FC FC Electrical Components & Controls 1 — Questions and Answers
Question 1: What is the typical control voltage used in residential furnace control circuits?
- 12V DC
- 24V AC (Correct answer)
- 120V AC
- 240V AC
Correct answer: 24V AC
Most residential furnace control circuits operate at 24V AC, supplied by a step-down transformer inside the furnace.
Question 2: What is the function of the furnace transformer?
- Powers the blower motor
- Steps 120V down to 24V for the control circuit (Correct answer)
- Converts AC to DC for the igniter
- Regulates gas valve current
Correct answer: Steps 120V down to 24V for the control circuit
The step-down transformer reduces 120V line voltage to 24V AC to power the thermostat and control board.
Question 3: Which furnace safety control opens the circuit if the heat exchanger area overheats?
- Pressure switch
- Rollout switch
- High-limit switch (Correct answer)
- Draft proving switch
Correct answer: High-limit switch
The high-limit switch monitors supply air temperature and opens the control circuit if the heat exchanger area exceeds a safe temperature.
Question 4: What does a rollout switch do when it trips?
- Shuts off the blower motor
- Shuts off the gas valve because flames have rolled out of the combustion chamber (Correct answer)
- Resets the thermostat
- Opens the fresh air damper
Correct answer: Shuts off the gas valve because flames have rolled out of the combustion chamber
A rollout switch detects flame rollout from the combustion area and interrupts the gas valve circuit to prevent a fire hazard.
Question 5: The flame sensor (flame rod) in a furnace operates on which principle?
- Resistance change with temperature
- Flame rectification of AC current to DC microamps (Correct answer)
- Thermocouple millivoltage generation
- Optical infrared detection
Correct answer: Flame rectification of AC current to DC microamps
The flame rod uses flame rectification, where the ionized flame conducts AC asymmetrically, producing a measurable DC microamp signal.
Question 6: What minimum flame signal (microamps DC) is typically required for a furnace flame sensor to keep the gas valve open?
- 0.1 µA
- 0.5–1.0 µA (Correct answer)
- 5–10 µA
- 25 µA
Correct answer: 0.5–1.0 µA
Most furnace control boards require at least 0.5–1.0 µA of DC flame signal to confirm ignition and hold the gas valve open.
FC FC Electrical Components & Controls 1
What is the typical control voltage used in residential furnace control circuits?