You should know, your problem may not be isolated to the supply/return in your home.

I rarely say this because it could be said about any posted problem here but, in this case, I think it would be best for you..... You need to have a qualified professional do a complete heat load calculation and duct sizing and routing diagnosis. It may be you only need an air balance adjustment or it could mean replacement of your duct system. They will also look at what your home is made of, R values of wall and ceiling insulation along with a substantial list of other factors.

There are a lot of questions to be answered so I'll start here...How many supplies do you have, what size are they, where are they located in each room? Is the unit and distribution through a crawl space, basement, attic? I'm not a fan of flex duct. If it's sagging or not installed properly, it can add a lot of resistance and leakage to distribution and return.

The total size of your returns should, at minimum, be equal to the total size of your supply registers but not too large either. If the return is too much larger than that, you will start pulling cold air in winter (drafts around windows and doors), hot and moist in summer from outside.

Off the cuff, I believe your unit may be properly sized, considering the size house you described. That is a really general statement and without a lot of answers, can't be taken as fact. However, that too depends on where you are located, your insulation and as mentioned several other factors.

If the supply registers are in the floor than the return should be close to the ceiling so the air actually circulates across more than just the floor.

If you want to experiment I would go to home depot or lowes and get the cardboard for a fridge and set it over the return and make it pull some air from 7' or so and see if that helps. It seems crude but I have heard people who tried it and were really surprised the difference it makes.

I have returns near the floor and supplys on the floor with 11 ft ceilings and I don't notice any stratification. Certainly this is not the OPs primary problem.

That return is small so I would say it couldn't hurt. Usually in a marginal system, there are enough leaks that if you just seal it better, you would find significant comfort. By marginal - I mean poorly installed or old. Also, most attics could use some air sealing and that may give you a degree or two for very little money.

In the end, it is pretty unlikely that upping the return is going to fix your problems.

You need to call, qualify a contractor as to how they check & fix duct system airflow problems.

Tech: needs to Make sure there are NO duct air leaks.

Tech: needs to Make sure the blower blades & evaporator fins are clean!

Return air filter area should be sized at 1100-cfm/300-fpm Velocity is 3.6-sf or 528-sq.ins.,/ .55% 960-sq.ins of total physical face area. The percent of (Ak) free-air-area varies depending on filter selected.

Make sure the supply is sized similarly so the pressures balance.

Having an ACCA Manual D performed would be very helpful...

udarrell

Here is a link that might be useful: Return Air Filter Type Selection Sizing & Performance

Thinking on your problem, I have another thought for you to look at. Is the thermostat near the central return? If so, the thermostat may be showing a different temperature than the rest of the house actually is. Even though what COUNTRYBOYMO suggests may seem off center, it will give you some indication of what that and what else might have to be done should you "have a man out". I don't like central returns but sometimes they just can't be helped. I hope you know that you will need to keep all doors to rooms open or, have high and low air vents in them.

Thanks everyone for your help. I'll try to give you more info on this matter.
1st off
I am located in the Peidmont of NC
I'd love to get a pro in to look at the duct etc. but like most people I can't afford it and am an extreme DIY'er ha ha.
My house is a 3 Bedroom -1 Bath Home. It;s a cinder block house. I know hte R rating isn't very good b\c of it but dont think this is the problem?
The Ceiling had blown in insulation so guessing that is good?
I have 9 - 4"X10" all in hte floor under the windows where possible.
IF I measured right the total sq inches of all is 360?
THe Supply is 14X20 (sorry for misleadin in first post) anyway If I did it right it would be 280 Square inches.
All the duct is under the house in the crawl space.
again it's all flex but not really saggin much.
Also when I crawled under there today I Noticed the Duct was sweating? (Should it be?)
THe ducts also look pretty new( I have jsut been in the hosue 2 years)
I haven't tried hte Firdge Box deal but can but I thoguht about taking the 14" return duct and pull it out of the return & pull it up trhought the floor (for a day) to see if it would make a differnece and add a filter over it. Would you suggest doing this or would it prove anything. (well other then I;m desperate ha ha ) . Hope I've answered all the questions. if not please ask again. I am hoping I can do something this weekend to help that will cool this heat house down haha. again thanks for all your time, help & knowledge

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.

For this weekend. Well maybe not that fast. Seal the crawlspace by closing the vents and ensuring that there is adequate plastic on the ground. Then run a dehumidifier in the crawlspace. That will lower the humidity in your house and minimize the duct sweating.

You should not have duct sweating in the crawlspace but my last house had that and it was still comfortable inside. The sweating if just in a few locations maybe a good sized leak that can be sealed with mastic.

There are literally a thousand DIY things you can do that will likely have a greater benefit than messing with the return. If your return is really a problem, there will be noise at the return.

Mastic and spray foam in your crawlspace will pay back in less than a year and almost certainly improve your comfort level. The drying of your crawlspace will pay off in reduced bugs in the house and reduced humidity and reduced mold etc etc.

The unit thta I have is a RUDD 2.5 ton unit. SO I dont think it's for a mobile home? The sweat is on all the supply ducts not just in places. I was thinking that not enought air moving (maybe coming in from the return ) so the AC had to work harder & making hte air colder is why it's sweating. Under the house it was cool 69-70 degrees. II have felt around most the connections & dont feel a lot of air leaking. they are a few small leaks I'll fix. IF I do have to get someone tocome out do you know approx what they mgiht charge? I was looking on line & found this http://hvac-talk.com/vbb/showthread.php?t=71026 ) states "2 ton 800/2 = 400 sq.inches
3 ton 1200/2 = 600 sq. inches" Does that sound right if so my return grill was just 280 sq inches? So wouldnt that be t osmall for a 2.5 ton.
also one more thing the return on hte unit itself is 14" round. Should you go with a larger return duct maybe 16" and that make a difference?
SORRY for keeping on bu I know something has to be causing it b\c hte air coming out is Cold. jsut maybe not enough is coming out to cool the entire house?

THanks again

Most returns are undersized without too much of a problem. I have one that is loud with expensive filters but I don't have any other issues with that system. Definitely up the return size. The duct is probably too small too and is a more insidious issue than the actual grill since you won't necessarily hear it as much.