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Old 09-02-2010, 11:09 PM   #16
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Changing duct size


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My 20x30 is actually more cubic inches than I need for my unit.
How do you figure that?

A 20X30 return grille isn't right for a 3 ton unit. Isn't yours 3.5 tons?

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Old 09-03-2010, 06:38 AM   #17
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Changing duct size


well the spec sheet calls for 2 12x16 so 20x30 is more cubic in isnt more better than less
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Old 09-03-2010, 06:59 PM   #18
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Changing duct size


20X30X2=1200CFM, with some noise, and a some what high resistance to air flow.
20X30X1.8=960CFM without noise.
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Old 09-03-2010, 11:17 PM   #19
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Changing duct size


So I should go up to a 20 x 36 I guess that is about the biggest I can go. But wont the size in the exsisting wall just allow the same amount through or no.
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Old 09-04-2010, 02:51 AM   #20
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Changing duct size


If your blower is strong enough. It can pull the same amount through. Doubtfull you have a 2 horse power blower motor though.

Blowers can only move so much air against so much resistance. So the idea is to minimize the amount of resistance the blower has to work against.

What size is the duct behind the grille.
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Old 09-04-2010, 07:02 AM   #21
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Changing duct size


It is a 12x28 cavity - 4 2x4's running up the corners. When I put the filter in should it feel like it is going suck the filter throught the opening? It seems like there is tons of suction the filter almost colllapsis. blower is 3/4-4-psc 559 9.5 amps full speed cooling speed med-high
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Old 09-04-2010, 07:17 AM   #22
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Changing duct size


No it shouldn't. When it does, that is also another indication. That your return is under sized.

Proper return should only have the air moving through the air filter at 300 FPM.

A 12X28 opening would be moving 1400 CFM at 600 FPM. While that FPM is fine for the duct work. A 20X30 return air filer grille has the air moving through it at 500FPM. And at that velocity, it will suck the filter in like you said.
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Old 09-04-2010, 08:23 AM   #23
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Changing duct size


oh ok so I should go ahead and find a bigger grill opening. I My living room is now cooling to the same temp as the return air area now even 1 deg cooler I will see when it is dead heat outside middle of the day. I still am going to put another vent in that way I can open my other dampers up some more and let everything evenout. So for sure if I upsize my filter it will not harm my unit?
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Old 09-04-2010, 08:38 AM   #24
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Won't harm it at all.

Darn hard to have too much return air. And if you did. It still wouldn't harm the unit.Lack of return air however, can harm a unit.

On a proper return. You should hear no noise at the return grille. A tissue paper should barely move when held 2" away from the return grille.
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Old 09-04-2010, 08:41 AM   #25
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Changing duct size


so should I go 20x32 or 20x36 they make a 20x40 but I think thats overkill plus too much$. It is crazy that my little tiny house needs a bigger return but I guess they just put in a 20x30 cause it is more popular.
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Old 09-04-2010, 08:50 AM   #26
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Changing duct size


the toilet paper if I have 2 squares at 10" pulls towrd the reture at 6" it will suck right to the grill. But don't you want suction it seems like if it is sucking hard that that just means more air coming in I would think.
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Old 09-04-2010, 08:57 AM   #27
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Changing duct size


Your system is probably over sized, and doesn't really need 3.5 tons. Which is one of the reasons for the uneven temps in the house.

Its not the size of the house in sq ft that determines the amount or size of the return grille/grilles. But the CFM requirements of the system/unit.

In a small restaurant(800 sq ft). I have 6.5 tons of A/C in it. It has 3-22X22 egg crate ceiling returns in it. Each connected to a 16" flex duct. 900 CFM per flex duct. Done for noise reduction.
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Old 09-04-2010, 09:05 AM   #28
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Changing duct size


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Originally Posted by sjthompson View Post
the toilet paper if I have 2 squares at 10" pulls towrd the reture at 6" it will suck right to the grill. But don't you want suction it seems like if it is sucking hard that that just means more air coming in I would think.

No, it just means you ave a grille with a very high velocity air moving through it. but not as much volume as it should have.

Think of it this way:

A 2 seat car, with 2 people in it, traveling at 100 MPH going to a ball game.
And a mini van traveling at 60 MPH with 6 people in it, going to the same game, the same distance away.

The car has more velocity, but is moving less people then the mini van. So its doing less work, but working the engine harder.

Your return is the car. When it should be the mini van.

Your system is trying to move 105 pounds of air a minute through that return. But probably only moving 80 pounds a minute.
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Old 09-04-2010, 09:07 AM   #29
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Changing duct size


huh well I fiqured it was alittle big so should I even mess with it or wait until it croaks and change it all out to the new stlye rayon or whatever? If rework the duct could I achieve a balanced system? Also I could probly run 1 extra 6" in the living and a 4" in my laundry since its oversized and not hurt anything huh?
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Old 09-04-2010, 09:09 AM   #30
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Changing duct size


here is the area of return air
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