ESCO Air Conditioning Practice Exam 2025 - Free AC Certification Practice Questions and Study Guide

When topping off an operating system that contains a blended refrigerant, introducing the refrigerant as a gas into the high side using the throttling valve is the correct approach. This method is crucial because blended refrigerants consist of multiple components that can separate if the refrigerant is introduced improperly, altering the balance of the mixture and potentially affecting the system's performance.

When adding a gas into the high side, it helps to ensure that the refrigerant maintains its blend and volatility characteristics. Doing so minimizes issues related to fractionation, where the different components of the blend could evaporate at varying rates, leading to a situation where the remaining refrigerant does not match the intended composition.

In contrast, introducing the refrigerant as either a liquid or in the low side could result in incorrect mixing or a potential phase change that could harm the refrigeration cycle. Using the high side allows the refrigerant to enter the compressor in gas form, ensuring that it is appropriately vaporized before entering the condenser and maintaining the integrity of the refrigerant blend throughout the process.