System Performance, Efficiency and Troubleshooting
System Performance
Verify with air-side math, confirm with refrigerant-side math
A performance check requires both sides. On the air side, total capacity = 4.5 × CFM × Δh, sensible capacity = 1.08 × CFM × ΔT, and latent capacity = 0.68 × CFM × Δgrains. You need dry-bulb and wet-bulb at the return and supply to get enthalpy; dry bulb alone cannot tell you total capacity. On the refrigerant side, compare superheat, subcooling, suction saturation, and compressor amperage against the manufacturer's charging chart and performance data.
Temperature split
A 16-22°F evaporator air temperature difference is the usual residential band, but it is a screening tool, not a diagnosis. A high split usually means low airflow - filter, coil, blower, or duct restriction. A low split usually means low load (low entering wet bulb), excess airflow, or a refrigerant-side problem such as undercharge.
Traps
Taking temperatures too close to the coil where the air has not mixed; using dry bulb only; assuming a system is fine because the space is cool; and comparing compressor amps to nameplate without correcting for voltage and ambient. Also remember to measure entering wet bulb - it drives everything downstream.
Worked example
1,200 CFM measured, return air enthalpy 29.5 Btu/lb, supply air enthalpy 23.0 Btu/lb. Total capacity = 4.5 × 1,200 × 6.5 = 35,100 Btu/h - very close to the 36,000 Btu/h nominal rating, so the system is performing. If the same unit delivered only 24,000 Btu/h, the refrigerant charge or airflow is suspect.
Efficiency and Troubleshooting
Know the ratings and what they measure
SEER2 and EER2 rate cooling; HSPF2 rates heat pump heating; AFUE rates fossil-fuel and electric furnaces. SEER2 is tested at a higher external static pressure than the legacy SEER test, so SEER2 values for the same equipment are lower than SEER values - never mix the two in a comparison. Ratings are laboratory part-load numbers; installed performance depends on airflow, charge, duct leakage, and envelope load.
A diagnostic sequence beats guessing
Work in a fixed order: verify airflow and filter, then the temperature split, then superheat and subcooling, then electrical readings. Common patterns are worth memorizing. Low suction, high superheat, low subcooling: undercharge or a restriction - a temperature drop across the filter drier confirms a restriction. High head, high subcooling, high amps: overcharge or a dirty condenser. Low suction with low superheat: low airflow or an overfeeding metering device. Compressor hums and trips: capacitor, windings, or contactor.
Worked example
A 3-ton unit runs 1,200 full-load hours per year at SEER2 14.3. Annual Btu = 36,000 × 1,200 = 43.2 million. kWh = 43,200,000 ÷ 14.3 ÷ 1,000 = 3,021 kWh. At $0.16 per kWh the annual cost is about $483 - a quick way to price an efficiency upgrade.
Traps
Comparing SEER2 to SEER, blaming the charge for a low-airflow symptom, and condemning a compressor before checking the capacitor. Also remember that a restriction can produce normal subcooling upstream of the blockage, so measure temperatures on both sides of the drier.