We have dual ac in a 2 story home.The upstairs is overly cool and downstairs is hot.AC tech says the damper is not working due to static pressure, and that this 10 plus year old home duct work is undersized and needs to be increased.really is this the deal 4k to replace.He says the t-stats are fightinf each other and the pressure won't allow damper to function.HELP already spent 5k on new unit 2 weeks after moving in last week.HELP