Cold Air Return Size Question

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Old 12-17-18, 06:27 PM
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Cold Air Return Size Question

So I have a question about a cold return duct/vent. My house has a 18" width return duct vent in the main hall way. There is literally another return vent down the hall 10 feet away that is 12" in width. So we started a basement re model and in the basement the 18" vent is sucked back by the most jankie air return box that looks like a 5 year old could have done a better job fitting it and sealing it. so after removing this box it is clear that two hold were cut in the sub floor between the studs leaving a stud between the two holes. So going around the home each return vent has good suction but the 18" one. and by good suction i mean putting a one dollar bill on the vent and it is sucked to the vent. Now to the actual question i was told by a friend who is in the heating and cooling field he believes the whole ghetto box assembly in the basement could be removed with a only one of the holes in the floor being used to be a return vent, As he put it if not suction is happening, the box is leaking to much and not allowing a good pull down.

Any help is appreciated
 
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Old 12-17-18, 07:34 PM
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Welcome to the forums.

I am not a duct pro. My personal opinion is the more return the better.

If your friend is a pro....... have him measure the return static pressure with and without that second hallway register to see if it's needed. It may have been a less than perfect installation but it was still allowing return air in.
 
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Old 12-17-18, 07:47 PM
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It's going to be asked so....what is the size of your system. I'm guessing it is a split system. Do you have Heating and cooling? 18" X ?...12" X ?...

How many returns do you have? What you type indicates at least 3?

I'm not sure "the dollar bill test" in itself give any useful information. Also pics are always a plus.

Why do you doubt your HVAC friend?
 
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Old 12-17-18, 08:07 PM
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I may have worded it wrong but I was not told to delete the 18” return, but yet make it the same size as the12” one that is 10 feet away. To explain this better, imagine standing in the basement and you look up between the joist and see a 18” long by 10” constructed sheet metal box that was constructed and just looking at you can see how poorly it was made, not sealed in any fashion with big gaps. made by someone who just wanted to get it done and move on to the next house. So you remove that box and inside of the box in the subfloor is two rectangular holes, almost where you would put two boots for air returns, except they created this box. The wall is load bearing so they made two holes in between the studs. Now what I was saying is because this box was so poorly made it was sucking so much air from the basement and not sucking any upstairs like it was intended to.

now to answer your questions.
Yes it is a split system ac and furnace
each bedroom has a supply and return
each bathroom has supply. The kitchen living room have 6 supply and two returns one being the 18” that was not drawing and pressure.

I know the the dollar bill test is not a true test but it was something I always heard that an air return should have enough force to hold a dollar bill to its grill.

Its not not I don’t trust my friend but I was just curious and he now lives up north so him getting down here would be a couple week wait and I was just looking for some extra input.
 
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Old 12-17-18, 08:37 PM
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I am also not a pro so take this with a grain of salt. Air balancing that system would seem difficult at best with that many returns in the house. You'd need to know the size of your system (Tons) and the total size of the returns going into your furnace (or return plenum) to determine if your returns are adequate (kinda sounds like they are). My concern however with that many returns, you run into balancing out the paths of resistances for them all, i.e. maybe the basement return was closest thus less resistance thus more return air coming from there.

Out of curiosity, does the 18" duct return air from the Hallway and also the basement. i.e. is it open to both? You might have to wind up getting an anemometer to try and balance out your returns. Or just trial and error it yourself with some cardboard blocking part of the vents (though that is not as representative since your ducts after the cardboard remain the same size, you could wind u with some sort of venturi sound).
 
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Old 12-18-18, 09:06 AM
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I like the dollar bill test! It tells me you are probably undersized on return so removing one will hurt. (Returns should be sized with the least amount of resistance so the blower can pull the air.)
 
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