Grille Size vs Duct Size

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
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I can't *see* it from here, and your missing 90% of the required information to even be able to make an educated guess. Call your *LOCAL*, competent, licensed, insured, professionally trained, HVAC technician.
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The OP missed one important thing. That "funnel" is covered by a web of sometimes open, sometimes mostly-closed webbing called a "filter". It restricts the airflow, such that the filter area must be larger than the duct, unless you use deep-pleated filters (which it ain't likely your system is equipped with).
The competent local HVAC guy can tell you what the filter area needs to be for a specific air flow and filter type.
LLoyd
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"giant one in the kitchen"
Against Code
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kjpro @ usenet.com wrote:

No Returns in basements, unless gas (oil) furnaces, water heaters & gas dryers are tightly sealed off separate from the Return area.
Why wasn't a big Return Run put upstairs for cooling? Do you live in a hot climate? - udarrell
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thought the question might be simple.
The house was built about 1903 with a coal furnace. In the last hundred years it has had an upstairs, two additions and a bathroom added, and at some point (perhaps after the additions) the furnace was changed to forced air. The present unit is only about 10 years old. The filter is 20 by 30 inches and is inserted in a slot at the furnace, just before the air enters the fan chamber.
The room that is now the kitchen may not have been the kitchen a 100 years ago, so it may not have been built with the return in the kitchen. I have no way of knowing why the builders did not include a cold air return upstairs, but it may have been motivated by the desire to avoid disturbing the lathe and plaster walls.
I also do not know why a "*LOCAL*, competent, licensed, insured, professionally trained, HVAC technician" did not move the air return to ensure the system was up to code. I was not here when the furnace was installed. I know that it was installed by a "*LOCAL*, competent, licensed, insured, professionally trained, HVAC technician" because the company's faded sticker is on the side of the furnace.
I'm in northern KY. Midwest climate with mild winters and hot summers. The unit's ability to keep the house comfortable is not in question. I did mention that only one room runs slightly warm in summer and slightly cool in winter, but that was technically not relevant to my question.
I think I failed to ask my question correctly. If I were to ask if the opening to the duct could be made larger, I suspect the answer would be "Sure, make it as large as you want it. Just don't change the length or size of the duct itself." So if I ask it the other way, "If all things remain exactly the same, can the opening to the duct be reduced?"
I thought the answer would be easy, like, "Sure. Just don't make the total surface area of the opening smaller than the surface area of the pipe." Or perhaps there would be a rule of thumb like, "The surface are of the opening to the duct must be the same as the surface area of the duct multiplied by 1.25" or something like that.
I have read a huge amount of HVAC information over the past week, and can't seem to find the answer. So what am I missing?
I'm not trying to re-size the duct or make it longer or shorter. I am aware of friction coefficients and angles and turbulence and the limiting effect of air grilles.
I also know that surface area isn't really appropriate for talking about pipes, but I'm working with the size of the opening. If the size of the opening is larger than the size of the duct, then that looks like a funnel to me.
Am I so far out in left field that I don't even know I'm playing hockey, or am I at least in the ball park?
Udarrell, thank you for your response. If someone asked me how polymorphism can be used in his or her C++ program, and I responded, "call a college educated programmer with a PHD to write the code for you," I should think that I would be considered rude.
Stark
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But it is now and its against code.

Because the Homeowner was to cheap to do it right.

Has a faded sticker, so they have to be competent... LOL

You didn't like our answers, TOUGH SHIT.

The right book.

Point?
Most ball parks are fairly large.

Too bad for them.
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isn't very strong. You'll need to work on that. Find the right book about reading. I was describing the condition of the sticker, not the ability of the technician. It was the name printed on the sticker that brought me to the belief that a "competent" installer did the work...a long time ago, as suggested by the faded condition of the sticker.

comprehension.
near the ball. Okay.

some sleep. You obviously make some good points, but they don't need to be couched in scorn.
You're beginning to make me suspect I stepped in troll feces.
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Your close.... but its homeowner feces. Homeowners belong in alt.home.repair, not here. this is a forum for professionals *IN THE HVAC TRADE*.
Most homeowners/slumlords come in here expecting us to tell them how to fix their broken 30 year old, Jurassic, furnace for free, or are not happy with their heating and cooling system that was either self installed, or installed by the lowest bidder. They get pissy when they get told that its going to cost them money to get it done right. They expect their system is supposed to last forever, but they trade in their $50,000 SUV every other year. FWIW, the chance of some 'moaner coming on here being one of *MY* customers, or being in my service area, is about 1 in 4 BILLION.
Its kind of like the idiot that thinks they can price shop over the phone and call asking "How much is an air conditioner??" when they don't have a clue what they have or size or efficiency or.... Try calling a car dealership and ask them "How much is a car??". you get the idea.
Its not our problem when they get told what they don't want to hear. I have to deal with my own customer base and tell them when its time to replace their system....its not what they want to hear, but they know its what they *need* to hear. They know its going to cost them $$$$$$$$ to replace their system when its time. My customers also know they will get their moneys worth.....they don't care about low price, they just want it done right.
Until you can actually show somebody the difference, and the results, they won't know whats right and whats not. You can get it done by a *COMPETENT*, licensed, insured, professionally trained, Master HVAC Technician, or you can get it done cheap. Its your choice.
In the mean time, please go troll alt.home repair......not here.
Thanx
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keep adding features to a program, but don't want to pay for the work.

I received was "A better place to ask this would be alt.home.repair" I would have said thank you and moved on.

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I read and comprehend just fine... You again state, "It was the name printed on the sticker that brought me to the belief that a "competent" installer"
That says you thought the installer was competent because of a sticker, Period.
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But I like having a big grill in the kitchen--can fit quite a few more burgers...
--






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Simon Schnizzard wrote:

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StarkShark wrote:

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.
--
Zyp



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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. I also thought I would be asking HVAC guys in this group what they think I should do. The answer is obvious: hire them.
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.
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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
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"Lloyd E. Sponenburgh" <lloydspinsidemindspring.com> wrote in

question. It sort of caught me off guard.

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

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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
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"Lloyd E. Sponenburgh" <lloydspinsidemindspring.com> wrote in

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.
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wrote:

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?

Maybe you arent so dumb after all?

Your welcome, Bubba
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