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Grille Size vs Duct Size

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Grille Size vs Duct Size StarkShark 10-29-2007
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Posted by Lloyd E. Sponenburgh on October 30, 2007, 10:00 am

> Thanks for the civil answer, anyway. I'll probably do that.
>

You may be due some reading comprehension practice, too. There were
answers in all the natter that you missed; your jerking knee got in front
of your eyes.

Succinctly; With consumer filters, NO, you cannot reduce the size of the
filter box to the size of the duct, any more than you could stuff a wad
of cotton in your "straw" analogy, and expect to get the same amount of
air through it.

LLoyd

Posted by StarkShark on October 30, 2007, 10:10 am
"Lloyd E. Sponenburgh" <lloydspinsidemindspring.com> wrote in

>
>> Thanks for the civil answer, anyway. I'll probably do that.
>>
>
> You may be due some reading comprehension practice, too. There were
> answers in all the natter that you missed; your jerking knee got in
> front of your eyes.
You are quite right. I was not expecting to be attacked for asking a
question. It sort of caught me off guard.


> Succinctly; With consumer filters, NO, you cannot reduce the size of
> the filter box to the size of the duct, any more than you could stuff
> a wad of cotton in your "straw" analogy, and expect to get the same
> amount of air through it.
I think I understand that. The larger the surface area, the slower the
air moves through any given area of the filter, thus increasing filter
effectiveness and life, as well as reducing the resistance to air
movement. It's the opposite of the Venturi effect.

However, I'm not talking about reducing the filter size. I failed to
mention that the filter is at the furnace. I just wanted to reduce the
opening of the pipe that leads to the filter. From the perspective of
surface area, the opening to the pipe is three times the pipe itself.
It's like a pipe hanging in a doorway. Can't I make it a pipe hanging in
a window?
I know, I know. Call a pro. Sigh.

Thank you also for a reasonable reply.
Stark

>
> LLoyd



--

Posted by Lloyd E. Sponenburgh on October 30, 2007, 10:44 am

> However, I'm not talking about reducing the filter size. I failed to
> mention that the filter is at the furnace. I just wanted to reduce
> the opening of the pipe that leads to the filter. From the
> perspective of surface area, the opening to the pipe is three times
> the pipe itself. It's like a pipe hanging in a doorway. Can't I make
> it a pipe hanging in a window?
> I know, I know. Call a pro. Sigh.

If you can figure out the effective open area of the grille that will
cover it, sure. But the grill restricts air not only by its "closed"
area, but by friction effects (The vanes "zig-zag" so the air is
deflected going through, deliberately obscuring a visual line to the open
ductwork).

That's why competent pros have tables and nomograms describing all that
stuff. For cosmetic reasons, nobody wants exposed elements to be bigger
than they must be. A good HVAC technician can minimize the openings
without minimizing performance.

LLoyd

Posted by StarkShark on October 30, 2007, 10:18 pm
"Lloyd E. Sponenburgh" <lloydspinsidemindspring.com> wrote in

>
>> However, I'm not talking about reducing the filter size. I failed to
>> mention that the filter is at the furnace. I just wanted to reduce
>> the opening of the pipe that leads to the filter. From the
>> perspective of surface area, the opening to the pipe is three times
>> the pipe itself. It's like a pipe hanging in a doorway. Can't I
>> make it a pipe hanging in a window?
>> I know, I know. Call a pro. Sigh.
>
> If you can figure out the effective open area of the grille that will
> cover it, sure. But the grill restricts air not only by its "closed"
> area, but by friction effects (The vanes "zig-zag" so the air is
> deflected going through, deliberately obscuring a visual line to the
> open ductwork).
>
> That's why competent pros have tables and nomograms describing all
> that stuff. For cosmetic reasons, nobody wants exposed elements to be
> bigger than they must be. A good HVAC technician can minimize the
> openings without minimizing performance.
>
> LLoyd
"Effective open area" is where the trouble lies. I get it.
My grille is huge, with only straight vanes. I can see right in to the
duct. You are right...I don't want that ugly thing any bigger than it
has to be. I understand now why it's not as simple as I thought.
Thanks.



--

Posted by Bubba on October 30, 2007, 6:05 pm
wrote:

>
>> StarkShark wrote:
>>> The heating and cooling in my house is functioning well. One room
>>> upstairs is a little cool when the heat is on, a little warm when the
>>> AC is on, but that's about it. It's a forced air system that was
>>> installed to replace a gravity feed coal furnace.
>>>
>>> The system has two air returns. One in the living room, about 12
>>> inches square, and a giant one in the kitchen, 24 X 30, or 720 square
>>> inches. The duct in the kitchen is formed from two 10 inch deep
>>> joists. As I see it, the joists form a return duct about 28 X 10, or
>>> 280 square inches.
>>>
>>> With all else remaining the same, can I reduce the opening in the
>>> floor to about 10 x 30, or 300 square inches without harming the
>>> intake volume of the duct?
>>>
>>> To me, it looks like breathing through a straw. You can put a funnel
>>> on the straw, but it won't make you get any more air through the
>>> straw. So does the opening in the floor need to be so huge?
>>>
>>> I suspect it's so large only because the duct was that way when the
>>> furnace was installed.
>>> Thanks
>>
>> Just like writing code. Call your local HVAC guy and see what he
>> thinks you should do. It's simple. A lot simpler than finding
>> someone to write computer C++ Code.
>>
>
>I was trying to avoid paying a bunch of money for a 5 minute visit to get
>the answer to a question I /thought/ was simple.

Wouldnt it be so nice if everything in this world was free. Maybe if a
company sent out a newbie and he spent an hour to answer your so
called 5 min problem, then maybe you'd feel better about paying a
bill?

> I also thought I would
>be asking HVAC guys in this group what they think I should do. The
>answer is obvious: hire them.

Maybe you arent so dumb after all?
>
>I think I'll try that attitude in comp.lang hierarchy and see how far it
>gets me. Or maybe in a computer repair forum on the web: "You want to
>learn how to fix your hard drive? Call a pro and give up."
>
>Thanks for the civil answer, anyway. I'll probably do that.


Your welcome,
Bubba

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