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So I asked a long-winded version of this question in another thread, but haven't really received any good responses yet. I'm going to condense and try again...

There is an 8" duct feeding a 6x12 register in my kitchen, that I need to relocate. By doing nothing else, will changing the size of the register to say a 10x10 or 12x12 provide more air and/or better distribution at all? I am pretty sure I need more "washing" of the exterior windows/doors in the area, so I am relocating the register to the center of the room (and will get a 4-way type as well), but I also wondered if going any bigger would help any too. And if so, what would be the biggest size?

Just speaking in generalities here and it doesn't have to be exact...

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Given a circular 8" duct you are dealing with an airflow area of about 50 sq/in. You currently have 72 sq/in opening. I haven't seen a picture of your register - some are not as open as others so it is hard for me to know for sure but... with 72 sq/in opening the air would just dribble out.

This is fine for a floor vent that is unobstructed by walls and furniture but usually not what you are going for. You are usually looking to push the air out a few feet out into the room. A smaller register does this. Also somewhat closing a register does this - however this could push air at unadvantageous angles and you don't want air hitting walls and furniture.

Most registers I size are about the same sq/in's as the duct or a little less. I am looking for the air to be pushed out hard so that it creates airflow in the room. Big rooms with oversized registers can have hot spots away from the register. Now the downside of going to small with your register is that it is too loud. Unless you are severely undersizing your register, the noise is really the only issue.

So a bigger register will not help cool or heat your room better. Putting the register in a better location and making sure the air is shooting out in an efficient manner and at an efficient angle is key.

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  • Great - Thanks for the feedback! As I mentioned in my other thread, I am pretty sure the rest of the duct work is undersized for the A/C unit, so the velocity may be a bit higher than it should be as all my registers are a bit noisy. This particular duct is being fed by a 12" branch off the main, which is also shared by another 8" to an adjacent room... I normally keep the other register partially-closed, so could that justify a slightly larger register where I want it? In other words, I could divert most (or all) the CFM's from the 12" branch to this 8" duct, so maybe I should upsize a bit?? – Joe in Miami Apr 14 '15 at 18:56
  • If you have two registers on the same run the size of one will effect the other one in negative proportion for sure. This is also dependent on how much they are opened. If you want to be able to "play" with the airflow then bigger registers are the way to go. Make sure when buying the registers that you get the ones with the inside diffusers. – DMoore Apr 14 '15 at 18:59
  • Cool. Good tip. Yeah, the other register on this branch is cooling a small area that never really gets warm, so I can definitely divert some flow away from that area and not impact it very much. But the kitchen is only 11'x12', so should I really need anything larger than an 8" duct supplying that area anyway? Would converting it a 10" back at the Y-junction be too much for that size of space? – Joe in Miami Apr 14 '15 at 19:24
  • It all depends on the pressure at that point in the duct. I wouldn't put the Y in because this just never works out like you think it will. But it is feasible if there is enough current pressure that it could sustain to small registers. – DMoore Apr 14 '15 at 20:02

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