Home Inspector Forced-Air Furnace Inspection Questions and Answers — Questions and Answers
Question 1: While inspecting a gas-fired, forced-air furnace, a home inspector observes a consistently yellow, wavering flame at the burners. Which of the following is the MOST likely cause of this condition?
- A cracked heat exchanger
- Improper gas pressure
- Inadequate combustion air (Correct answer)
- A malfunctioning gas valve
Correct answer: Inadequate combustion air
A yellow, wavering flame is a classic sign of incomplete combustion. For complete and efficient combustion, a furnace needs the correct fuel-to-air mixture. When there is not enough oxygen (combustion air), the flame will burn yellow and produce soot and carbon monoxide. While other issues can affect the flame, inadequate combustion air is the most direct and common cause for this specific observation.
Question 2: A home inspector is evaluating a conventional metal flue vent pipe for a gas furnace and notices white, chalky residue and faint streaks on the exterior of the pipe joints. This observation is most indicative of which of the following problems?
- Normal oxidation of the galvanized metal
- Excessive heat from the furnace
- A blocked or partially obstructed flue
- Condensation and poor drafting of exhaust gases (Correct answer)
Correct answer: Condensation and poor drafting of exhaust gases
White, chalky residue (efflorescence) and water streaks on a metal flue pipe are signs that moisture is condensing inside the vent. This occurs when the exhaust gases cool down too much before exiting, often due to poor drafting, an oversized flue, or blockages. The condensation is acidic and can corrode the metal vent pipe over time, creating a safety hazard.
Question 3: During a furnace inspection, you identify a crack in the heat exchanger. What is the PRIMARY safety concern associated with this defect?
- Reduced heating efficiency and higher utility bills
- The potential for fire within the furnace cabinet
- The leakage of combustion gases, like carbon monoxide, into the conditioned air (Correct answer)
- Damage to the blower motor from excessive heat
Correct answer: The leakage of combustion gases, like carbon monoxide, into the conditioned air
The heat exchanger's primary function is to separate the combustion gases from the breathable air that is circulated throughout the home. A crack compromises this barrier, allowing dangerous byproducts of combustion, most notably carbon monoxide, to mix with the heated air and be distributed into the living spaces. This poses a serious health and safety risk.
Question 4: Which of the following furnace components is a safety device designed to shut down the burners if the temperature in the plenum becomes excessively high, preventing damage to the heat exchanger?
- Flame rollout switch
- Pressure switch
- High-limit switch (Correct answer)
- Gas valve
Correct answer: High-limit switch
The high-limit switch is a safety control that monitors the temperature inside the furnace plenum. If the temperature exceeds a preset limit, typically due to a dirty filter or a malfunctioning blower motor restricting airflow, the switch will open the circuit to the gas valve, shutting off the burners to prevent the furnace from overheating.
Question 5: A home inspector observes significant rust and corrosion on the exterior of a furnace's heat exchanger. This condition is MOST likely an indicator of:
- A refrigerant leak from the evaporator coil
- High humidity in the utility room
- Normal aging on a furnace older than 10 years
- Past or present condensation issues (Correct answer)
Correct answer: Past or present condensation issues
Rust and corrosion on a heat exchanger are primarily caused by moisture. This can result from condensation issues within the furnace system, such as a leaking humidifier, a clogged condensate drain line on a high-efficiency furnace, or acidic flue gas condensation from improper venting. While high ambient humidity or a refrigerant leak could contribute, direct condensation is the most common cause of significant corrosion on the heat exchanger itself.
Question 6: When inspecting a forced-air furnace system, an inspector notes that the metal flue pipe has a horizontal run of 8 feet before it enters the chimney. For proper drafting of exhaust gases, this horizontal pipe should:
- Be perfectly level to ensure even flow
- Have a downward slope away from the chimney
- Have a continuous upward slope towards the chimney (Correct answer)
- Be insulated to maintain high gas velocity
Correct answer: Have a continuous upward slope towards the chimney
The flue vent for a standard-efficiency furnace must have a continuous upward slope from the furnace to the chimney, typically a minimum of 1/4 inch per foot of horizontal run. This upward slope helps the hot, buoyant exhaust gases to vent naturally and prevents condensation from running back into the furnace.
Home Inspector Forced-Air Furnace Inspection Questions and Answers
While inspecting a gas-fired, forced-air furnace, a home inspector observes a consistently yellow, wavering flame at the burners.
Which of the following is the MOST likely cause of this condition?