The Human Body is a very complex system which occasionally needs repair. Choosing a good service provider to fix whatever might be wrong and help you repair and maintain your system can be a matter of life or death. You would pick the most knowledgable and licensed person to perform that service.
Likewise, your heating and cooling system is one of the most complex systems in your home. Choosing a Service Provider who is knowledgeable and licensed   will play a critical role in the comfort, air quality, and safety of your home.
While choosing, don’t be afraid to ask how many years experience they have and on what equipment. Also, make sure they are licensed and insured. They should be well trained to deal with important issues that come up in heating systems such as freon, electricity, gas and oil. In cooling systems, the issues often are electrical issues, or Freon.
A few decades ago, before home air conditioning when airflow was only marginally understood, the job was known simply as furnace repair!  Everyone knew hot air rises. Ductwork was most often an afterthought.
Today, Ventilation has become a technology of its own. When working with a furnace, knowing the proper operation of home ventilation and exhaust systems  can be the key to prevent serious injury or death. A malfunctioning system, or one that has been improperly installed, is dangerous and can cause Carbon Monoxide Poisoning.
Last of all, ask for a list of references. A trustworthy, established business will not be afraid to put you in touch with others who have used their service.