I have a 1973 mobile home. it is a double wide it is 1400 sq ft. and has a 4 ton coleman a/c unit 12" return. and 12" supply that y's to 2 10" that connect at the end of the trunk lines that is about 40' long one on each side of the mobile home the trunk lines are 10"x5" aluminum. 5 4x8 floor registers out of each trunk line. the problem I am having is only about 1 1/2 ton of air flow is comming out. so what I was going to do is change the return to 14" and then move the supply connections from the end of the trunk line to the middle so air flow would go both directions in the trunk and not have to only push one direction.(hopefully splitting the available air and being able to use more cfm from the unit) also i was thinking of making the return 14" connect to a "y" to 2 12 inch and placing one at each end of the home what is the maximum length of a return duct right now it is about 15' if i changed it to 2 12" it could be as mucch as 40' does this sound like it might help I do realise that this unit is oversized and i will never get 4 tons of air through that duct but that is the only thing I cannot change.