The U.S. Environmental Protection Agency (EPA) requires this certification since appliances that use refrigerants can harm the ecosystem if they're not properly installed or maintained. While the EPA mandates this area of HVAC work, requiring a technician to get a license is up to the state to decide. With that said, not all states demand contractors to obtain one.
An air conditioner's compressor contains a refrigerant. As it works, it sends this refrigerant through the system. As warm air blows across the coil that carries this refrigerant, the heat transfers to the refrigerant (cool always absorbs warm). A fan moves the cooled air through the ducting and out of vents that lead into the rooms of your house. The refrigerant returns to the compressor where the absorbed heat is moved outside. The refrigerant is then sent through the coil once again to continue the cycle. 

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