Residential HVAC Installation Practice Exam 2025 - Free HVAC Practice Questions and Study Guide

The best time to perform leak checking on a refrigeration system is when it is standing with dry nitrogen pressure of 150 psig in the system. This pressure test allows for the detection of any leaks in the system by pressurizing it with dry nitrogen, making it easier to pinpoint and repair any leaks before proceeding with further steps in the installation process.

Option A, performing leak checking during peak operation, would not be ideal as the system is actively running, which could make it harder to detect leaks due to the system's working conditions.

Option B, conducting leak checking after adding refrigerant, may not be effective as there may already be refrigerant in the system, which could interfere with the leak detection process.

Option D, performing leak checking during the defrost cycle, would not be suitable as the system is in a specific operating mode intended for defrosting, which may not be optimal for detecting leaks in the system.

Therefore, option C, performing leak checking when the system is standing with dry nitrogen pressure of 150 psig, is the best choice as it provides a controlled environment for effective leak detection in the refrigeration system.