Using dryer with rigid duct and 3" outdoor vent

The previous owner of my house had put in a 3" dryer outdoor vent. Why I don't know, perhaps this was the size years ago, but in any event I thought I would compensate for this by running all rigid duct from my dryer to the vent. I ran about 8 feet of 4" duct with 2 elbows: one at the dryer outlet in the back, and one at a 45 where the duct meets the outdoor vent. Then there is a 4"-3" adapter at the outdoor vent. Being the fact that I ran all smooth duct, is this enough to compensate for the 3" opening to the outside, or do I still need to get a 4" vent?
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On Tue, 6 Jan 2009 04:47:13 -0800 (PST), Mikepier

    I don't know the specs for your drier. They vary so what is OK with one might not be OK with another. I suspect you will be OK but remember that by providing that 4" vent you will reduce back pressure and that will mean faster drying, and longer drier fan life. Over all I would want the 4 Inch vent.
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If it will make you sleep more soundly at night, get the bigger vent. Otherwise quit overanalyzing things and do your laundry.
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*That little 3" vent is enough to cause resistance in the air flow. Give the factory a call and see what they say, but I say 4" all the way.
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I checked GE online for the specs on my dryer. Although this is not the exact model of mine (about 8 years old), it is equivelant. On page 2 shows the max duct length chart. Notice how there is a column for 4" cap and another for 2 1/2" cap. Does this mean since I'm using only 8 feet rigid duct w/2 elbows it should be fine? Or is this chart misleading. http://products.geappliances.com/MarketingObjectRetrieval/Dispatcher?RequestType=PDF&Name70213_dvlr223gg_r3.pdf
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Mikepier wrote:

http://products.geappliances.com/MarketingObjectRetrieval/Dispatcher?RequestType=PDF&Name70213_dvlr223gg_r3.pdf
Dunno. I've got thirty feet of 4" PVC ducting (I guess you'd call that 'rigid') with six elbows (2 are 45) and my dryer does what it's supposed to do. 'Course the dryer is 15 years old and the ducting has been in place for only two years (previously it dumped into the attic! Goddamn roofers), so maybe the whole shebang will deteriorate over time...
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http://products.geappliances.com/MarketingObjectRetrieval/Dispatcher?RequestType=PDF&Name70213_dvlr223gg_r3.pdf
*That 2.5" opening is the flap on the vent opening, not the duct diameter.
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Seems odd they would make a 4" dryer vent with only a 2.5" flap. What is the purpose of that?
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wrote:

Seems odd they would make a 4" dryer vent with only a 2.5" flap. What is the purpose of that?
*You are looking at it wrong. The flap swings open 2.5". The actual opening area is much greater. It might be 2.5 inches by 4.25 inches wide.
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On Tue, 6 Jan 2009 04:47:13 -0800 (PST), Mikepier

You'll get better air movement with a 4" vent. I was not aware there are 3" dryer vents, but I've seen plenty of 6" vents.
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On Tue, 6 Jan 2009 04:47:13 -0800 (PST), Mikepier

From a fluidics standpoint, the cross sectional area is proportional to the airflow. So,
area = Pi*r^2 area (of 3") = 7.06 area (of 4") = 12.5
A 77% more flow in 4" compared to 3".
Reducers, turns, smoothness, length, pipe size, lint buildup all effect airflow.
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Looks like I'll be changing it to a 4" duct soon. It's not that hard, I just have to make the 3" hole now 4".
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I changed it over the weekend, and it seems dryer performance has improved.
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