One day a frustrated tech called and told Richard that he was working on a system with a low suction pressure that was frosting up. He further explained that his new digital gauges were telling him that the superheat was 0, so he added refrigerant. However, no matter how much refrigerant he added, the superheat would not increase.
Richard asked him, “how much refrigerant do you have to add to make the fan blow harder?” There was no response, so Richard asked again, “Tell me, I really don’t know. How much refrigerant does it take to make the fan blow harder?” Finally, the tech responds: “Your question makes no sense! There is no relationship between the amount of refrigerant in the system and how hard the fan blows.” Richard then replies “So why are you trying to fix an airflow problem by adding refrigerant?”
While it is true that an undercharge can cause an air conditioning system to frost, the most common cause of a frosting air conditioning coil is actually low airflow. Always look at airflow issues first when trying to remedy a frosting air conditioning system. Common airflow issues include a dirty air filter, a dirty evaporator coil (caused by dirty air filters), closed registers, and poor ductwork.
One tip-off is superheat. A system with airflow issues will operate with a low superheat while an undercharged system will operate with a high superheat. Note that if the coil is frozen over it will need to be defrosted before any pressures or temperatures are checked. The ice covering the coil makes its own airflow restriction.