According to ASHRAE Guideline 3, during a standard vacuum test, the system should be checked for leaks if the pressure rises from 1 mm Hg to above what level?

• A. 1.5 Hg
• B. 2.5 Hg
• C. 3.0 Hg
• D. 4.0 Hg

Answer: (B)

In a standard vacuum test, you’re checking whether the system can maintain a deep vacuum. Start with a very low pressure, around 1 mm Hg, and watch how the pressure changes. If it rises above 2.5 inches of mercury, that escape of air into the system indicates a leak. That 2.5 inHg threshold is chosen because small, normal fluctuations from outgassing or temperature changes stay well below it, while a rise past this point reliably signals a leak that needs to be found and repaired. Lower readings wouldn’t clearly differentiate a true leak from ordinary vac­uum stability, and much higher readings wouldn’t promptly identify leaks.