Air Conditioning and Refrigeration Gas
Core concepts
Candidates must know refrigerant families and their safety classifications: CFCs (phased out), HCFCs (e.g. R-22, being phased out), HFCs (e.g. R-134a, R-410A), HFOs (e.g. R-1234yf), and natural refrigerants (ammonia R-717, CO2 R-744, hydrocarbons). Know the ASHRAE safety group letters (A1 low toxicity/low flammability, A2L lower flammability, A3 higher flammability, B1/B2 higher toxicity). Understand the vapour-compression cycle: evaporation (absorb heat at low pressure), compression, condensation (reject heat at high pressure), expansion. Know superheat and subcooling as diagnostic measures - superheat too high means undercharge or restricted flow; subcooling indicates charge in the condenser. Understand the ozone depletion potential (ODP) and global warming potential (GWP) concepts and why R-410A (GWP ~2088) is being replaced. Know recovery, recycling and reclamation requirements, and never vent refrigerant to atmosphere. Know pressure–temperature relationship: each refrigerant has a unique saturation curve, so a gauge reading means nothing without the refrigerant identity and temperature.
Traps
A frequent trap is confusing superheat and subcooling, or applying an R-22 pressure chart to R-410A. Another is assuming all HFCs are non-flammable - A2L refrigerants are mildly flammable. Watch for questions on evacuation levels (measured in microns) and the need for deep vacuum and nitrogen pressure testing, not oxygen.
Worked rule of thumb
Target superheat at the evaporator outlet is typically 5–8 K (about 9–14 °F) for a properly charged system; if measured superheat is far above target, suspect undercharge or a restriction. Always pressure-test with dry nitrogen and evacuate to below 500 microns before charging.