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Old 01-02-2010, 07:52 AM   #1
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Single or Dual Return Duct


My next question.

Can return air have two ducts from the same filter box?

My 3 ton heat pump has a 20x25x1 air return filter. The sheet metal box has two 10" flex ducts and each have about a 7' run back to the air handler. Air handler is in the crawl space under the house.

Does it matter that the air return is split into two 10" ducts?

Is one larger correctly sized duct better and two?

Thanks.

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Old 01-02-2010, 08:17 AM   #2
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Its ok to have it ran back in 2 ducts.

But, 2-10" flex are too small for the supposed 1200CFM that your 3 ton heat pump needs to move.

2-12" would be border line.
1-10" and 1-14" would be border line.
2-14" would be ok.
Or 1-16".

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Old 01-02-2010, 08:27 AM   #3
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Hi BT,

I should have expected the worse. I suspect the installer had plenty of 10" duct so they ran two vs using the correct size.

One more item to add to the fix list.

Best to have more return air capability than too little? Maybe an 18"?

Thanks.

Last edited by Earnie; 01-02-2010 at 08:30 AM.
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Old 01-02-2010, 08:40 AM   #4
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For that short of a run, 18" is ok.

Your air filter, is a bit small also.
It puts your velocity through the air filter at 500 foot per minute. Air filters tend no to do a great job at velocities above 500FPM.

500 FPM is ok, but thats max.

Your filter grille should be at least a 25X25, which would get you down to 430FPM.
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Old 01-02-2010, 09:14 AM   #5
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25"x25". Why am I not surprised.

I went and measured available space. 20x25 is the largest grill box that would fit in the location it's at.

I did read somewhere else that a person had a similar issue. They had a sheet metal company make a V frame so they could install two filters in a V shape. They then had two slightly smaller filters in the same grill box.
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Old 01-02-2010, 09:27 AM   #6
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Guessing the hold up is the 20" side.

Is there room for a 20X30. That would get you down to 450FPM.

Depending on your configuration where the 2-10" currently connect to the air handler. Might be able to have to return brackets installed.
Or 1 single large one.
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Old 01-02-2010, 03:58 PM   #7
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You are correct BT. The side dimension is the limitation.

I do have extra space in another part of the house. Adding a second grill and filter looks to be the way to resolve the single undersized filter.

Would a 14" x 20" x 1" work as the second filter?

So I would end up with:

20" x 25" x 1" with two 10" flex ducts.

14" x 20" x 1" with one 10" flex duct.

This would give the air handler return three 10" flex ducts.

Is this a correct generalization? Air returns should have plenty of CFM but low FPM to lower noise and allow the filter to work. Room supplies should be lower CFM and higher FPM to get the air out into the rooms.

Just came back from the local home improvement store. Looked through a whole wall of filters of all sizes and manufacturers and NONE stated any FPM limitations or working ranges.
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Old 01-02-2010, 05:15 PM   #8
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3-10" will be ok. Still a tad restrictive. Changing one of those 2-10" to a 12" would help.

Your generalization is basically correct.

Air filters, are rated at 300, and 500 FPM, usually.
You can sometimes find their ratings on the manufacturer's website.
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Old 01-02-2010, 05:19 PM   #9
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I'll look into changing one 10" to a 12".

Thanks BT.

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