Make your job easier with our handy refrigerant charging and diagnostic HVAC app HVAC Buddy. If you are servicing equipment the best way to begin diagnosis and determine the proper refrigerant charge is to use HVAC Buddy, a set of gauges, and a thermometer. Superheat for fixed capillary or Subcool forTXV/EXV systems. Calculates targeted vs. Actual Subcooling.