Air Conditioning and Heat Pump Fundamentals
Core concepts
A candidate must be fluent in the refrigeration cycle: compression, condensation, metering, evaporation, and the return of superheated vapor to the compressor. Understand pressure–temperature relationships, superheat and subcooling, latent versus sensible heat, and the difference between a heat pump in heating and cooling modes, including the reversing valve and defrost. Know sensible heat ratio, wet-bulb and dry-bulb, and how airflow affects capacity.
Traps in questions
Watch for confusion between superheat (measured at evaporator outlet) and subcooling (at condenser outlet). Questions often swap the direction of heat flow in heating mode. Remember that a heat pump's capacity falls as outdoor temperature drops, and that defrost cycles temporarily interrupt heating. Do not assume higher refrigerant charge always means better performance; overcharge raises subcooling and can flood the compressor.
Worked example
If suction pressure corresponds to a 40°F evaporating temperature and suction line temperature is 52°F, superheat is 12°F. Rule of thumb: target superheat at the evaporator is typically 8–12°F for a fixed-orifice system, adjusted by indoor wet-bulb and outdoor dry-bulb.
Study tip
Trace the cycle on a pressure–enthalpy chart and label each state point; this anchors both conceptual and diagnostic questions.