Refrigerant Cycle and Charge Diagnostics
What this domain tests
This domain tests whether you can read the refrigerant cycle from pressures and temperatures, choose between superheat and subcooling charging, and separate charge faults from airflow faults.
Core rules
- Convert gauge pressure to saturation temperature with the correct PT chart. R-410A and R-32 have small glide; R-407C has larger glide, so use bubble and dew values correctly.
- Fixed-orifice systems are charged by superheat; TXV systems by subcooling. Target superheat for fixed orifice can be estimated as (3 × indoor WB − 80 − outdoor DB) / 2.
- Superheat = suction line temperature − evaporating saturation temperature. Subcooling = condensing saturation temperature − liquid line temperature.
- High superheat with normal subcooling suggests low airflow or a restriction; low superheat suggests overcharge or a flooded evaporator.
- Condensing temperature should sit roughly 10–20°F above outdoor air; evaporating temperature roughly 20–35°F below return wet-bulb, depending on the coil.
Worked example
R-410A fixed-orifice system, indoor 63°F WB, outdoor 95°F DB. Target superheat = (3 × 63 − 80 − 95) / 2 = 7°F. Suction pressure is 118 psig, about 40°F saturation; suction line temperature is 60°F, so superheat is 20°F, which is 13°F above target. Before adding refrigerant, check airflow and indoor wet-bulb; a dirty filter or low blower speed produces the same high superheat.
Traps
- Charging by suction pressure alone.
- Using the wrong PT chart or ignoring glide on a blend.
- Measuring superheat before the system stabilizes.
- Adding charge to fix high superheat caused by low airflow.