Controls, Electrical, and Safety Devices
This domain tests the interface between control logic, electrical circuits, and safety. Understand line voltage vs. low voltage control circuits, the role of the transformer, and how a thermostat completes the R-to-W, R-to-Y, R-to-G circuits. Know the sequence of a call for cooling: thermostat closes Y and G, contactor pulls in, compressor and condenser fan start, blower runs. Be able to read a ladder diagram and trace a circuit through switches in series and loads in parallel.
Safety devices to know: high-limit switch, flame sensor and rollout switch, pressure/draft switch, condensate overflow switch, and fuse/breaker protection. Understand that a rollout switch is a manual-reset safety indicating flame rollout - never bypass it. Know that a flame sensor proves flame via microamp signal and that a dirty sensor causes short cycling on lockout.
Traps: assuming a tripped breaker means a bad breaker rather than a short or grounded compressor; confusing normally open and normally closed contacts; and forgetting that control transformers are often fused on the secondary. Also watch for questions that treat a safety device as a control device.
Rule of thumb: Troubleshoot in order - power, control signal, safety string, load. Worked example: no cooling, thermostat calls but contactor doesn't pull in. Check 24V at contactor coil; if absent, trace back through the safety string (float switch, high-pressure switch) to find the open device rather than replacing the contactor.