Home Inspector Test Home Inspector Forced-Air Furnace Inspection 3 — Questions and Answers
Question 1: When inspecting a high-efficiency condensing furnace, the inspector finds the PVC vent pipe is not supported and sags between connection points. Why is this a defect?
- PVC pipe must be replaced with metal for all furnace applications
- Sagging can allow condensate to pool and block flue gas flow (Correct answer)
- The pipe diameter is likely incorrect for the furnace BTU rating
- Condensing furnaces do not require venting
Correct answer: Sagging can allow condensate to pool and block flue gas flow
Condensing furnace vent pipes must be properly supported and pitched to drain condensate; sagging creates traps that can block the flue and cause the furnace to shut down or back-draft.
Question 2: The thermocouple or flame sensor in a gas furnace is designed to do what if it fails to detect a flame?
- Increase gas flow to relight the burner
- Signal the control board to lock out and stop gas flow (Correct answer)
- Activate the draft inducer to purge the combustion chamber
- Open the main gas valve wider
Correct answer: Signal the control board to lock out and stop gas flow
The flame sensor sends a signal proving ignition; if it fails to sense a flame within a set period, the control board shuts off the gas valve to prevent unburned gas accumulation.
Question 3: A home inspector tests the furnace and notices it cycles on and off very frequently without reaching the setpoint temperature. This is called:
- Short cycling (Correct answer)
- Delayed ignition
- Rollout
- Downfiring
Correct answer: Short cycling
Short cycling occurs when the furnace turns on and off more frequently than normal, often due to overheating, a dirty filter, or a faulty limit switch.
Question 4: What is the function of the high-limit switch in a forced-air furnace?
- It controls the maximum thermostat setpoint
- It shuts off the burner if the supply air temperature exceeds a safe threshold (Correct answer)
- It limits the volume of gas supplied to the burners
- It prevents the blower from exceeding maximum RPM
Correct answer: It shuts off the burner if the supply air temperature exceeds a safe threshold
The high-limit switch monitors heat exchanger temperature and shuts down the burner if temperatures exceed safe limits, protecting against overheating.
Question 5: An inspector finds a furnace filter that is heavily clogged with debris. What is the most likely consequence if left uncorrected?
- Increased gas pressure at the burners
- Reduced airflow causing overheating and possible heat exchanger damage (Correct answer)
- The igniter will fail to reach ignition temperature
- The thermostat will not call for heat
Correct answer: Reduced airflow causing overheating and possible heat exchanger damage
A severely restricted filter reduces airflow across the heat exchanger, causing it to overheat, which can trigger the limit switch and over time crack the heat exchanger.
Question 6: What material is NOT acceptable for use as a furnace flue connector in a Category I (negative pressure, non-condensing) gas appliance?
- Type B double-wall vent pipe
- Single-wall galvanized steel connector pipe
- PVC plastic pipe (Correct answer)
- Stainless steel single-wall connector
Correct answer: PVC plastic pipe
PVC is only approved for Category IV (positive pressure, condensing) appliances; using it on a Category I furnace is a code violation and safety hazard.
Question 7: When a home inspector operates the furnace and notices a strong smell of aldehydes or a sharp acrid odor from the supply registers, what should be suspected?
- A dirty air filter
- A cracked heat exchanger allowing combustion gases into the airstream (Correct answer)
- Duct leakage pulling in attic air
- An oversized furnace for the space
Correct answer: A cracked heat exchanger allowing combustion gases into the airstream
Aldehydes and acrid odors in supply air during furnace operation are classic indicators of a compromised heat exchanger leaking combustion gases.
Home Inspector Test Home Inspector Forced-Air Furnace Inspection 3
When inspecting a high-efficiency condensing furnace, the inspector finds the PVC vent pipe is not supported and sags between connection points.
Why is this a defect?