I am finishing off my basement and need to know about the cold air return vent. The place that I am considering putting it is about three feet from the main return. Is this too close? Does it need to be at least a certain distance away? Since it is so close to the main return, will it effect the other cold air returns in the main level of the house?

If the return air grille is properly sized, it will have no effect on the rest of the return air ductwork. However, if there was no air supply to the basement before, and you are adding some now, then the entire system may need to be re-balanced, and the capacity of the air-handler to serve the additional space should be examined.

Be very careful heating finished basements. If there is no insulation on the concrete walls moisture can condense in the winter. Cold walls above ground level and hot air makes for mold. Mold can be a huge problem behind finished walls.
I know this doesn't adress your question but your question reminded me of the dumb mistake I made. You need a return in the basement to keep air from stagnating even if you add no heat. If adding no supply air you need something like a laundry shute to keep the basement from going negative and sucking outside air at every nook and cranny.

Thank you both for your comments.
Another question: I will be putting three heat/air conditioner vents in the ceiling in the finished area. The space is about 30 x 20. When I do the cold air return, should it be at the botton of the wall or can it be at the top of the wall? The space that I have designated for it is on a wall between all three of the vents.
I am also having a small office area that is about 12 x 15 with one heat/air conditioning vent in the ceiling. Will I need a cold air return for this room also? The room will probably never be closed off from the main room.
I do have the isulation against the concrete walls in between the studs. I used R13 with the Kraft paper.

Is there anything else that I need to know or that would be helpful?

Since the return is intended to collect cooler air, it should be low on a wall. You should have no problem with the office if the door is never closed, but for such a small room, you could undercut the door an inch, and that would allow the return air to pass from the office into the main room when the door is closed.

The location of the return air grille near the supply diffusers may be OK, it it may not, depending on the velocity and throw of the air from the diffusers. If velocity is low and throw is short, there is the risk of short-circuiting, where the supply air is drawn into the return before it can properly condition the space. If the diffusers are adjustable, adjust them so that the supply air is directed away from the return as much as possible.

I can move the location of the cold air return to a different wall so I can have the opening at the bottom. It will be next to a bathroom wall which I have put up insulation as soundproofing. Is this okay? Can the duct work be touching the insulation? Also, for the size of the room that I have, do you know what is the appropriate size for the cold air vent?

In talking about insulation, can I put insulation in the basement ceiling next to the existing duct work to the main level of the house? If so, does it have to be a special kind if it touches the duct work?

Thanks for all the great information.

Absolutely yes on the insulation contacting the duct work, with one caveat: If the duct work is supply, then there should be no contact to any kraft paper on the insulation. If you read the printing on the face of the kraft paper, you will learn that the fiberglass insulation is non-flamable, but that the kraft paper is flamable, and needs to be covered with a non-flamable surface, such as gypsum board. While it is not likely that the supply ducts will ever get hot enough to ignite the paper, it makes no sense at all to skate that close to the edge if the ice, as it were. It's best just to trim the paper back a couple of inches.

Fiberglass duct insulation is sold at all HVAC supply houses. It comes in rolls of varying R-value, depending on the thickness of the unrolled, expanded fiberglass. The exterior is typically fiberglass-reinforced mylar, and is UL approved for the application for which it is designed.

Insulating ducts helps prevent transitional losses of the conditioned air stream to other areas not intended to be conditioned. It also muffles the transmission of sound through the ducts to other areas of the dwelling. It is critical, however, that all duct joints and seams be sealed well, both to prevent whistling, and to prevent loss of the conditioned air to non-conditioned space behind walls or above a ceiling.

The size of the return air grille is a function of the air volume in cubic feet per minute (CFM), and the velocity of the air eb=ntering the vent. If the vent is too small, the velocity will be high, causing noise, and the system will starve for air, since it is unable to draw the return air back at a high enough rate.

If your space is 30x20 feet, 8 feet high, that is 4800 cubic feet. Let's say you want 6 air changes per hour, that's 28,800 cubic feet per hour, divided by 60 minutes, or 480 cubic feet per minute. That is a ballpark figure of the amount of air that must be supplied, although there are other factors. Usually the return air quantity is somewhat less than tha supply quantity, to pressurize the space slightly and avoud having to account for infiltration in the calculations. So, you might return about 450 cuboc feet per minute, and you want to keep the velocity low to mitigate noise...say the desired velocity is 150 feet per minute. The return air grille then should be 450 divided by 150, or about 3 square feet of FREE area, which is usually from 60 to 75 percent of gross area, which should then be anywhere from 4 to 5 square feet gross area.

All of this depends on how much air will actually be supplied to the space, however. If there's only enough for, say, 10 air changes per hour, the numbers will be very different.

Don't you need to figure opening(s) for the combustion air? Here in Kansas the codes require a certain amount of air in the room housing your furnace and hot water heater to assure proper combustion. If the room is not big enough then air grills need to be added to allow air in from the conditioned room(s). Sometimes these openings will also act as cold air returns.

If the heating equipment is in an enclosed space, combustion air should be taken from outside, rather than use air that one has paid good money to condition and then send it up the flue.

I think he was talking about return air, not combustion air. It only makes sense to re-use the return air (which you have already cooled or heated) than to continually use outside air. You can't heat or cool the World.. If the return air is from the same (only) Heating/Cooling System) then there should be no effect on the output, once the basement space has met the Setpoint.

Well, he SAID "combustion air", so I presume he MEANT "combustion air".

Just had to add my 2 cents worth.a former gas co. employee....cold air return must be so many feet away from your furnace. Also there is a rule of thumb, which I cannot remember, that you have so many square inches of return air vent for your BTU's. I cannot remember the formula as I have been away from it too long. I had two cold air returns but in my furnace. In the old days their was a return vent in every room, now days they just put the big ones in. I had one put on each end of my house, so I could close my french doors to my kitchen and the utility side of the house without starving my furnace. Your heating bill will be more if you starve the air. They guy you said you have to have it "balanced" is correct. Wish I knew more or could remember more to help you, but hope this small amount of information will help you on your search. I do know if the air return is too close to the furnace and you have a heat vent, it cuts it off from the rest of the house. The gas bills to customers with a properly installed furnace would be so much lower than the ones installed by homeowners or contractors who did not understand the workings of a heating system. One customer went to Vo-Tech before he built his home so he could install his own heating system. You may check with a teacher at Vo-Tech to get some free info. In our area, only a few of the installers understand it all has to be balanced. Best wishes, Gas Lady AKA, donkeybrain

Hey Hetzel, If you will read the original post, he said "Cold Air Return", not "Combustion Return".. I didn't mean to upset you. Please accept my apology. And don't take everyting so seriously, my little buddy...

If you read the post that I was replying to, it said "combustion air" and not "cold air return", hence my reply that conditioned air should not be used for combustion air. That post is two above my reply. Sorry for your misunderstanding..