I am in Florida in a 40 year old house. My 10.5 year old Lennox heat pump with air handler installed in garage attic has never been particularly reliable -- has always required service almost every year. With 10 year parts/labor from my local AC company, all was fine. With the $1500 tax credit, I would like to replace the system even though it is currently running fine. I got quotes from my current company, one other big local outfit, and Sears.
A few years ago, I added 300 sq ft of living space (80 sq ft space plus 220 sq ft space). I need to up the size of my system from 3 ton to 3.5 ton and I want to add new ceiling vents in those 2 rooms.
I am considering replacing all the duct work in the attic with new flex ducts. The system now is very unbalanced with rooms at the far end of the house getting very little air. I thought I needed new ducts to solve this problem. 2 of the quoting companies did recommend replacement and the 3rd said the ducts were fine and that they could add 2 new vents and still balance the whole system. To my layman's mind, that seems unlikely. BTW, I have seen the duct work sweat a lot of water right through the paper covered insulation in the summer. It seems like modern use of distribution boxes and dampers is better than what I have now.
So my main question right now is, do you think I can really get away with current duct work and get 2 new vents off of it, or is replacement of all duct work really the only option?