NATE AC Service: Refrigerant Charge, Airflow, and Electrical Verification
The Air Conditioning Service domain tests whether you can measure, diagnose, and correct a residential split system's charge, airflow, and electrical components - not simply recite the refrigeration cycle. Items are scenario-driven: you are handed gauge readings, line temperatures, indoor conditions, static pressure, and amp draws, and asked what those numbers prove and what you would do next. The exam rewards technicians who verify in a fixed order: airflow first, then charge, then electrical.
Charge verification depends on the metering device
- Fixed orifice / piston: charge by superheat, typically 8–12°F at the outdoor unit, with the thermocouple on the suction line 6–12 in. from the compressor and insulated from ambient.
- TXV / EEV: charge by subcooling, typically 8–12°F at the liquid line, per the manufacturer's data plate.
- Superheat = suction line temperature − saturation temperature from suction pressure.
- Subcooling = saturation temperature from liquid pressure − liquid line temperature.
- Never adjust charge until airflow is confirmed correct; low airflow mimics an undercharge and will send you chasing refrigerant.
Airflow targets
- 350–450 CFM per ton, 400 nominal; verify at the blower with a flow hood or by temperature split.
- Temperature split (delta T) 16–22°F, taken at the return and supply after 15 minutes of steady operation.
- Total external static pressure around 0.5 in. w.c. for typical residential equipment; higher values point to restrictive filters, undersized returns, or a dirty coil.
Worked example
R-410A, 3-ton TXV system. Suction pressure 120 psig ≈ 41°F saturation; suction line 53°F gives 12°F superheat - acceptable. Liquid pressure 350 psig ≈ 107°F saturation; liquid line 97°F gives 10°F subcooling - in range. Return 75°F DB / 63°F WB, supply 58°F DB gives a 17°F delta T. Conclusion: charge and airflow are both acceptable; do not add refrigerant.
Traps that cost marks
- Chasing subcooling on a fixed-orifice system, or superheat on a TXV system.
- Taking line temperatures with an uninsulated probe or a long hot manifold hose, which skews superheat high.
- Diagnosing a dirty condenser as an undercharge: high subcooling with normal-to-high superheat indicates a restriction or poor heat rejection, not low refrigerant.
- Ignoring indoor wet-bulb when comparing delta T to a target chart.