Final answer:
The recommended duration to run a vacuum pump after achieving 29.9 inches Hg vacuum on an R-134a system depends on the system and moisture levels but is typically 15 to 30 minutes for small systems. Consulting the service manual or a qualified technician is essential for specific recommendations.
Step-by-step explanation:
When evacuating an R-134a system, the duration for which the vacuum pump should be run after the low-pressure manifold gauge pulls 29.9 inches (760 mm) Hg vacuum can vary based on the system size and amount of moisture.
However, for a complete and thorough evacuation, it is recommended to run the vacuum pump for an extended period even after achieving the desired vacuum level to ensure that all moisture is removed. Industry best practices suggest maintaining the vacuum for a minimum duration, often recommended by system manufacturers or in accordance with applicable regulations or standards, which can commonly range from 15 minutes to 30 minutes for small systems.
For larger systems or those with significant moisture, the vacuum may be held for several hours. It is crucial to consult the system's service manual or a qualified technician for the specific duration appropriate for the system in question.