hear them pretty well, even with the good 4" honeywell filters.
zl700
13 years ago
Having read your post carefully, it states that the 18" with 20 x 20 was added as an extra return to the existing one right?

So the answer depends on what is existing and how much more return air you needed. Perhaps the 18" was used for its low static or length of run, and the 20 x 20 grille was an adequate addition to the existing return air.

Also you never mention it being a filter grill so with the 4" filters mentioned, sounds like it is back at the unit?

Perhaps it is not the amount or size of the return but also the supply or plenums.

Much more info would be needed to give you a accurate answer.
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oceanliner
Original Author
13 years ago
Sorry for the confusion.

My grills do not contain any filter since the filter is located inline in the air handler.

My system has two 18" returns: an existing 18 inch return and a newly added 18 inch return. The existing 18" return is on a 24x36 grill and is connected to the end of the plenum (My system runs at 1750 CFM on high. Although both returns are 18", I noticed that the existing return is a lot more noiser than the new one (returns are side-by-side) which indicates that the existing return is pulling lots more air than the new one.

For the new return, is a 20x20 grill (without filter) too small for a 18 inch pipe(10 feet)?
david_cary
13 years ago
Ok so there is no exact answer. Yes a 20x20 flows less than a 10 foot 18 inch pipe. They are all just resistance points/paths. You would get more flow from a larger grill and it would be a better match to that size pipe.

But - if the return is not making much noise, then it probably isn't a significant resistance to flow. And the larger grills are larger visual grills. If you don't care, then change it out but it may not make any significant difference.

Is 20x20 grill large enough for 18 inch return?

Comments (4)

zippyhvac

zl700

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Is 20x20 grill large enough for 18 inch return?

Comments (4)

zippyhvac

zl700

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