FC FC Heat Exchangers & Blower Systems 1 — Questions and Answers
Question 1: What is the primary function of the heat exchanger in a gas furnace?
- To filter combustion gases
- To transfer heat from combustion gases to the circulating air without allowing gases to mix (Correct answer)
- To store heat between cycles
- To regulate gas pressure
Correct answer: To transfer heat from combustion gases to the circulating air without allowing gases to mix
The heat exchanger is a sealed metal chamber that transfers heat from hot combustion gases to the circulating supply air while keeping them completely separated.
Question 2: Which test method uses a chemical tracer gas introduced into the combustion side to detect heat exchanger cracks?
- Combustion analysis
- CO ambient test
- Tracer gas leak test (e.g., nitrogen with halide detector) (Correct answer)
- Thermal imaging only
Correct answer: Tracer gas leak test (e.g., nitrogen with halide detector)
A tracer gas test introduces a detectable gas (such as nitrogen with a halogen additive) into the combustion chamber to identify cracks by sensing tracer gas in the air stream.
Question 3: What happens to supply air CO levels when a cracked heat exchanger is combined with a high-temperature operating condition?
- CO levels remain unchanged
- CO levels in the supply air increase significantly due to combustion gas infiltration (Correct answer)
- CO levels decrease as combustion improves
- CO levels vary only with outdoor temperature
Correct answer: CO levels in the supply air increase significantly due to combustion gas infiltration
Cracks in the heat exchanger allow combustion gases with elevated CO to enter the supply airstream, especially when draft pressure is favorable.
Question 4: What is the most common cause of premature heat exchanger failure in residential furnaces?
- Oversized gas valve
- Restricted airflow causing repeated thermal stress from overheating cycles (Correct answer)
- Hard water condensate
- Excessive flue gas velocity
Correct answer: Restricted airflow causing repeated thermal stress from overheating cycles
Chronically restricted airflow (dirty filters, closed registers) causes the heat exchanger to overheat repeatedly, leading to metal fatigue and cracking.
Question 5: Which inspection technique is commonly used by technicians to visually check for heat exchanger cracks during operation?
- Shine a bright flashlight inside the heat exchanger only
- Observe the burner flame for movement or disturbance when the blower turns on (Correct answer)
- Use a borescope camera through the flue outlet
- Measure flue gas temperature with a thermocouple
Correct answer: Observe the burner flame for movement or disturbance when the blower turns on
When the blower starts, air pressure changes can cause a cracked heat exchanger to deflect and disturb the burner flames, visible as flame rollout or flickering.
Question 6: A condensing furnace has a primary and secondary heat exchanger. What is the role of the secondary heat exchanger?
- To preheat the combustion air
- To extract additional latent heat from water vapor in the flue gas, condensing it to liquid (Correct answer)
- To cool the flue gas before venting
- To filter particulates from the flue gas
Correct answer: To extract additional latent heat from water vapor in the flue gas, condensing it to liquid
The secondary heat exchanger extracts latent heat from water vapor in the flue gas as it condenses, boosting furnace efficiency above 90% AFUE.
FC FC Heat Exchangers & Blower Systems 1
What is the primary function of the heat exchanger in a gas furnace?