At 400 cfm per ton, 1,200 cfm is needed at a minimum. But for returns, it is always recomended to go larger. I use an old formula which is simple and always works for me. Take the total system required cfm X .36, then add that to the total cfm. Example= 1,200 cfm X .36= 432, 432 + 1,200= 1632 cfm. An 18" round duct, or three 12" round ducts will work. Then the return grill area in the cieling or wall should be- 14 X 14, 12 X 16, ect for the sake of noise the bigger the better. The first number in the grill size is the width, the second is the hiegth- so a 14 X 20 would be 14" wide and 20" tall- the stamps in the grill running the 14" width.

I personally would really like to help you but it would be way too difficult and time consuming without a lot of up front technical information. Most of us like to "do it right" from the ground up. I think you'll find that fooling with this and that will take up a lot of time to maybe some but,no real efficient end. I understand your need to pull the purse strings. But if your duct work is sweating, that is certainly part of your problem. Is this flexible duct inside a fiberglass insulation sleeve or when you say flexible duct, is it various sizes of dryer vent? If so, you may have to start replacing it anyhow if the insulation got wet, as money allows. I would suggest doing so with rigid insulated ducting. Join the duct run of the month club and by the end of next year you might be finished. This is obviously an add-on unit before you got there. There is no way for us to determine if it was sized properly for the home and if the coils are matched properly. It almost sounds like a mobile home unit adapted to your house. The guy before you might have been saving money too and cheated on the sizing. Your blower unit speeds will also need to be known. I think your best bet would be to go to the book store, library, on line and educate yourself on air distribution. For example, the size of a room will determine if that room should have one or two supply registers and/or returns and what size they should be. You'll need to know the pro's and con's of mounting the supply's or returns high or low. Although the learning will not necessarily be that hard, it is a must have. Just the little information I mentioned here is a drop in the bucket for all the things that must be known and considered. Either you learn or pay someone who has already learned or you can use a lot of coat hanger wire and duct tape and hope for the best. Go ahead and shove the return up through the opening if you can, that's better than doing nothing to try to make things better. If you have that much play, you are losing a lot of air flow due to resistance and excessive length. You shouldn't have any sags at all. Even metal ductwork not properly installed can cost you in proper distribution. For example, a 90 degree elbow placed in the wrong place and not properly installed, can cause you to lose 50-60 percent of air flow, due to blockages and resistance caused by a vortex of the air within that bend. Just too much to really effectively cover here, at least in my opinion.

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.

Too many photos. You only need 1 photo of the dining room and don't need to photograph every bedroom and only 1 or 2 of the front of the home. The living room photo, move that orange/brown leather chair before you take the next photo. It hides the room. You can slide it back after the photo. Also, you need contact info and asking price on the web site. Also, you need several bullet points that describe the house and/or area on the web site. Maybe talk about the square footage and lots size also. maybe taxes and age of house and major features (new furnace, roof, fireplace, x bedrooms, 2 car heated garage, etc)? Don't list small crap that everyone has, like garage opener and such. What made you BUY the place? Often that is a selling point as well that you can market. List on craigslist.com as well and post 4 photos and write about a paragraph or so. And repost on craigslist about every 10 days. Don't get lazy and not post the photos, thinking the web site link will do. Folks need to see the pix on craigslist or they won't go to your web site. Also ref to your web site from craigslist. Get a custom made For Sale sign for your front yard. Go to a sign shop. They make them for under $100. It is well spend. You pick the color, font, sign size. I usually go bigger than the standard R.E. sign. Pick a color that will stand out against the green grass or white snow. I like electric blue and yellow. You can even use an icon, such as a golf club if that was the type of home you were selling. Put major features (finished basement? acre lot? waterfront?)on the sign and # bedrooms, bathrooms and your phone number. Get flyers for the front yard. Keep them in the little box that you put next to/on the sign. Keep the flyers stocked. realize neighbors will take them, especially the first week. Don't worry, it is natural curiousity. Also reference the web site on the flyer, saying for more PHOTOS go to www.sitexyz. Make sure you have at least one front photo on the flyer. Have price and address in bold either at the top or right below the photo in large/bold. Have your contact info easy to see on flyer. I sold several FSBOs, so am giving you advice that truly will help you. Also, I suggest a discount broker to get into MLS. But then you have to pay an agent that brings the buyer (2.5-3%). If you do this, you need to be able to not pay this commission if you sell to someone that doesn't have an agent. In my case, this occurred and I simply paid the $400 or so to get the listing into MLS, and nothing more. Good luck. it is hard work. Then screening the callers is not easy either. Neighbors might lie to get in to see the place. they say their uncle(or mother, brother, etc) is looking to move and they are looking for him. BS 97% of the time. Return all calls pronto. Good luck.