NATE AC Service cheat sheet

Superheat = suction line temperature minus evaporator saturation temperature; target 8-12°F at the evaporator outlet for a fixed-orifice (piston) system with a normal load.

Subcooling = condensing saturation temperature minus liquid line temperature; target 8-12°F for a TXV system, measured at the condenser outlet or liquid line at the valve.

TXV systems are charged to subcooling, fixed-orifice/piston systems to superheat; adding refrigerant raises subcooling and lowers superheat.

Evaporator saturation temperature from a gauge reading must be converted using the low-side pressure-temperature chart for the refrigerant in use; R-410A and R-22 charts are not interchangeable.

Standard airflow is 400 CFM per ton of nominal capacity; a 3-ton system moves about 1,200 CFM, and low airflow lowers suction pressure and raises superheat.

Total external static pressure is measured across the blower with both return and supply taps; typical residential design is 0.5 in. w.c., with 0.8 in. w.c. or more indicating duct or filter restriction.

Delta-T across the evaporator coil is typically 16-22°F for a properly charged, correctly airflow-adjusted system; low delta-T suggests low charge, high airflow, or a dirty coil.

Temperature split check: measure return air dry-bulb at the filter grille and supply air at the nearest register after 15 minutes of steady operation; readings taken before stabilization are invalid.

Compressor amp draw compared to nameplate RLA confirms electrical health; a compressor drawing well below RLA with low head pressure indicates a low charge or a failing compressor valve.

Superheat = suction line temperature minus evaporator saturation temperature; target 8-12°F at the evaporator outlet for a fixed-orifice (piston) system with a normal load.

Subcooling = condensing saturation temperature minus liquid line temperature; target 8-12°F for a TXV system, measured at the condenser outlet or liquid line at the valve.

TXV systems are charged to subcooling, fixed-orifice/piston systems to superheat; adding refrigerant raises subcooling and lowers superheat.

Evaporator saturation temperature from a gauge reading must be converted using the low-side pressure-temperature chart for the refrigerant in use; R-410A and R-22 charts are not interchangeable.

Standard airflow is 400 CFM per ton of nominal capacity; a 3-ton system moves about 1,200 CFM, and low airflow lowers suction pressure and raises superheat.

Total external static pressure is measured across the blower with both return and supply taps; typical residential design is 0.5 in. w.c., with 0.8 in. w.c. or more indicating duct or filter restriction.

Delta-T across the evaporator coil is typically 16-22°F for a properly charged, correctly airflow-adjusted system; low delta-T suggests low charge, high airflow, or a dirty coil.

Temperature split check: measure return air dry-bulb at the filter grille and supply air at the nearest register after 15 minutes of steady operation; readings taken before stabilization are invalid.

Compressor amp draw compared to nameplate RLA confirms electrical health; a compressor drawing well below RLA with low head pressure indicates a low charge or a failing compressor valve.