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Old 05-01-2020, 05:24 PM   #1
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Question about duct sizing...


I need to run some 4" round pipe to a 4" round 90 degree boot. When ordering the oval pipe, am I ordering a 4" oval or a 5" oval since mashing a 4" round gives a 5" oval (see image)?


Hopefully that makes sense.


VB,
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Old 05-01-2020, 09:03 PM   #2
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Re: Question about duct sizing...


Why do you need oval pipe at all?

Which part is oval?

Oval of the same diameter has worse airflow characteristics than round/circle.
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Old 05-02-2020, 06:52 AM   #3
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Re: Question about duct sizing...


The round side is 4".
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Old 05-02-2020, 10:35 AM   #4
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Re: Question about duct sizing...


The standard round to oval will measure 3-1/2 x 4-7/16 on the oval side. It's normally used to run duct inside a wall bay and use off the shelf duct. The oval duct has slightly less cross section than a round duct of the same trade size. (approx 3% less)
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Old 05-02-2020, 10:54 AM   #5
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Re: Question about duct sizing...


Quote:
Originally Posted by surferdude2 View Post
The standard round to oval will measure 3-1/2 x 4-7/16 on the oval side. It's normally used to run duct inside a wall bay and use off the shelf duct. The oval duct has slightly less cross section than a round duct of the same trade size. (approx 3% less)

Thanks for the reply.


So If I need to order oval 12', do I order 5" oval? I think the answer is yes...
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Old 05-02-2020, 12:13 PM   #6
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Re: Question about duct sizing...


If you already have the ell, measure the oval side. If it measures approximately as I said above, you will need 4" oval duct to fit.

If you don't have the ell already, order a 4x4 round to oval ell and 4" oval duct.

Tip/hint: The circumference of a round duct and a comparable size oval duct is exactly the same.

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Old 05-02-2020, 01:06 PM   #7
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Re: Question about duct sizing...


Who sells 4" oval? I'm not seeing a lot of options out there...
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Old 05-02-2020, 01:18 PM   #8
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Re: Question about duct sizing...


Friction rate is not determined by cross sectional area, but by shape and internal material surface area.

I doubt 4" oval would flow much air at all, regular 4" is generally used for under 40 cfm. (depends on the friction rate)
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Old 05-02-2020, 01:35 PM   #9
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Re: Question about duct sizing...


I have never used any 4" oval duct on anything. I suppose there is some application for it in radon runs to the roof via interior wall cavities but I'm happy to say I've never had to do that. lol
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Old 05-02-2020, 01:42 PM   #10
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Re: Question about duct sizing...


yah even regular 4" i've only seen serving small bathrooms and walk in closets.
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Old 05-02-2020, 01:48 PM   #11
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Re: Question about duct sizing...


So, maybe I'm looking for the wrong pipe then and sending you guys on a wild goose chase...

I'm plumbing a dryer vent which is 4" but is going through a Dryerbox 400 with an oval opening of 4 7/8" x 3 1/8".



So is that 5" oval pipe or 4" oval pipe?
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Old 05-02-2020, 02:00 PM   #12
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Re: Question about duct sizing...


That doesn't seem like a regular oval duct shape.

Rigid metal duct comes with without the seam joined, so you may be able to it and form it into the right oval shape.

I don't know if you'll need 4 or 5" as the inlet of the box doesn't look like a standard size.
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Old 05-02-2020, 03:28 PM   #13
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Re: Question about duct sizing...


The wall box you have is for a retrofit install where the duct is already in the wall and may just be compressed 4" round duct. You can't use that box on an existing 2x4 wall unless you remove the wall board to install the 4" round duct in a collapsed fashion. You are expected to feed the existing hard duct through that oval shaped hole and connect it to the flex duct from the dryer. edit: They mention the need for a shoe horn type of tool... I don't doubt the need.

The recessed opening provided by the box allows the flex duct to bend without crimping and allows the dryer to be pushed back nearly against the wall. I see no use for the ell you picture if you are doing a standard install. No transition is needed since the flex will fit the 4" duct even if it's deformed.

A standard installation requires a 2x6 wall or 1" furring on an existing 2x4 wall. There is a different wall box for that use. It has a round hole.

If you do any hard duct install, be sure to keep it smooth inside... no screws penetrating... snap joints together and wrap with foil tape. Even well installed dryer venting is a hazard at best and needs to be kept clean.

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Old 05-02-2020, 04:45 PM   #14
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Re: Question about duct sizing...


So, using a 4" round duct will indeed work with the box as they suggest.

I'm wanting to use a 90 degree oval attaching to the 4" round. What size oval duct 90 degree do I need to purchase, a 4" or a 5"?

Something like this.... I can't use round it has to be oval.
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Old 05-04-2020, 07:10 AM   #15
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Re: Question about duct sizing...


Quote:
Originally Posted by surferdude2 View Post
The standard round to oval will measure 3-1/2 x 4-7/16 on the oval side. It's normally used to run duct inside a wall bay and use off the shelf duct. The oval duct has slightly less cross section than a round duct of the same trade size. (approx 3% less)

Will a 4" round connect directly to a 4" oval? The measurements look close to what you posted but since I don't work in the industry I'm guessing.
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