Refrigerant Recovery, Recycling, and Handling
The Regulatory Frame
School-bus A/C work falls under refrigerant handling rules that prohibit venting refrigerant to the atmosphere. Technicians must be certified, use EPA-approved recovery/recycling equipment, and follow proper evacuation and charging procedures. Questions test whether you know that recovery is required before opening a system, that recovery cylinders must not be overfilled (typically 80% by weight), and that refrigerant must never be mixed.
Recovery, Recycling, Evacuation, Charging
Know the sequence: recover refrigerant, then recycle (if the machine does), then evacuate to remove moisture and non-condensables, then charge by weight or per manufacturer spec. Evacuation pulls a deep vacuum and holds it to confirm no leaks. A vacuum that rises after the pump is isolated indicates a leak or moisture boiling off. Non-condensables (air) raise high-side pressure and reduce capacity.
Traps to Watch
A classic trap: a question describes topping off a system without evacuation. That is wrong - you must recover, evacuate, and charge to spec. Another trap is mixing R-12 and R-134a; contaminated refrigerant must be handled as a separate stream. Also watch for questions implying you can "just add a little" to a leaking system; EPA rules and good practice require locating and repairing leaks. Recovery cylinder questions often hinge on the 80% fill limit and never using a disposable or unapproved cylinder.
Worked Rule of Thumb
If a system holds a vacuum of about 500 microns (or manufacturer-specified level) for 10-15 minutes with the pump isolated and does not rise significantly, it is considered leak-free and dry enough to charge. A steady rise toward atmosphere means a leak; a rise that then stabilizes at a higher micron level often means moisture is still boiling off and more evacuation time is needed.