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I have a one-story house, and it looks like it would be easy to run new ducts myself in the attic. What should I know before trying to design a duct system myself?

I have seen ductwork that's metal rectangular with a main supply trunk that has branches coming off, I've seen it made from ductboard, I've seen pre-insulated circular ducts routed all over the place with plenum connections, etc. Which of these should be considered for a house such as mine?

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"The best" is highly subjective and not very meaningful as a question. –  The Evil Greebo Sep 10 '12 at 12:58
    
Why do you want to run new ducts? What problem are you addressing? –  Matthew Sep 10 '12 at 16:21
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You need to know how to calculate the required CFM - cubic feet per minute - that your ductwork will allow. On a room per room basis you need to determine how much air flow is needed, how large the vent needs to be, and thus how large of a duct you require. Then you need to determine how large of a trunk you require and at what points you need to reduce it after branching off legs from the trunk.

Finally you need to also know where to position the return air and how large it needs to be, in order to support the output from your HVAC.

I don't know how to calculate those things - I just know that's what you need to know.

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+1, it also helps to know how to calculate cross-sectional areas. (width * length for rectangles, Pi * radius^2 for circles). I once had a "professional" HVAC installer tell me than two 12" rounds were "way bigger" than an 18" round... –  Matthew Sep 10 '12 at 16:01
    
Well of course, everyone knows that 226 is bigger than 254... ;) –  The Evil Greebo Sep 10 '12 at 16:22
    
@TheEvilGreebo... you'd be surprised. The plan called for an 18" round and we happen to walk in on him installing two 12's because he didn't have the elbow he needed for the 18. We tried to explain it to him for like 15 minutes by doing the math on paper, etc... only to have to say "you're not getting paid unless you use an 18 or double 14s" –  Matthew Sep 10 '12 at 16:29
    
Well double 12's ARE way bigger WRT how much space you waste installing them side by side compared to a single 18... so he was right, after all! ;) –  The Evil Greebo Sep 10 '12 at 16:31
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That depends on your attic space or bulkhead though, to be fair. double 12s occupy a 12x24 space. If you were trying to fit into a bulkhead, this may be preferable to the 18x18 space... though just using a 12x24 rectangular would be even better and could meet spec... though this did not apply to my particular circumstance. –  Matthew Sep 10 '12 at 16:40
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