My house is 1600 sqft. It is 9 years old. The central unit is going out: bad contacts, high pressure, amps are very high, etc. I live in Las Vegas. I am going with an American Standard 15 SEER. Should I stay with the 4 ton? Would the duct work effect the unit if I installed a 5 ton if originally it has a 4 ton? Any other info. would be appreciated.