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Old 06-01-2005, 04:30 PM   #1
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Duct sizing question

Background: My house is a very old (1852) two-story home, about 1100-1200 sf. I am adding central heat and air and retrofitting the house with ducts.

It's a 3 ton split system heat pump with the air handler in the attic. I began this project with someone who thought he knew exactly what he was doing. However we've run into issues with duct sizes.

On the supply side, is a 16" duct connecting to the air handler the proper size for a 3 ton unit? The main line that connects to the branches that serve the individual rooms is 8". What size should connect the two?

Any help would be appreciated.



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Old 06-04-2005, 12:42 PM   #2
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did you buy a complete system ,duct to ?it depends the smaller the pipe the more noise it makes and the better chance of freeze up.whats the problems you have ?I'm in canada, rule of thumb is 400 cfm per ton ,now this varies in dfferent places .now using this rule of thumb you would need about 1200 cfm at your fan that would be about 1000 fpm in your main trunk .You want just over half that(fpm) in your runs .An 8" pipe is good for about 250 cfm@ just over 600fpm I dont really understand your last question ,but hear it is you should have a flex connector between the unit and the duct (if your buddy is a hack he'll try and talk you out of it :D )it doesent have to be used ,but it will reduce noise transfer.now you have 16x 12 duct?or bigger if it is insulated, the branches come off and the main trunk is made smaller to increase the preasure so it can travel further.8" runs makes me think your buddy is a comercial sheet metal guy ,or a refrige apprentace ?these branch runs arent insulated flex are they?If they are ,there equivalent to a smaller pipe diameter we are only aloud to use it to connect to the grill .if you have a large room ,you should really break it down to more 6" runs to difuse the air better .8" will work but it used more for office space (also more affordable)now this is based on 1200 cfm ,the unit you have is probably a variable speed to make it more versitile whats the return air layout?how many runs in tottal ?hope this helps


Last edited by murray; 06-04-2005 at 12:45 PM.
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