Why am I seeing frost or ice on the suction line (the black insulated line) of the outdoor AC equipment and is this a problem? First a quick overview on the difference and workings of the liquid line and suction line of an AC system.
When a refrigerant gas in its vapor form goes to the compressor, it is compressed and heats up (high pressure) and then enters the condenser where it begins to give off heat to the outside and turn into a liquid. When it exits the condenser, it is in liquid form and is still warm and under high pressure. Then it goes through the expansion device and starts to turn into a vapor and picks up heat as it goes through the evaporator. When it exits the evaporator, it’s in vapor form due to being at a lower temperature because of low pressure vs the liquid line which is under higher pressure.
Perhaps that overview made no sense and was to quick of an explanation. Not to worry. I often have to think about it in detail to remind myself of the concepts. Most folks mistakenly think that an AC works by creating cold air. Not true. Rather, they work by removing the heat inside a building and transferring it outdoors. The below graphic depicts how the refrigeration cycle works.
There are various reasons a suction line may be icing up and yes it indicates a problem. The cause could be: a) restricted air flow, b) dirty evaporator coil, or c) low refrigerant charge. There are other probable causes. Bottom line is an HVAC specialist is needed to diagnose and correct the condition.