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Question 1. Which of the following best describes the specific gravity of natural gas? A) 0. B) 1. C) 1. D) 2. Answer: A Explanation: Natural gas has a specific gravity of approximately 0.60, meaning it is lighter than air. Question 2. The heat content of propane is closest to which value per cubic foot? A) 500 BTU B) 1,000 BTU C) 2,500 BTU D) 4,000 BTU Answer: C Explanation: Propane (LP) delivers roughly 2,500 BTU per cubic foot, significantly higher than natural gas. Question 3. In the combustion triangle, which element is NOT one of the three required for combustion? A) Fuel B) Oxygen C) Heat D) Water vapor Answer: D Explanation: The three elements are fuel, oxygen, and heat; water vapor is a product, not a requirement. Question 4. Primary air is mixed with the fuel __________. A) After ignition
B) Before ignition C) Inside the heat exchanger D) In the vent pipe Answer: B Explanation: Primary air combines with the gas before the flame is established, ensuring proper mixture. Question 5. The typical air-to-fuel ratio for natural gas is about: A) 5: B) 10: C) 15: D) 20: Answer: B Explanation: Roughly 10 cubic feet of air are needed for each cubic foot of natural gas for complete combustion. Question 6. Which burner type uses a series of small holes to create a wide, low-intensity flame? A) Inshot burner B) Ribbon burner C) Slotted burner D) Rotating burner Answer: B Explanation: Ribbon burners have narrow slots that produce a broad flame pattern ideal for low-intensity applications. Question 7. The “spud” in a furnace burner primarily controls: A) Air pressure B) Gas flow rate C) Ignition voltage D) Draft speed
Question 11. Which ignition method uses a high-frequency spark to light the gas? A) Standing pilot B) Intermittent pilot – spark ignition C) Hot surface ignition (HSI) D) Flame rectification Answer: B Explanation: Intermittent pilots with spark ignition create a spark each time the furnace calls for heat. Question 12. Flame rectification works by: A) Measuring flame temperature with a thermocouple B) Detecting a change in resistance across the flame rod C) Using a photodiode to sense light intensity D) Monitoring pressure changes in the vent Answer: B Explanation: The flame conducts a small current, allowing the control board to confirm flame presence. Question 13. The standard manifold pressure for natural gas on a furnace is: A) 1.0 W.C. B) 3.5 W.C. C) 7.0 W.C. D) 11.0 W.C. Answer: B Explanation: Natural-gas furnaces are typically set to 3.5 inches water column (W.C.) manifold pressure. Question 14. For propane, the typical manifold pressure setting is: A) 3.5 W.C.
B) 7.0 W.C.
C) 11.0 W.C.
D) 15.0 W.C.
Answer: C Explanation: Propane furnaces use a higher pressure, commonly 11 inches water column. Question 15. In a 24-V thermostat circuit, the “W” terminal is used for: A) Cooling signal B) Fan control C) Heat call D) Power supply Answer: C Explanation: The “W” (white) terminal signals the furnace to initiate heating when the thermostat calls for heat. Question 16. A heat anticipator set too high will cause which symptom? A) Short cycling B) No heat output C) Continuous blower operation D) Fan motor overload Answer: A Explanation: An overly high anticipator delays the thermostat’s “off” signal, leading to short-cycling. Question 17. Which safety device opens when the heat exchanger temperature exceeds the limit? A) Pressure switch B) Rollout switch C) Limit switch D) Flame sensor
Question 21. Category I venting is characterized by: A) Positive pressure, condensing vent B) Negative pressure, non-condensing vent C) Direct-vent, sealed combustion D) High-temperature PVC venting Answer: B Explanation: Category I uses negative pressure (draft) and is non-condensing, typical of standard B-vent systems. ****Question 22.** The minimum clearance required for a vent termination in a garage is: A) 6 inches above the floor B) 12 inches above the floor C) 18 inches above the floor D) 24 inches above the floor Answer: C Explanation: Code mandates at least 18 inches clearance to prevent ignition of gasoline vapors. Question 23. A barometric damper in a vent system serves to: A) Increase draft pressure automatically B) Reduce noise from the vent C) Modulate airflow based on outdoor pressure D) Filter combustion gases Answer: C Explanation: Barometric dampers adjust vent opening to maintain proper draft despite changing atmospheric pressure. Question 24. The most common tool for measuring gas pressure at the manifold is a: A) Voltmeter
B) Manometer C) Thermometer D) Multimeter (current mode) Answer: B Explanation: A manometer directly reads pressure in inches water column, the standard for gas manifolds. Question 25. When measuring static pressure in a duct, a value above .5 W.C. indicates: A) Normal operation B) Excessive airflow restriction C) Low fan speed D) Faulty thermostat Answer: B Explanation: Static pressure above .5 inches water column suggests duct blockage or undersized ducts. Question 26. A combustion analyzer is primarily used to detect: A) Voltage fluctuations B) Carbon monoxide and excess oxygen levels C) Air filter pressure drop D) Inducer motor RPM Answer: B Explanation: Combustion analyzers measure CO, CO₂, O₂, and stack temperature to assess combustion quality. Question 27. The relationship between temperature rise (ΔT), BTU input, and CFM is expressed by: A) CFM = BTU / (1.08 × ΔT) B) BTU = CFM × ΔT × 0. C) ΔT = BTU / (CFM × 1.5)
Explanation: The furnace receives 120 V AC, so the multimeter must be set to AC voltage. Question 31. The most likely cause of a furnace that won’t start but shows 24 V at the thermostat is a: A) Faulty inducer motor B) Open pressure switch C) Bad limit switch D) Clogged air filter Answer: B Explanation: If the pressure switch remains open, the control board will not proceed to ignition despite correct thermostat voltage. Question 32. In a direct-vent furnace, the combustion air is taken from: A) Inside the living space B) The attic C) Outside the building through a sealed pipe D) The basement Answer: C Explanation: Direct-vent units draw combustion air from outside via a sealed vent, eliminating indoor air depletion. Question 33. What is the primary purpose of a draft hood on a vent pipe? A) To increase gas pressure B) To smooth airflow and reduce turbulence C) To filter combustion gases D) To provide a mounting point for the blower Answer: B Explanation: Draft hoods streamline the flow, reducing turbulence and improving vent drafting.
