Technician Quick Tip:
The most widespread problem with high efficiency units is overcharging. This can cause starting problems, lack of capacity and premature compressor failures. Remember, you cannot overcharge a unit too increase its capacity! Some reasons a unit might be overcharged are dirty coil, slow fan, plugged filter, the unit is connected to an indoor coil with too small a piston or capillary, or the installer or mechanic has a desire to see the "right" pressure (the right pressure for high efficiency units are low).
Low suction can result in low heads and a mechanic might try to overcome this with additional charge. It appears that most overcharging is done in an attempt to raise the head/suction pressure to what the installer or service mechanic considers adequate pressure. If the unit has a fixed metering device adjust the charge down to the recommended superheat found on the units label or in the installation instructions. If no manufactures recommendation superheat exists charge the unit to 15º of superheat as a default. Superheat is simply the refrigerant gas temperature above the refrigerants saturation point. To check superheat compare your actual refrigerant line temperature returning to the compressor (use a accurate thermocouple type or digital thermometer) to your refrigerants saturation point