I need to learn more about the refrigeration side of things...I have been avoiding it for too long!!
These were my exact words earlier this year.
So, I started out watching a few You Tube videos explaining the principles,
then I stumbled on this website:
http://epatest.com/ although I know there are a few others similar.
Within 2 weeks I not only learned the basics but got my EPA 608 certification to legally purchase and dispose of refrigerants.
Total cost was about $25.00, everything is done on-line, material is easy to understand, plenty of practice questions and open book exams.
I would highly recommend this course to anyone who likes to work on their own machines. (it's also simply a feather in the cap for us old guys)