FC FC Electrical Components & Controls 2 — Questions and Answers
Question 1: Which terminal on a conventional thermostat wiring system carries the 24V common (return) signal?
- R terminal
- C terminal (Correct answer)
- W terminal
- Y terminal
Correct answer: C terminal
The C (common) terminal completes the 24V circuit from the transformer secondary, providing a return path for connected devices.
Question 2: What is the purpose of the pressure switch on an induced-draft furnace?
- To verify gas pressure is adequate
- To confirm the inducer motor is creating proper draft before allowing ignition (Correct answer)
- To sense indoor air pressure for comfort control
- To detect duct static pressure
Correct answer: To confirm the inducer motor is creating proper draft before allowing ignition
The pressure switch verifies that the draft inducer has established proper negative pressure before the control board allows the ignition sequence to proceed.
Question 3: A furnace control board flashes a fault code 3 times (pause) 1 time. How should a technician interpret this?
- Three faults occurred, then one more
- The code is 31 and corresponds to a specific fault in the manufacturer's chart (Correct answer)
- The board is counting 3 minutes to reset
- The LED is malfunctioning
Correct answer: The code is 31 and corresponds to a specific fault in the manufacturer's chart
Furnace control boards use flash codes where the number of long flashes followed by short flashes encodes a specific fault code (e.g., 3-1 = code 31).
Question 4: What is the function of the blower door interlock switch on a furnace?
- Controls blower speed
- Cuts power to the furnace if the blower compartment door is removed (Correct answer)
- Monitors filter restriction
- Senses blower motor temperature
Correct answer: Cuts power to the furnace if the blower compartment door is removed
The interlock switch de-energizes the furnace when the blower door is open, preventing operation with exposed moving parts and electrical components.
Question 5: What type of motor is most commonly used in high-efficiency furnace blowers for variable-speed operation?
- Split-phase induction motor
- Shaded-pole motor
- ECM (electronically commutated motor) (Correct answer)
- Universal motor
Correct answer: ECM (electronically commutated motor)
ECM motors use permanent magnets and electronic controls to vary speed efficiently, reducing energy consumption compared to PSC motors.
Question 6: When testing a furnace capacitor with a digital multimeter set to capacitance, a reading significantly below the rated MFD value indicates:
- The capacitor is functioning normally
- The capacitor has failed and should be replaced (Correct answer)
- The motor needs lubrication
- The capacitor is overcharged
Correct answer: The capacitor has failed and should be replaced
A capacitor reading more than 10% below its rated microfarad value indicates internal degradation and the component should be replaced.
FC FC Electrical Components & Controls 2
Which terminal on a conventional thermostat wiring system carries the 24V common (return) signal?