Heating, Ventilation, and Air Conditioning Systems
The refrigeration cycle and service discipline
HVAC questions test the refrigeration cycle and safe service practice. Refrigerant enters the compressor as low-pressure gas, is compressed to high-pressure hot gas, condenses to liquid in the condenser while shedding heat, passes through an expansion device (fixed orifice tube or thermostatic expansion valve) that drops pressure and temperature, then boils in the evaporator while absorbing cabin heat, and returns to the compressor. An accumulator on orifice-tube systems or a receiver-drier on expansion-valve systems stores refrigerant and removes moisture.
Diagnose with pressures and temperatures, not guesses. Low side too low with a high side too low suggests an undercharge or restriction; both sides high suggests poor condenser airflow or overcharge; a low side that pulls into vacuum suggests a blockage. Superheat is evaporator outlet temperature minus saturation temperature at the low-side pressure; too little superheat risks liquid slugging, too much means starved evaporator. Subcooling checks charge on expansion-valve systems.
Service procedure: identify the refrigerant, recover rather than vent (certification is required by regulation), recycle, evacuate to remove moisture, then charge by weight. Use the correct oil - PAG oils are hygroscopic and vary by system; electric compressors use specific oils and non-conductive refrigerant. R-1234yf is mildly flammable (A2L class), so handling, leak detection, and equipment differ from R-134a.
Also know the air-side: blower motor and resistor, blend door actuators, cabin filter, heater core flow, and mode doors.
Trap: adding refrigerant to a system with a leak instead of finding it with dye, electronic detection, or pressure testing.
Rule of thumb: a healthy vent temperature is roughly 17–22 °C below ambient in moderate humidity; high humidity reduces that margin.