FC FC Electrical Components & Controls

Question 1

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

Accepted Answer

24V AC

Answer

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?

Accepted Answer

Steps 120V down to 24V for the control circuit

Answer

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?

Accepted Answer

High-limit switch

Answer

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?

Accepted Answer

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

Answer

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?

Accepted Answer

Flame rectification of AC current to DC microamps

Answer

The flame rod uses flame rectification, where the ionized flame conducts AC asymmetrically, producing a measurable DC microamp signal.

(FC) Furnace Installation and Maintenance Certification Practice Test