Refrigerant Charging: Mistakes to Avoid in HVACR Systems

Refrigerant charging is one of the most common operations in HVACR maintenance, but it's also one where too-hasty assessments can compromise performance and efficiency. An insufficient or excessive amount of refrigerant can alter operating pressures and temperatures, reduce cooling capacity, and lead to incorrect diagnoses.

Therefore, checking the charge shouldn't be based solely on the pressure reading on the pressure gauge. To obtain reliable data, it's necessary to evaluate the system's operating conditions, verify proper airflow, and use parameters such as superheating and subcooling according to the manufacturer's instructions.

With the increasing diversification of refrigerants and the use of glide blends, these skills become even more important for refrigeration technicians.

Refrigerant charging: first you need to check the system

One of the most common mistakes is to interpret an abnormal pressure as the immediate consequence of an incorrect quantity of refrigerant.

In reality, pressures and temperatures depend on numerous variables . Before intervening on the charge, it is therefore necessary to verify that the system is operating under the expected conditions.

Among the aspects to be checked are first of all:

• air flow through the exchangers;
• cleaning filters and batteries;
• correct functioning of the fans;
• internal and external temperatures;
• stabilization of the system before measurement;
• operating conditions intended by the manufacturer.

Incorrect airflow over the evaporator , for example, alters the heat load of the exchanger and can affect both the pressures and temperatures measured. Adjusting the refrigerant flow without first identifying this anomaly can therefore lead to overloading or undercharging the circuit.

The time required for the system to stabilize is also important. The technical source recommends waiting approximately 15-20 minutes of operation before taking measurements, to allow pressures and temperatures to reach sufficiently stable conditions.

The general principle remains, however, to always follow the procedures and values ​​indicated by the equipment manufacturer.

Superheating and subcooling: which parameter should you use?

Pressure alone does not allow you to accurately determine whether the amount of refrigerant in the system is correct.

Two fundamental parameters are superheat and subcooling .

The choice of the main parameter also depends on the type of expansion device used.

In systems equipped with a thermostatic expansion valve (TXV) or electronic expansion valve (EEV) , the device intervenes to regulate the evaporator's power supply and maintain superheat within certain values. In these configurations, subcooling is typically one of the main parameters for checking the charge.

In fixed orifice systems, however, superheat plays a more indicative role because the refrigerant flow rate is not modulated in the same way.

In any case, there is no universal value that can be applied to all systems. The correct value depends on the configuration, environmental conditions, and the manufacturer's technical specifications.

Another parameter that can affect superheating is the wet-bulb temperature of the air entering the evaporator . As humidity varies, the thermal load on the evaporator changes, also influencing the expected superheat value.

It is therefore essential for the technician to use appropriate tools and compare the collected data with the tables and procedures provided for that specific system.

Refrigerant mixtures: pay attention to dew point, bubble point and glide

The transition to lower GWP refrigerants is increasing the presence of refrigerant blends on the market, making it even more important to properly understand the relationship between pressure and temperature.

In pure refrigerants , the phase transition occurs at a given saturation temperature for a given pressure. In zeotropic mixtures , however, the different components can evaporate or condense at different temperatures.

Three fundamental concepts arise from this:

• bubble point , temperature at which the evaporation of the liquid begins;
• dew point , temperature at which evaporation ends and the refrigerant is completely in the vapor phase;
• temperature glide , difference between bubble point and dew point temperatures at the same pressure.

To avoid measurement errors, the dew point is used as a reference in calculating superheating, while the bubble point is used for subcooling.

Confusing these values ​​can lead to misinterpreting the state of the circuit and, consequently, modifying a charge that may actually be correct.

As new blends enter the HVACR market, knowledge of the related pressure-temperature tables becomes an increasingly important skill for installation and maintenance.

F-Gas: Leak prevention remains a key responsibility

In addition to correct charging, there is also a particularly important regulatory issue for the European market: the prevention of refrigerant emissions .

EU Regulation 2024/573 requires individuals performing certain installation, maintenance, servicing, repair, leak testing, and recovery activities to hold the required certifications. The new framework also extends training and certification to relevant alternatives to F-gases, including natural refrigerants where applicable.

The Regulation also requires technicians and companies involved in the affected activities to take precautionary measures to prevent leaks . When a fluorinated gas leak is detected, it must be repaired without undue delay; for systems subject to periodic inspections required by the legislation, the effectiveness of the repair must then be verified.

Proper charge management is therefore not just a key aspect of system performance. It's also part of a broader refrigerant management approach that includes leak detection, recovery, maintenance, and technical expertise.

For refrigeration engineers, especially in a market characterized by increasingly diverse refrigerants in terms of pressure, glide, flammability, and operating characteristics, the ability to correctly interpret system data remains one of the key tools for avoiding unnecessary interventions and maintaining system reliability and efficiency.