Refrigerant Overcharge Troubleshooting and Prevention
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When we charge systems, we typically add refrigerant via the suction line when the system is running. (You can only add refrigerant when the system is off if the system is under vacuum and has no refrigerant at all.) It’s easy to overcharge systems with accumulators, as the accumulator may store liquid refrigerant before it gets to the compressor. People also may overcharge systems if they don’t let the system equalize, if it’s cold outside, or if the airflow is poor.
You can’t tell if a system is low on refrigerant from suction pressure alone, and some technicians may overcharge a system with refrigerant if they see low suction pressure. Technicians are also more likely to overcharge a system if they don’t weigh in the charge with a scale and don’t give a system time to equalize. Systems that have different stages of operation also need to be run at the highest stage during charging; failure to do so may lead you to overcharge a system.
When you add refrigerant, it boils in the suction line and goes to the compressor, where it becomes pressurized. It becomes a subcooled liquid in the condenser and goes to the metering device. As the pressure drops in the metering device, some of the refrigerant will boil off. The metering device ensures that the evaporator is fed with refrigerant (and maintains superheat in the case of a TXV).
When there is too much refrigerant in the system, some of that refrigerant will back up against the metering device and “stack” in the condenser. So, the head pressure increases, and the condenser becomes less efficient. As a result, the main symptoms of refrigerant overcharge are high head pressure and high subcooling. (However, a clogged condenser coil may also result in high head pressure and high subcooling. A visual inspection will often allow you to distinguish dirty condensers from overcharge; finding the differential between the liquid line temperature and the ambient temperature can help.)
Low suction pressure is NOT a telltale indicator of low charge and likely indicates a restriction or poor airflow. So, we need to focus on multiple refrigerant readings and focus on the state of the airflow if we come across low suction pressure. Suction pressure is also not a good charging indicator.
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