There are several factors to keep in mind when selecting and working with a heating and cooling (HVAC) contractor. The following tips can help you make an educated decision and receive quality service:

Ask for a certified HVAC technician. Keep in mind that not all states require certification.

Your contractor should be licensed, well trained, and experienced to provide quality installations. Ask your contractor about his or her training, experience, and membership in contractor associations. Ask if he is insured and bonded.

Good contractors own and use refrigerant recovery equipment and are certified to handle refrigerant in cooling systems. Ask for proof of certification if applicable to your state.

Contractors should know how to properly size your home heating and cooling system (e.g., determine whether you need a two-ton or three-ton cooling system). Don't use a contractor who wants to size your unit solely on the square footage of your house. Contractors should calculate equipment size using computer software or professional guidelines such as the Air Conditioning Contractors of America's "Manual J." To gather necessary information, the contractor should spend at least a half hour poking around your house, taking measurements, and asking questions. He or she needs to measure ceilings, floors, windows, and walls, and check insulation throughout the home. Systems that have been sized properly to fit your home provide better humidity control, cycle on and off less frequently, and cost less than oversized systems. Insist on getting a copy of the load calculations (or computer printout). These can be useful for comparing bids.

Ask your contractor to inspect your ducts for leaks, incomplete connections, and compatibility with the rest of your system. Ideally, your contractor should use diagnostic equipment and fix leaks using a quality duct sealant (duct tape is not sufficient). He or she may also recommend changes to your duct system. Since as much as 30% of the efficiency of your system is a result of your duct work, overlooking duct improvements may compromise comfort and cost you money.
If your house or water heater uses combustion (i.e., it burns something like natural gas or fuel oil), you should have a house pressurization test performed to make certain there is no danger of "backdrafting." Backdrafting is when the fumes from the combustion process are pulled back into the home, threatening the health and safety of occupants.
Your air conditioner or heat pump condenser should always match the indoor coil. Your contractor should replace both coils at the same time for maximum efficiency.

Have your contractor select an install location that provides for ease of maintenance. Make sure the inside coil can be reached for cleaning. Depending on the model, the contractor may need to install an access panel. The coil should be cleaned every two years. The air filter should also be easy to remove and should be cleaned or changed whenever it is dirty. (This can be monthly during peak season.)
If possible, have the contractor place outside air conditioning units on the north or east side of the house, out of direct sunlight. Leave plenty of room for free air flow on all sides, and at least four feet at the top. Keep the area free of debris and shrubbery.
Always obtain a written contract or proposal before allowing your contractor to install a new system and be sure to ask about warranties.
Remember, the contractor who gives you the lowest bid may not be the best choice for you. Paying slightly more may get you better equipment and better service. Carefully evaluate a contractor's proposal to ensure you get the equipment and service that best meets your needs.
Most contractors offer maintenance agreements to keep your system running smoothly for years to come. Also ask about the warranty length, what it covers and if there are extended labor warranties available.