Which gas is used to purge the flush after flushing lines and coils?

• A. Nitrogen
• B. Oxygen
• C. Argon
• D. Helium

Answer: (A)

When purging after flushing lines and coils, you want a dry, inert gas to displace air and moisture and prevent any chemical reactions inside the system. Nitrogen fits this role perfectly because it is dry, non-reactive with refrigerants and oils, and widely available and inexpensive in HVACR shops. Using nitrogen helps keep oxygen and moisture levels low, reducing the risk of corrosion, oxidation, or hydrolysis that could harm seals and components.

Oxygen would introduce reactive gas that can cause rust and other issues, so it’s not suitable. Argon and helium are also inert but are more expensive or less practical to supply in typical service environments, making nitrogen the standard choice for purge gas.