Upgrade from 5" to 6" branch duct, and return register vs duct size


  #1  
Old 08-24-16, 03:03 PM
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Upgrade from 5" to 6" branch duct, and return register vs duct size

Three separate questions, relating to new ductwork going in after asbestos abatement.

I know there are many factors that go into properly sizing a system, but would there generally be harm in going from 5" round duct to 6" round duct? The kitchen is always the hottest room of the house due to its location and big windows, and we're trying to get a little extra air to it. I'll note that the old 5" duct appears to be from the previous HVAC system that somebody just hooked a new system up to in the mid-90's. In fact, ALL of the ductwork (supply and return) seems to be legacy, as much of it was covered in asbestos). I have no idea if it was professionally sized or not, but since one of the start collars fell into the trunk because it wasn't secured, I'm guessing a professional wasn't involved.

Second ... if a return trunk has a register opening of 168 square inches, but the duct only has an area of 96 square inches, is the limiting factor the duct? In other words, could one shrink the register opening to 96 square inches with no change in performance?

Third... if a return trunk has a register opening of 234 square inches, and a duct size of 240 square inches... but is also has the above mentioned 96 square inch duct feeding into it, what's going to happen?

Specifically in this case, it's a floor return register that goes into a 10x24 trunk. But right under the floor register is also a 12x8 duct from another return. I would attach pictures, but with half the sheet metal missing due to the asbestos work, they'd be pointless. So here's a crummy drawing.

And yes, that 12x8 rectangular duct is the one I was talking about replacing with 8" round in another thread.

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Old 08-26-16, 12:20 PM
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6 in duct will only gain you about 30 cfm. can you do a 7in? the 10x24 duct will only move about 1100 cfm of air.
 
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Old 08-26-16, 06:07 PM
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It's iffy. It's coming off of a 8x16 trunk (off of the 8" side), and going through a 6.5" space. Could probably make it work with some luck.

If it matters, I also have two 5", one 6" and one 3.5x12" off of the trunk. Should have enough CFM to spare to go up to 7"?
 
 

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