NATE Core: HVACR Fundamentals
What this domain tests
The NATE Core exam is the prerequisite knowledge test for NATE certifications. It checks HVACR fundamentals rather than a single specialty: electrical safety and basic circuits, hand and test instruments, heat-transfer principles, the refrigeration cycle, air properties, and field math. Questions often combine a measurement with a rule - for example, reading a pressure, converting it to saturation temperature, and calculating superheat.
Core rules and concepts
- Ohm's law: E = I × R; power: P = E × I. In series circuits current is common; in parallel circuits voltage is common, and total resistance is less than the smallest branch.
- Gauge pressure is measured above atmosphere; absolute pressure equals gauge pressure plus atmospheric pressure (about 14.7 psi at sea level).
- Sensible heat changes temperature; latent heat changes state. One BTU raises one pound of water 1°F. One ton of refrigeration equals 12,000 BTU/h.
- Refrigeration cycle: the compressor raises pressure and temperature; the condenser rejects heat; the metering device drops pressure; the evaporator absorbs heat.
- Superheat = suction-line temperature − saturated suction temperature. Subcooling = saturated liquid temperature − liquid-line temperature.
- Standard air is about 0.075 lb/ft³. Sensible airflow: BTU/h = 1.08 × CFM × ΔT. Total airflow: BTU/h = 4.5 × CFM × Δenthalpy.
- Safety: use lockout/tagout, PPE, and refrigerant recovery. Pressure-test with nitrogen, never oxygen.
Worked example
A suction line reads 52°F. The PT chart for the measured suction pressure shows 40°F saturation. Superheat = 52°F − 40°F = 12°F. The reasoning: superheat is measured at the evaporator outlet and uses the saturated temperature from the pressure, not the suction-line temperature alone. Confirm the target against the manufacturer's charging chart.
Traps that cost marks
- Treating psig as psia; add about 14.7 psi at sea level.
- Subtracting the wrong temperature for superheat or subcooling.
- Using dry-bulb temperature where wet-bulb or enthalpy is needed.
- Adding parallel resistances directly instead of using reciprocals.
- Assuming 400 CFM/ton alone proves correct charge.