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Old 10-09-2012, 07:24 PM   #1
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Return drop sizing


I need to get a furnace replaced and I've had a bunch of estimates. Some want to replace the drop and others done. The ones who don't are not being considered. I need enlarged sized ductwork since I have a 80k BTU furnace need and a 3ton AC need. In any case, the ductwork is sized too small and needs to be enlarged.

My question is, for a furnace/AC need as I have, and for a 16x25 filter frame, what is a properly sized return drop? Some places want to replace it with a 20x14 and others want to use a 20x8. One wants to use a 22x10. These numbers all all over. Are they too small? I have some numbers in my head, and none of the estimates jive with what I understood the need to be.

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Old 10-09-2012, 07:48 PM   #2
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Return drop sizing


22X10 is ok.

What size is your old furnace.

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Old 10-09-2012, 09:20 PM   #3
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Return drop sizing


144 squareinches per ton.. equals-432square inches minimum for the return

Last edited by Nickhvactech; 10-09-2012 at 09:40 PM.
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Old 10-09-2012, 09:36 PM   #4
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Return drop sizing


I think your thinking of the return grille. 144 sq in per ton.

A 10X20 sheet metal duct will move 1200CFM at a friction rate of .042" per 100 foot of total equivalent length, at a velocity of 864PM, which is more then acceptable.
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Old 10-09-2012, 09:41 PM   #5
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Return drop sizing


I understand the question now!! Reading too quickly
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Old 10-10-2012, 09:29 AM   #6
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Return drop sizing


At this point, my next question is:

Is one 16x25 filter frame inadequate??
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Old 10-10-2012, 04:11 PM   #7
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Return drop sizing


16X25 return air filter grille, or 16X25 return air filter bracket.

for a bracket, the velocity is 433FPM, which is ok, a little higher then preferred. For a return air filter grille, its too small.

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