Electrical Power, Motors, and HVAC Controls
Ohm's law and the control circuit
Most HVAC electrical troubleshooting happens on the 24-volt control circuit. Know Ohm's law (E = I × R), the difference between a series safety switch and a parallel load, and how to read a ladder diagram: power flows from the transformer through the thermostat and safeties to the contactor coil. Thermostat terminals are conventional: R (24V hot), C (common), W (heat), Y (cooling contactor), G (fan), O/B (reversing valve). A missing C wire breaks smart thermostats and is a common service call.
Motors carry nameplate FLA, a service factor, and locked-rotor amps. Running amps above nameplate suggests overload, low voltage, bad bearings, or restricted airflow. Overload protection (internal or external) protects the motor; the circuit breaker protects the conductors - they are not interchangeable. ECM blowers hold constant airflow by varying speed and torque, so static pressure problems show up as high amp draw rather than reduced airflow.
Traps: confusing LRA with running amps; misreading a normally-closed safety as open circuit; replacing a control fuse without finding the short; forgetting that a heat anticipator or setup parameter changes cycling; assuming a VFD solves a mechanical imbalance.
Worked example: a 24V control circuit with a 12Ω coil draws I = 24 ÷ 12 = 2A. If the transformer is rated 40VA, that coil alone consumes 48VA - the transformer will overheat until the circuit is corrected.
Always lock out and tag out, verify with a meter, and confirm the supply voltage matches the nameplate before condemning a component.