Electrical Controls, Motors and Troubleshooting
Circuits and controls
Control circuits are series strings: 24 volts from the transformer passes through the thermostat, every safety, and the load. Any open safety - high limit, rollout switch, pressure switch, auxiliary limit - kills the whole string, so voltage-drop testing across each device finds the culprit faster than guessing. Terminals are R for 24 V hot, C for common, W for heat, Y for cooling, G for fan, and O/B for the reversing valve. Voltage divides in a series circuit; current divides in parallel.
Motors
PSC motors require a run capacitor. ECM and variable-speed motors use an electronic module and are often programmed for constant torque or constant airflow. Discharge and isolate a capacitor before measuring microfarads, and compare with the nameplate rating, typically within 6-10 percent. Amp draw above nameplate points to overload, bad bearings, a weak capacitor, or restricted airflow. Locked rotor amps are the startup value; running amps should be far below it.
Troubleshooting discipline
Verify power at the disconnect, then control voltage at the transformer, then follow the schematic. Board LEDs are clues, not verdicts. Check the cheap things first: filter, capacitor, connections, thermostat wiring.
Traps
Testing a capacitor in circuit, assuming a blown low-voltage fuse means a bad board (it usually means a shorted wire), confusing a sequencer with a contactor, and forgetting that a 40 VA transformer supplies only about 1.6 A at 24 V.
Worked example
A gas valve coil measuring 48 ohms on a 24 volt circuit draws 24 / 48 = 0.5 A, comfortably within transformer capacity.