Heat Pumps and Electric Heating - Calculating cfm of round duct.

I was told that if I run a continuos loop from and back to my plenum on my supply side using round ducting that the cfm doubles in comparison to having my duct work terminate somewhere. Is this true? I had a bid done for a 3.5 ton heat pump system using a 12" duct loop system. The square inches for a 12" duct is 113" i believe which is only good for about a 2 ton unit unless its doubled.
Help!

3.5 ton heat pump system using a 12" duct loop system. The square inches for a 12" duct is 113" i believe which is only good for about a 2 ton unit unless its doubled.

Have to ask why do they want to do this loop. Is it in a slab??? are why???

But yes a 12" round is 113" sq Also that 12" is good for 500 CFM But you need 1400 CFM off of that 3.5 ton.
For one line off the 3.5 ton you need a 18" metal or a 20"flex round

ED;)

Yes it is in a slab. So, is running a loop the same as running 2 trunks? They told me there was better heat distribution this way.
Thanks

If thats a full loop Id say its right at it for that 3.5 ton. Dont know just what your set up there is. But I like the ducts in the slab heat. We use the PVC heat pipes for it. But make a loop around the home with the Ht/AC in the center of the home and spider pipes out to the loop That way the whole floor is warm in winter.

ED;)

One last question. I am having a hard time finding metal or pvc ducting on the internet 12" or bigger. I'd would like to do this on my own. Any suggestions?
Thanks for the help, this web site is very handy.
Mike

http://www.general-plastics.com/report_boca.asp?table=2

http://www.kroyind.com/heatairduct.html


http://www.usplastic.com/catalog/product.asp?catalog%5Fname=USPlastic&category%5Fname=13834&product%5Fid=16848

Might look at them

ED;)