FC FC Electrical Components & Controls

Question 1

What is the typical control voltage used in residential furnace control circuits?

Accepted Answer

24V AC

Answer

12V DC

Answer

120V AC

Answer

240V AC

Question 2

What is the function of the furnace transformer?

Accepted Answer

Steps 120V down to 24V for the control circuit

Answer

Powers the blower motor

Answer

Converts AC to DC for the igniter

Answer

Regulates gas valve current

Question 3

Which furnace safety control opens the circuit if the heat exchanger area overheats?

Accepted Answer

High-limit switch

Answer

Pressure switch

Answer

Rollout switch

Answer

Draft proving switch

Question 4

What does a rollout switch do when it trips?

Accepted Answer

Shuts off the gas valve because flames have rolled out of the combustion chamber

Answer

Shuts off the blower motor

Answer

Resets the thermostat

Answer

Opens the fresh air damper

Question 5

The flame sensor (flame rod) in a furnace operates on which principle?

Accepted Answer

Flame rectification of AC current to DC microamps

Answer

Resistance change with temperature

Answer

Thermocouple millivoltage generation

Answer

Optical infrared detection

Question 6

What minimum flame signal (microamps DC) is typically required for a furnace flame sensor to keep the gas valve open?

Accepted Answer

0.5–1.0 µA

Answer

0.1 µA

Answer

5–10 µA

Answer

25 µA