If the air conditioning contractors cut corners on the ductwork, then the entire efficiency of a new unit is greatly reduced. In Phoenix, homes built after 1980 used flex ductwork as the preferred means of air distribution. Flex ductwork is good because it is easy to install, required little preparation and can be done by less skilled workers. On the down side, unless an air conditioning contractor does a proper load and duct design calculation using Manual J and D, a number of problems can arise because of cutting corners. Problems such as poor airflow, high room pressures, excessive dust and conditioned and heated air being lost to the outside are all too common in existing homes. Many times if an air conditioning contractor has done work or replaced an AC unit in the past, they really will mess a system up. I think one of the reasons good home inspectors and energy auditors are so skeptical of HVAC contractors in Phoenix is because we see the outrageous work done and what it is costing the homeowner.