I am adding a basement bedroom and need help sizing the duct work. I used the program Residential 5.0 to obtain the heating and cooling loads. The program is telling me I need .3 Tons, with the summer cooling load being 3,800 and winter heating being 4,400. The respective flow rate requirements are 100 cfm and 36 cfm.

I have downloaded the program Marinsoft Duct Calculator which I believe I need to properly size the ducts.

I'm in the process of determining the current size of my forced air furnace and AC unit. The house is only 800 sq. ft upstairs and I have added an additional 400 sq. ft in the basement. I live in St. Paul, MN so it gets plenty cold in the winter.

So I have a few questions:

What should I be concerned with when sizing the ducts?

What should the flow rate be in cfm and the velocity in fpm at the supply vents? I'm thinking of having 2 6" supply’s.

How do I size the return?

Can I tap into the existing trunk? I'm assuming I need to do some type of calculation to determine this.

Any website or books someone could point me to would be great so I can learn more.