Size of Basement Cold Air Return?

[markman100]

Although I'm a first time poster, I read this board time and again to solve DIY problems. Thanks to all those have contributed. Now I have a question about the size of the cold air return.
I'm finishing my basement (25'x25')
Breakdown:
- family room (25'x12')
- bedroom (12'x12')
- bathroom (6'x10')

I'm running four 6" pipe from the main duct between floor joists to 12" from the exterior wall in these rooms per recommendatins here.

How large should the opening be for the cold air return for this size space?

A little background:
The main cold air return (12"x20") descends from the upper 2 floors to the basement where it makes a 90 deg. turn just above the floor into the bottom of the furnace. I'd like to build a stud wall next to the return, pan it out and put a return grill near the floor. Near the top of the pan out I'd like to connect into the return. I read that this will cut down blower noise. What size of grill? What size should I make the holes going from the panned out stud wall to the main return duct?
I've read that if I make the basement return too large then air from the 2 upper floors won't be circulated as much, also it can create a negative pressure situation that can cause toxic gases from the furnace to circulate in the house. I've also read where one cold air return is enough if there is 1.5 inches of space below the doors.
Thanks in advance.

[Jay11J]

As long the furnace is in a room itself, and has fresh air, You'll be fine.

i would put in a single return in the bedroom.

Nothing in the bathroom!

and one in the family room.

[markman100]

Thanks for the info. Is it important that the return grills be a certain size?

[Jay11J]

Just do a 6 x 12 (one wall cavity) and have it near the floor to pull the cold air off the floor in the winter.

[Ed Imeduc]

Use a 6"X 30" grill for down there as you are going to use it in the wall space .So use two stud spaces for down there . That will let you have about 98 sq " back to the furnace. Dont forget to cut the doors down there off 2" off the floor. On the bottom to let air out of the other rooms

ED ;)

[markman100]

Use a 6"X 30" grill for down there as you are going to use it in the wall space .So use two stud spaces for down there . That will let you have about 98 sq " back to the furnace. Dont forget to cut the doors down there off 2" off the floor. On the bottom to let air out of the other rooms

ED ;)

Thanks Ed,
I'm thinking I should have a return in the bedroom. I also read that 50% of return air should come from basement. Your thoughts? If so, should I reduce the size of the 6"x30" return?

[Ed Imeduc]

Go for the two wall studs with the 6 X 30 thats only 98sq" in the wall. then a 6X14 for the bed room thats 49"sq. On a real good job a 3 ton AC needs around 240sq" out and in

ED ;)

[markman100]

One more question Ed if you don't mind.
I will place the 6x30 vent in the bottom of 2 stud spaces which are panned. At a height of 6 ft. I'll connect the panned space to the vertical return a few inches behind it. How best to do this? What size hole to cut? Should I use 2 6" flex pipe or does this defeat the purpose of 6x30 opening down below?
I read its quieter if I connect the panned space to the main return up high.

I really want to thank you for your assistance. I'm a photographer and answer readers questions online and in a newsletter. I know how time consuming this can be.

[Ed Imeduc]

Its a lot in just what works best for you here. I dont know if a 6" L90o would drop down enought to go into the wall for you. Are yes a flex 6" is ok dont have to have insulation on it. Are take a piece of 6" pipe on one end cut up into it 2" ever 1" all the way around it. Now bend every other tab out 90o. Put in 6" hole in drywall and reach in and bend the rest of the tabs over inside the drywall. Now take a 6"L90o and on the female end do the same only about 3/4" deep on the cuts. bend every other one out 90o. put up in the cold air duct in a 6" hole. reach in and bend them over . Turn the the L in the duct to line up with that 6" pipe . Now as its the cold air you can use duct tape are put a drawband on it and we are done . Do one in each stud space:D