Refrigerant Charging, Airflow, and Efficiency Verification
Charging and efficiency verification questions combine refrigerant behavior, airflow measurement, and performance ratings. The exam expects you to choose the correct charging method and prove the system meets airflow and temperature-rise targets.
Charging methods
- Weigh-in is the most accurate method when the line set and charge are known.
- TXV/EEV systems are charged to target subcooling at the outdoor and indoor conditions. Fixed-orifice systems are charged to target superheat.
- In heating, use the manufacturer's heating-charge chart or approach method; do not apply cooling subcooling targets. Defrost and low ambient affect readings, so stabilize conditions first.
- Verify airflow before charging. Most residential systems target about 350–450 CFM per ton, with 400 CFM/ton as a common nominal value; always follow the blower table and static-pressure limit.
Efficiency verification
- Measure total external static pressure and compare to the blower performance table.
- Use temperature rise = output Btu/h ÷ (1.08 × CFM) for electric heat.
- Check duct leakage, filter pressure drop, refrigerant line insulation, and cabinet sealing.
- Confirm rated performance concepts: SEER2 for cooling, HSPF2 for heating, and that matched components are required for the rating.
Worked example
A 3-ton air handler moves 1,200 CFM and the electric strip heat output is 36,000 Btu/h. Temperature rise = 36,000 ÷ (1.08 × 1,200) = 36,000 ÷ 1,296 ≈ 27.8°F. If measured rise is 20°F, airflow is likely high or the heater is not at rated output; check the blower tap, duct static, and heater amps.
Traps
Do not charge by suction pressure alone. Do not verify airflow from thermostat fan setting. Do not use cooling targets in heating mode, and do not ignore the difference between nominal and actual CFM.