Is the 4x12 duct the main duct for the second floor? It is too small if it is. A 3 ton AC needs 1200 CFM of air. This means the area of the duct should be about 175 sq. inches. You have 48 sq. inches. This duct needs to made bigger. The flex ducts should also be checked for leakages.

A 4 ton blower would puch 1600 CFM. You should not exceed 1200 CFM with a 3 ton unit. This is not going to solve your problems.

Thanks for explaining about the blower Mike, that's what one contractor proposed.

It looks like there are two ducts for the second floor. The other one (also 4x12 I think) feeds two short metal supplies that put out decent air.

We think the problem duct is coming from a 4x12 on the first floor.

Mike is right in that you cannot compensate for too small of a duct system by trying to force more air through.

Think of it as water instead of air. If you pipe diameter (duct size) is 3 inches round, and to get the correct flow of water (air) you need a larger pipe, then trying to force the water to flow faster doesn't really solve the problem.

Thanks, forgot to ask is there a way to make the duct bigger as Mike has suggested? The 4x12 runs between the walls on the first floor up into the front attic space, then changes to the flex duct when it goes up into the upper attic and out to each bedroom. The house is a cape cod.

It may not be feasible to increase the size of the existing duct with opening up walls. Another option is to run another duct through an existing chase, or create a chase inside a closet.

one of the contractors mentioned adding another duct, but we couldn't find a place to hide it for one of the bedrooms on the 2nd floor. Might work for the room on the opposite side of the house. I really don't want to open up walls.

I don't necessarily have to do this part right now, correct? I've been without a/c for almost a week, and really want to make a decision and get it scheduled!

You can, but discuss it with the contractor so it can part of the proposal.