FC Heating System Maintenance & Troubleshooting 3 — Questions and Answers
Question 1: What is the primary purpose of an induced-draft (power vent) blower on a mid-efficiency gas furnace?
- To increase combustion air temperature before mixing with gas
- To pull flue gases through the heat exchanger and expel them outdoors (Correct answer)
- To pressurize the combustion chamber for better mixing
- To recirculate exhaust gases for higher efficiency
Correct answer: To pull flue gases through the heat exchanger and expel them outdoors
The induced-draft blower creates negative pressure in the flue, drawing combustion gases through the heat exchanger and venting them safely outdoors.
Question 2: A technician finds that the condensate drain on a 96% AFUE furnace is blocked. What is the MOST serious consequence if it is not cleared?
- Reduced combustion efficiency only
- Condensate backs up into the secondary heat exchanger, causing a pressure switch fault and lockout (Correct answer)
- The blower motor overheats
- Flue gases reverse into the living space immediately
Correct answer: Condensate backs up into the secondary heat exchanger, causing a pressure switch fault and lockout
Blocked condensate backs up into the secondary heat exchanger and pressure switch tubing, tripping the pressure switch and shutting down the furnace.
Question 3: When performing an annual furnace tune-up, which task should be completed LAST?
- Check gas pressure at the manifold
- Clean the burners
- Verify CO levels at supply registers with the system running (Correct answer)
- Inspect the flue vent connection
Correct answer: Verify CO levels at supply registers with the system running
Carbon monoxide verification should be done last with the system fully assembled and running under normal operating conditions to detect any leaks introduced during servicing.
Question 4: A furnace pressure switch trips on each heating call but resets when the draft inducer is turned off and back on manually. What should the technician inspect?
- Thermostat wiring for shorts
- Inducer motor RPM, venting for blockages, and pressure switch hose for cracks (Correct answer)
- Igniter resistance
- Gas valve solenoid voltage
Correct answer: Inducer motor RPM, venting for blockages, and pressure switch hose for cracks
Intermittent pressure switch faults often point to weak inducer motor performance, blocked venting, or cracked pressure switch hoses that cause loss of differential pressure.
Question 5: A gas furnace produces a yellow-orange flame instead of a steady blue flame. Which condition does this indicate?
- Excess primary air
- Insufficient primary air causing incomplete combustion (Correct answer)
- High gas pressure
- Overheating of the heat exchanger
Correct answer: Insufficient primary air causing incomplete combustion
A yellow-orange flame indicates incomplete combustion due to insufficient primary air, which can also produce dangerous levels of carbon monoxide.
Question 6: How often should the condensate trap and drain line on a high-efficiency condensing furnace be inspected or flushed?
- Every 5 years
- Only when blockage symptoms appear
- At least once per heating season (Correct answer)
- Every 10 years or at manufacturer intervals
Correct answer: At least once per heating season
The condensate trap and drain line should be inspected and flushed at least annually to prevent blockages that can trigger pressure switch lockouts.
Question 7: A technician notices scale and rust deposits inside the secondary heat exchanger of a condensing furnace. What is the BEST corrective action?
- Apply pipe sealant to reduce moisture contact
- Flush with a mild acid solution approved for HVAC condensate systems
- Replace the heat exchanger if deposits are severe (Correct answer)
- Increase the furnace thermostat setpoint to reduce condensation
Correct answer: Replace the heat exchanger if deposits are severe
Severe scale and rust deposits in the secondary heat exchanger restrict airflow and reduce efficiency; replacement is required when damage is significant.
FC Heating System Maintenance & Troubleshooting 3
What is the primary purpose of an induced-draft (power vent) blower on a mid-efficiency gas furnace?