FC Heating System Maintenance & Troubleshooting 2

Question 1

A furnace cycles on and off frequently (short-cycles) without reaching the set temperature. What is the MOST likely cause?

Accepted Answer

Clogged air filter restricting airflow

Answer

A clogged air filter restricts airflow over the heat exchanger, causing it to overheat and trip the high-limit switch repeatedly.

Question 2

When inspecting a gas furnace heat exchanger, which method is considered most reliable for detecting cracks?

Accepted Answer

Observing flame pattern disturbance when blower starts

Answer

A cracked heat exchanger typically causes the burner flame to waver or blow out when the blower motor starts, indicating combustion gas is mixing with supply air.

Question 3

A technician measures a static pressure drop of 1.2 in. W.C. across a 1-inch pleated filter rated at 0.5 in. W.C. maximum. What should be done?

Accepted Answer

Replace the filter immediately

Answer

A pressure drop more than double the filter's rated maximum indicates a severely clogged filter that must be replaced to restore proper airflow.

Question 4

Which component in a forced-air furnace is responsible for preventing the blower from running when the heat exchanger is too cold on startup?

Accepted Answer

Fan-limit control

Answer

The fan-limit control delays blower operation until the heat exchanger reaches an adequate temperature, preventing cold air circulation at startup.

Question 5

A furnace ignites but the flame shuts off after 8–10 seconds. The most probable cause is:

Accepted Answer

Failed flame sensor (dirty or corroded)

Answer

A dirty or corroded flame sensor cannot conduct enough microamps to prove the flame, so the control board locks out and extinguishes the burner.

(FC) Furnace Installation and Maintenance Certification Practice Test

(FC) Furnace Installation and Maintenance Certification Practice Test

FC Heating System Maintenance & Troubleshooting 2