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Duct Sizing Question Richard 12-08-2006
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Posted by Richard on December 8, 2006, 7:00 am
If I'm using 8" round duct to facilitate
my returns, shouldn't the register boots
and the takeoffs have the same area as
the 8" pipe?

I have the same question with the supply
boots as well. If the area of 8" round is
100 square inches, why then are there so
many boots with dimensions like:

4 x 10, 6 x 10, 8 x 10 ?

It appears none of these boot openings or
takeoffs have adequate cross-sectional areas
to facilitate the round duct size I'm running to
them. Won't the air volume be limited by the
smallest piece in the branch?

I do realize this question is probably stupid, but
my brain short circuited on this for some reason.

Thanks in advance for any help.


Richard



Posted by daytona° on December 8, 2006, 7:19 am
were is you air duct calculator?
8" rd duct only has a 220 cfm capability with .10 static
so, (2) 6" returns will be the max or you will get whistling


> If I'm using 8" round duct to facilitate
> my returns, shouldn't the register boots
> and the takeoffs have the same area as
> the 8" pipe?
>
> I have the same question with the supply
> boots as well. If the area of 8" round is
> 100 square inches, why then are there so
> many boots with dimensions like:
>
> 4 x 10, 6 x 10, 8 x 10 ?
>
> It appears none of these boot openings or
> takeoffs have adequate cross-sectional areas
> to facilitate the round duct size I'm running to
> them. Won't the air volume be limited by the
> smallest piece in the branch?
>
> I do realize this question is probably stupid, but
> my brain short circuited on this for some reason.
>
> Thanks in advance for any help.
>
>
> Richard
>



Posted by Steve Scott on December 8, 2006, 8:58 am
8" round is about 50 sq in, not 100. The max velocity you want in a
return run in residential is 700fpm. We try to keep face velocity at
the grille under 500fpm.

wrote:

>If I'm using 8" round duct to facilitate
>my returns, shouldn't the register boots
>and the takeoffs have the same area as
>the 8" pipe?
>
>I have the same question with the supply
>boots as well. If the area of 8" round is
>100 square inches, why then are there so
>many boots with dimensions like:
>
>4 x 10, 6 x 10, 8 x 10 ?
>
>It appears none of these boot openings or
>takeoffs have adequate cross-sectional areas
>to facilitate the round duct size I'm running to
>them. Won't the air volume be limited by the
>smallest piece in the branch?
>
>I do realize this question is probably stupid, but
>my brain short circuited on this for some reason.
>
>Thanks in advance for any help.
>
>
>Richard
>


--
Try BlackHole -- The ultimate
compression utility!





Posted by on December 8, 2006, 9:44 am

>8" round is about 50 sq in, not 100. The max velocity you want in a
>return run in residential is 700fpm. We try to keep face velocity at
>the grille under 500fpm.



please explain how you go about doing that when a variable speed
furnace blower will vary its delivered cfm from 50 to 100%. :)




>
>wrote:
>
>>If I'm using 8" round duct to facilitate
>>my returns, shouldn't the register boots
>>and the takeoffs have the same area as
>>the 8" pipe?
>>
>>I have the same question with the supply
>>boots as well. If the area of 8" round is
>>100 square inches, why then are there so
>>many boots with dimensions like:
>>
>>4 x 10, 6 x 10, 8 x 10 ?
>>
>>It appears none of these boot openings or
>>takeoffs have adequate cross-sectional areas
>>to facilitate the round duct size I'm running to
>>them. Won't the air volume be limited by the
>>smallest piece in the branch?
>>
>>I do realize this question is probably stupid, but
>>my brain short circuited on this for some reason.
>>
>>Thanks in advance for any help.
>>
>>
>>Richard
>>


Posted by Steve Scott on December 8, 2006, 9:58 am
You're a PITA, Fish. :) But to answer your question you design for
the max air flow, but then you knew that.

IMO, the trick really comes into play when you have AC with floor
registers and a variable speed system. As far as I can tell, you're
going to have pooling. Good case for radiant floor heat and AC with
ceiling registers.

On Fri, 08 Dec 2006 14:44:27 GMT, gofish@gonefishin.net wrote:

>
>>8" round is about 50 sq in, not 100. The max velocity you want in a
>>return run in residential is 700fpm. We try to keep face velocity at
>>the grille under 500fpm.
>
>
>
> please explain how you go about doing that when a variable speed
>furnace blower will vary its delivered cfm from 50 to 100%. :)


--
Try BlackHole -- The ultimate
compression utility!





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