Procedures for Recharging Air Conditioning Systems with Refrigerant
Refrigerant charging might be the least understood practice in the air conditioning industry next to setting airflow. Although there are several methods, typically there is only one correct method for the type of unit you are working on.
1) Total Superheat Method (Fixed Orifice / Piston / Capillary Tube / Non-TXV)
Temperature must be above 12°C outdoors and above 21°C indoors with a indoor wet-bulb temperature above 10°C. The evaporator coil cannot operate below freezing or an overcharge and possible compressor damage could result.The total superheat method is the most accurate method of charging fixed orifice or capillary tube systems. These systems are critically charged and require correct airflow and accurate test instruments to properly and accurately charge the system
2) Condenser Subcooling Method (TXV)
Temperature must be above 15°C outdoors and above 21°C indoors with a indoor wet-bulb temperature above 10°C. If not already in operation, assure that the system is leak free and a proper evacuation has been performed to assure proper degassing and dehydration of the piping and the evaporator coil. Attach a high accuracy digital gauge like the Digital Manifold to the service ports. Make sure the gauge has been zeroed, if required barometric pressure offset adjusted to current conditions, and the proper refrigerant type selected
3) Weight or "Weigh In" Method
Can be done correctly under any ambient conditions. Typically this method requires an already evacuated system that is ready for charge, or removal of the existing charge so the proper charge can be weighed in.
4) Approach Method (Lennox)
In order to verify charge by the approach method, the evaporator airflow must be properly set, and the evaporator coil and condenser coil completely clean. Find the required approach in the manufacturers installation instructions, which will be specific to the condenser you are servicing. Measure the outdoor air temperature entering the condenser from top to bottom of the coil outside of line of site of the sun and obtain an average condenser entering air temperature. Measure the liquid line temperature with an accurate clamp probe thermometer.
Determine the temperature difference between the liquid line temperature and the outdoor air temperature. This temperature difference is the approach temperature. Adding refrigerant will decrease the approach and removing refrigerant will increase approach. Allow 10-15 minutes of operation for the approach to stabilize after refrigerant is added or removed.