Air Conditioning: Airflow, Charge Verification, and Psychrometrics
The Air Conditioning portion of the Master LC-AC exam targets light commercial DX cooling: proving airflow, verifying charge against the metering device, and using psychrometrics to decide which of the two is wrong. Expect numbers and a diagnostic order, not memorized gauge readings.
Airflow first
- Nominal design is 400 CFM per ton, with 350–450 CFM/ton as the usable band. Three tons means about 1,200 CFM.
- Confirm airflow with external static pressure and the blower performance table, not by hand-feel at a register.
- Low airflow raises the temperature split and drops suction pressure; it counterfeits an undercharge. Fix airflow before you touch refrigerant.
Then charge, by metering-device type
- Fixed orifice or bleed-port piston: charge to superheat.
- TXV or EEV: charge to subcooling, typically 8–12°F, always per the nameplate.
- Target superheat ≈ (3 × indoor wet-bulb − 80 − outdoor dry-bulb) ÷ 2. Indoor wet-bulb drives it; dry-bulb does not.
Worked example
A 3-ton R-410A split runs 1,200 CFM with return air at 75°F DB / 63°F WB and supply air at 63°F DB. Split = 12°F. Sensible capacity ≈ 1.08 × 1,200 × 12 = 15,550 Btu/h. The expanded rating table shows roughly 25,000 Btu/h sensible at these entering conditions, so the unit is delivering about 62% of rating. Because airflow is already verified, the loss is on the refrigerant side: check superheat or subcooling, then compressor amperage against nameplate RLA. Had airflow measured 850 CFM, you would correct the blower first and re-test.
Traps that cost marks
- Charging by suction pressure alone.
- Taking superheat at the service valve rather than at the evaporator outlet.
- Using dry-bulb where the charging chart requires wet-bulb.
- Judging charge during the first few minutes, or at outdoor ambient below about 65°F without the manufacturer's low-ambient procedure.