Most important thing about having a new unit installed is the first day it's installed. If done right you'll have a maintenance free system for 15+ years. Get it wrong and you'll have to chronically replace parts and have an early unit death.
  1. Follow these guidelines if you have flex ductwork and ask your AC contractor to install this size return at a minimum (you can never have enough returns in your house)
    20" for a 5 ton unit, 18" for a 4 ton unit, 16" for a 3 ton, 14" for a 2 ton
  2. Ask for disc dampers on all the supply collars off of the supply plenum
    You can control the airflow to each duct this way
  3. Ask the sales rep to register your unit for you
  4. Ask if the attic hatch trim will need to be removed so you know upfront
  5. Ask upfront if they will need to add refrigerant to the system after install
  6. Get it in writing that for every additional foot past 15 feet of lineset, the installing technician will add .35 pounds of refrigerant
    A follow up question to the previous one, the tech should always add refrigerant if your refrigerant lines are longer than 15 feet (or the distance from the condenser to the air handler is more than 15 feet)
  7. Ask for a new fused disconnect if your unit is on the roof
  8. Ask for the ductwork to be sealed and get it in writing as part of the install
    Best to ask for pictures from the installers too