Selecting an HVAC Contractor
   

Request a load calculation. A good contractor will take measurements in your house and ask questions to determine the best unit size for your home.

Request a calculation of your energy savings. Your contractor should be able to calculate your utility bill savings and total lifetime costs for a recommended unit.

Inspect ducts. Ask your contractor to inspect ducts for leaks and compatibility with your system.
  

Consider a house pressurization test. Have your house tested for "backdrafting," which occurs when furnace and appliance combustion fumes are pulled back into the home and could threaten your health and safety.
   

Replace both indoor and outdoor coils. When replacing an air conditioner or heat pump, be sure to replace both indoor and outdoor coils for maximum efficiency and reliability.
   

Obtain a written contract. Always obtain a written contract or proposal before allowing your contractor to install a new system. Ask about warranties for labor and parts.
   

Weigh the costs. The lowest price may not always be the best price. Paying slightly more now may get you better equipment, service and cost savings in the years to come due to lower operating costs.
   

Install for easy maintenance. Make sure the air conditioning unit is installed for easy maintenance.

(Source: Energy Star Home Heating and Cooling System Checklist, U.S. Environmental Protection Agency)


More resources:  

http://www.furnacecompare.com/heating-contractors/how-to-negotiate.html