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re: duct basics
10 sep 2003
mark or sue  wrote:

> there a certain velocity you don't want to go below?

i'd say don't go above 400 feet per minute, to keep fan power low. big ducts,
or no ducts at all, just open doors and stairways and floor grates and slow
ceiling fans, if not thermosyphoning. work with nature, not against it. put
heat returns near outside walls in wintertime, and so on. 

>is a 30 degree dt reasonable estimate for cooling...

sure... 400 cfm per ton is about 12k btu/h/400btu/h-f = 30 f. 
>is it reasonable to use a 6" sheet metal branch duct when you want about
>3500 btu and a 7" duct when you want 5000 btu of heating or cooling?

hvac criminals move heat energy. let's move power in btu per hour.

a 6" round duct has pi(3/12)^2 = 0.375 ft^2 of cross section.
at 400 ft/m, it can move 400ft/mx0.375 ft^2= 150 cfm of air.
with a 30 f dt, it can move about 30fx150btu/h-f = 4500 btu/h.


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