duct size - http://www.hvacmechanic.com/ Forums
Original message
| nrd
 |  "duct size"
, posted Wed 25 Jun 21:18  
I need to update my round duct work FROM 7" diameter to something that is avaiable now either 6" diameter or 8" diameter. I have a 2 story house with no cold air returns upstairs. The round ducts are what feed the upstairs. Which size should I use? 6" or 8" ?
nrd
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Replies:
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| Freon
| "Re(1):duct size" , posted Thu 26 Jun 10:38
The size duct you need will depend on how many cfm are needed to the 2nd floor. Assume you have 6" supply ducts to each register in every 2nd floor room. Count all the 2nd floor registers and multiply by 110. That will give you the cfm needed to adequately supply those registers. (2@6" registers in the 2nd floor will need an 8" supply to the 2nd floor). Now you can size the duct going to the 2nd floor. If in doubt, always go bigger, never smaller and avoid any 90 elbows. Use metal ducting. You need every thing on your side for adequate air flow.
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| | nrd
| "Re(2):duct size" , posted Sat 28 Jun 23:55
Thank you for the sound advice. The formula helps and going bigger make sense. Thanks Again
nrd
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| theduke03
| "Re(1):duct size" , posted Wed 25 Jun 21:22
7". Why update?
"My dad was the most feared furnace fighter in Northern Indiana."
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| | nrd
| "Re(2):duct size" , posted Wed 25 Jun 21:25
I need to add boster fans in the ducts to get cool/heated air upstairs better
nrd
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| | theduke03
| "Re(3):duct size" , posted Wed 25 Jun 21:36
That sounds quite brutal. Why not add a return duct to the upstairs. That is the real solution, you can blast all the cold air to the 2nd floor but without return the hot/humid air has nowhere to go, so it remains.
"My dad was the most feared furnace fighter in Northern Indiana."
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