FC Heating System Maintenance & Troubleshooting 2 — Questions and Answers
Question 1: A furnace cycles on and off frequently (short-cycles) without reaching the set temperature. What is the MOST likely cause?
- Oversized heat exchanger
- Clogged air filter restricting airflow (Correct answer)
- Low thermostat setpoint
- Faulty flue pipe
Correct answer: Clogged air filter restricting airflow
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?
- Listening for rattling sounds during operation
- Observing flame pattern disturbance when blower starts (Correct answer)
- Checking CO levels at the supply register
- Measuring heat exchanger surface temperature
Correct answer: Observing flame pattern disturbance when blower starts
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?
- Replace the filter immediately (Correct answer)
- Increase blower motor speed
- Install a bypass duct around the filter
- Clean the filter with compressed air
Correct answer: Replace the filter immediately
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?
- High-limit switch
- Fan-limit control (Correct answer)
- Pressure switch
- Rollout switch
Correct answer: Fan-limit control
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:
- Faulty draft inducer motor
- Failed flame sensor (dirty or corroded) (Correct answer)
- Cracked heat exchanger
- Undersized gas supply line
Correct answer: Failed flame sensor (dirty or corroded)
A dirty or corroded flame sensor cannot conduct enough microamps to prove the flame, so the control board locks out and extinguishes the burner.
Question 6: What is the correct procedure for cleaning a flame sensor rod?
- Wipe with a damp cloth and reinstall
- Lightly abrade with fine steel wool or emery cloth (Correct answer)
- Soak in vinegar for 30 minutes
- Sand with 60-grit sandpaper
Correct answer: Lightly abrade with fine steel wool or emery cloth
Light abrasion with fine steel wool or emery cloth removes oxide buildup from the sensor rod without damaging it.
Question 7: A homeowner reports that one room is significantly colder than others even though the furnace runs continuously. Which maintenance item should be checked first?
- Thermostat calibration
- Duct damper positions and duct integrity in that zone (Correct answer)
- Furnace heat exchanger condition
- Gas valve pressure setting
Correct answer: Duct damper positions and duct integrity in that zone
Closed or leaking duct dampers and duct disconnections are the most common cause of uneven heating in a specific room.
FC Heating System Maintenance & Troubleshooting 2
A furnace cycles on and off frequently (short-cycles) without reaching the set temperature.
What is the MOST likely cause?