The builder doesn't create the ideal situation for everything when the leave an unfinished basement 'roughed-in'. We have an architech friend who knows the yahoo that designs the homes we're buying. While they are reputable, they do have a lot of design flaws - and these are greatly exaggerated when trying to finish an unfinished basement that is roughed-in/half done. There is a possiblity of moving the duct into the main basement, but it would mean creating ductwork that goes perpendicular to the joists, thus lowering the ceiling in that area. It may not be a big deal, but it's something I was hoping to avoid to save money (wasn't planning on hiring an HVAC guy when the basement gets done). The bedroom has one suppy vent - and the basement area has one also. My contractor has already told me I'll probably need to put in electric baseboard heaters, as that's not enough to heat a below-grade basement. And the basement finishing bill keeps getting higher!!!

You'll hear a lot of debate about this one - some will say you can't do it without calculating a Manual J etc but the reality is there's a lot of boilerplate hvac systems, at least where I am in Canada. I have two returns in the upper floor of my house only a couple of metres apart, and none in the finished basement and none in any of the bedrooms, which I've been told is an 'impossibility' - well here in Canada, it's not and the system works well enough - the master bedroom is a bit cooler with the door closed, but it's above a poorly insulated garage. I see a lot of basements with the return in the ceiling. Warm air does rise, though, and in the summer you might want some of that cooler air in the basement circulating upstairs, so I'm planning to do exactly what you mentioned, put the return near the floor, backing onto the furnace room. If there was some sort of big issue with changed airflow, you could probably block off an equivalent of the downstairs return vent in the upper return, to equalise the air flow, but to this layperson, the SUPPLY of cold air is not as much of an issue if you're not getting cavitation (a vacuum effect because the pump isn't able to suck in as much air as it's trying to deliver.) Take some temperature readings over a few hours in different spots in the house, and make note whether the furnace is running a lot more than it was previously. The 'duct work' typically is either the space between floor joists or a boxed duct around the same size, or, running down the wall, typical stud wall spacing, 16." That's what I see here in British Columbia, again, I'm not an HVAC tech. Don't go excessively large. If you go through the furnace room wall you won't need much in the way of ducting and if you need to you can use thermopan (silver faced cardboard) instead of drywall or OSB to form the 'duct' from the floor up to the return (make sure you cut into the return, not the warm air section and make triple sure you're not going to damage any wires, gas lines, a/c lines etc. As for the return, the standard size grille is about the width of the space between studs I guess so around 16?" If you need to tweak it, you can block sections of it or the upstairs one inside under the grille if you need to. If you're careful, you're not going to damage anything irrevocably.

Copper or brass pipe is permitted if the gas is certified to contain equal to or less than 0.3 grains of hydrogen sulfide per 100 cubic feet of gas and it requires a written confirmation from the gas supplier. IRC- 2414.5.2 & UPC-1210.1.1 Copper tubing used for GAS must be specifically labelled as "Suitable for Gas" Gas lines may not be run through an Air Duct, Chimney, Vent or Plenum. Copper Pipe requires Brazed or approved screwed fittings. IRC-2414.10.2 & UPC-1211.2 Flared fittings may not be used in concealed locations.

Proviso I'm not an hvac professional. But, at base, it sounds like you've got a pressure differential between the inside/outside of the house or possibly the cold attic - the lower floors are pulling in air because air will move to lower-pressure areas. Trying to track down exactly where the air is coming in from may be a bit of a fool's errand, in that if that pressure differential exists, the lower floor will pull in the air from somewhere - outdoors through doors or windows, or through ductwork from elsewhere. Close off one entirely, you may find it just moves elsewhere. So perhaps focus slightly differently, on what bothers you. Is it the feeling of draftiness due to cold / warm air, affecting comfort? Is temperature okay otherwise? If so, one way to address would potentially be closing partially those vents (with a simple louvre) or redirecting the cold air flow in a different way in the room. (I'm kind of assuming you don't have any other significant issues or complaints about your hvac system). I have had this issue in a few rooms seasonally (note these are not A/C system vents, my heat is hydronic). We felt the draft and was uncomfortable in certain places. We replaced some vent covers with types that partially close or can be redirected. It does not completely block the air, just keeps it from the feeling of draft. Overall it has resolved the issue (and of course important that we did not want to stop the exchange of fresh air, just moderate it some). Note, since you've said it happens differently with windy weather outside, I strongly suspect it is differential pressure within the house (wind blows and there's all kinds of effects of faster air/lower pressure around the house). It's possible there are other ways to address such as improving air flow within the conditioned space (so instead of cold air moving to reduce the pressure imbalance, it's warmer air doing so). But this is a hideously complex subject where the house has to be looked at as a system and I'm not going to guess how to do this or diagnose over internet. Oh, another small thought - why did you turn off the fresh air system? What is that system? It's possible this is part of the issue, that the fresh air system was partially intended or designed to help deal with/moderate the pressure imbalances. Did this problem start occurring more when you stopped using that system?