FC FC Heat Exchangers & Blower Systems 2 — Questions and Answers
Question 1: What is AFUE and what does a rating of 96% mean for a furnace?
- Annual Fuel Utilization Efficiency; 96% of fuel energy is converted to usable heat (Correct answer)
- Air Flow Utilization Efficiency; 96% of airflow reaches the registers
- Average Furnace Unit Efficiency; the furnace is 96% reliable
- Actual Flue Usage Efficiency; 4% of flue gas is recycled
Correct answer: Annual Fuel Utilization Efficiency; 96% of fuel energy is converted to usable heat
AFUE (Annual Fuel Utilization Efficiency) measures seasonal heating efficiency; 96% AFUE means 96 cents of every dollar of fuel is converted to heat.
Question 2: What is the purpose of lubricating PSC blower motor bearings annually?
- To increase blower speed
- To reduce friction, prevent overheating, and extend motor bearing life (Correct answer)
- To reduce electrical noise
- To prevent moisture from entering the motor
Correct answer: To reduce friction, prevent overheating, and extend motor bearing life
Lubricating sleeve or ball bearings in PSC motors reduces friction heat, prevents premature wear, and extends motor service life.
Question 3: What is the consequence of running a furnace blower with an unbalanced or dirty squirrel cage wheel?
- Improved airflow from centrifugal effect
- Vibration, noise, reduced airflow, and premature motor and bearing failure (Correct answer)
- Lower static pressure
- Increased heat exchanger temperature
Correct answer: Vibration, noise, reduced airflow, and premature motor and bearing failure
Dirt buildup on blower wheel blades causes imbalance, vibration, and airflow reduction that stresses the motor shaft and bearings.
Question 4: When measuring blower motor amperage, a reading above the nameplate FLA (Full Load Amps) indicates which condition?
- The motor is running efficiently
- The motor is overloaded due to restricted airflow or a failing motor (Correct answer)
- The capacitor is oversized
- Normal operation at high static pressure
Correct answer: The motor is overloaded due to restricted airflow or a failing motor
Current draw above nameplate FLA indicates the motor is working harder than designed, typically from restricted airflow or internal motor problems, risking overheating.
Question 5: What is the function of the blower delay-off control at the end of a heating cycle?
- To prevent short cycling of the thermostat
- To keep the blower running after the burner shuts off to recover residual heat from the heat exchanger (Correct answer)
- To cool the combustion chamber rapidly
- To purge the flue of remaining combustion gases
Correct answer: To keep the blower running after the burner shuts off to recover residual heat from the heat exchanger
The blower continues running for 1–3 minutes after burner shutdown to extract residual heat stored in the heat exchanger and deliver it to the living space.
Question 6: A variable-speed furnace blower running at reduced speed during heating mode provides which benefit?
- Faster recovery from large temperature setbacks
- More even temperature distribution and quieter operation with lower energy use (Correct answer)
- Higher static pressure to overcome dirty filters
- Faster heat exchanger warm-up
Correct answer: More even temperature distribution and quieter operation with lower energy use
Running at lower speeds distributes heat more evenly, reduces noise, and consumes significantly less electricity than full-speed operation.
FC FC Heat Exchangers & Blower Systems 2
What is AFUE and what does a rating of 96% mean for a furnace?