What is the most important factor in choosing an AC company to come to your residence?

look them up make sure they have good reviews on the web

Number (1) - Check to make sure they are a licensed, bonded and insured contractor. You can do this, if you live in Arizona by visiting the ROC website at Page Not Found or File Not Found | Arizona Registrar of Contractors.
Here you can verify whether or not they are licensed and you can view any outstanding complaints against that contractor/the resolution.
Number (2) - Visit your local BBB website and search for that contractor. Look to see if they are an accredited Better Business Bureau member and see what their rating is and check out any complaints/resolutions.
Number (3) - Check with Arizona Corporation Commission if they promote their business as an LLC or a corporation and make sure they are.
Number (4) - Reviews! I can’t emphasize how much a good review means to me.

reputation

Assuming you are only looking at licensed contractors; their corporate culture is the most important factor. Do they believe in providing the best trained, background checked employees that will properly diagnose and repair the problem at a fair price - and guarantee the work? In order to find that out you will probably need to rely on personal referrals, reviews and the BBB. I wouldn’t hesitate to ask those questions when you call an AC company.

Check to make sure they are a licensed, bonded and insured contractor. BBB is a good place to see if they have any complaints. Also check for any reviews they may have on the internet. you want a honest reliable company.

Lic and insurance

The content of the proposal will say much about the contractor. Is it specific? Does it say they are buying and installing goods? Did they outline risks/extra costs? Do they follow the attorney gen of the states guidelines? Are they willing to clarify terms?

Reviews and referrals

Actual pictures of the work in progress. This will show the cleanliness, protection, prove the work shown was actually their job(to a degree), and yes the finished product.

Ask for recommendations, verify license status and trust your instincts.

Be sure they are licensed and insured. Check for reviews

Lic and insurance

The content of the proposal will say much about the contractor. Is it specific? Does it say they are buying and installing goods? Did they outline risks/extra costs? Do they follow the attorney gen of the states guidelines? Are they willing to clarify terms?

Reviews and referrals

Actual pictures of the work in progress. This will show the cleanliness, protection, prove the work shown was actually their job(to a degree), and yes the finished product.

Reposted by Josh under company name.

Many states that license HVAC contractors require that the applicant have a minimum amount of on-the-job or training experience — typically 2 to 5 years — working with HVAC systems.

That experience is critical since an HVAC contractor will likely need to have a competent understanding of how things like electrical wiring, refrigerant, combustion systems, and airflow through heating ducts all work together to add heating or cooling comfort to a home.

Not only is comfort important, but safety is critical – knowing the proper operation of home ventilation and exhaust systems when working with a furnace can be the key to prevent death or injury from events like carbon monoxide poisoning, which can occur if an HVAC system is malfunctioning or improperly installed.

When interviewing candidates, don’t forget to ask about:

Licensed, bonded and insured: Many states and municipalities mandate that heating and cooling contractors meet minimum education and on-the-job experience requirements, as well as pass written exams in order to be licensed to work in the heating and cooling industry.

In order to qualify for and continue to hold that license, HVAC companies frequently need to hold a minimum level of insurance and bonding. Any professional that works in your home should carry liability insurance to protect the homeowner in the case of accidental damage or injury.

References: A professional heating and air conditioning company should have a list of recent customers that you can refer to in order to verify that the company in question performed the work in a timely manner and to the customer’s satisfaction. It’s important to actually call those references to get relevant details about how their projects fared.

Estimates: Due to the high cost of repairing, replacing or installing new HVAC equipment, it’s essential to get at least three written estimates from three different companies.

The project’s cost is likely your most important consideration – and costs may vary widely from one company to the next - but it’s also very important to ensure that the project’s overall scope, details and completion dates are thoroughly documented.

Brands carried: When seeking estimates for new or replacement heating or cooling equipment be sure to ask prospective companies what specific manufacturers or equipment brands they carry. Unlike other home improvement contractors, HVAC companies often only carry a small selection of brands. If you’re doing your own price comparison research on new equipment, you may need to limit your hiring choices to companies that specialize in your equipment brand of choice.

Experience with your system: If your heating or cooling systems features cutting-edge energy-efficiency design, such as a geothermal system, or relies on an old-school operating system, such as steam-driven radiators, make sure the company you choose has relevant experience with your particular system.

Tax credits or rebates: Depending on the type of home heating or cooling equipment you’re installing or upgrading, your project may qualify for federal or state tax credits or rebates. Be sure to ask about what incentives are available for your project and make sure the project is properly documented to meet federal, state or local incentive requirements.

Payment schedule: On a big job, especially when installing new equipment, you may need to make a deposit and make progress payments or planned payments to satisfy the bill. If you use a payment plan or apply for financing through the HVAC company’s line of credit, make sure you read all the fine print and know what you’ll be financially responsible for.

Service contracts: If you’re hiring a company for maintenance or having new equipment installed, enrolling in a service company’s service contract program may give you added peace of mind. Service contracts are just that, a contract that ensures your HVAC system is serviced regularly by the company.

http://www.angieslist.com/articles/how-hire-hvac-expert.htm