Question 34. The maximum allowable CO level in flue gases for a properly operating furnace is: A) 100 ppm B) 400 ppm C) 800 ppm D) 1,200 ppm Answer: B Explanation: Codes typically limit CO in exhaust to 400 ppm for safety. Question 35. A CO detector placed in a bedroom should be installed at a height of: A) 6- 12 inches from the floor B) 3- 5 feet from the floor C) Near the ceiling, within 6 inches D) Directly on the wall at floor level Answer: B Explanation: CO detectors are most effective when mounted 3- 5 feet above the floor where CO concentrations are likely to accumulate. Question 36. The term “excess air” in combustion refers to: A) Air that is hotter than the flame B) Air that is not needed for stoichiometric combustion C) Air that bypasses the heat exchanger D) Air that contains moisture Answer: B Explanation: Excess air is the additional oxygen beyond the theoretical requirement, used to ensure complete combustion. Question 37. Which of the following is a typical symptom of an over-fired furnace? A) Low flame height
D) Circulator pump Answer: A Explanation: The inducer motor creates draft to purge the combustion chamber and move flue gases before ignition. Question 41. In a high-efficiency condensing furnace, the secondary heat exchanger primarily extracts heat from: A) Exhaust gases above 140 °F B) Condensate water C) Primary combustion air D) Ambient room air Answer: B Explanation: Condensing furnaces recover latent heat from the water vapor in exhaust gases, using the secondary heat exchanger. Question 42. The typical pressure required to open a gas valve on a natural-gas furnace is: A) 2 W.C. B) 3.5 W.C. C) 5 W.C. D) 7 W.C. Answer: B Explanation: Most natural-gas valves are set to open at 3.5 inches water column. Question 43. When using a multimeter to test a transformer’s secondary voltage in a furnace, you should set the meter to: A) 200 V AC B) 600 V AC C) 2 V DC D) 20 mA AC Answer: A
Explanation: The transformer secondary typically provides 24 V AC, so the 200 V AC range is appropriate. Question 44. A “locked rotor” condition in an inducer motor is indicated by: A) Motor runs at full speed but no pressure is generated B) Motor hums but does not start turning C) Motor runs in reverse direction D) Motor starts and stops rapidly Answer: B Explanation: Locked rotor means the motor windings are energized but the rotor is stalled, producing a humming sound. Question 45. The purpose of a “draft inducer” in a furnace is to: A) Provide combustion air to the burner B) Remove flue gases and create a negative pressure in the vent C) Supply fresh air for the blower motor D) Increase the thermostat voltage Answer: B Explanation: The inducer creates a draft that pulls exhaust gases out and ensures proper venting. Question 46. Which of the following is a sign of insufficient draft in a vent system? A) Flame lifts off the burner ports B) Burner flame is too small and blue C) Furnace runs continuously without cycling D) Excessive condensation in the vent pipe Answer: A Explanation: A weak draft causes the flame to lift, indicating inadequate suction.
B) Two levels of heating output for better efficiency C) Simultaneous heating and cooling D) Redundant safety switches Answer: B Explanation: Dual-stage furnaces can operate at low and high fire levels, improving comfort and efficiency. Question 51. The most common cause of a furnace “short-cycling” problem is: A) Oversized heat exchanger B) Undersized blower motor C) Faulty thermostat placement in a cold draft D) Excessive static pressure in the ductwork Answer: C Explanation: A thermostat in a cold draft can cause frequent on/off cycles as it rapidly reaches setpoint. Question 52. In a furnace, the “rollout” switch is typically located: A) On the gas valve housing B) Near the blower motor C) Adjacent to the heat exchanger, sensing excessive surface temperature D) Inside the thermostat housing Answer: C Explanation: Rollout switches monitor the heat exchanger surface and trip if temperature exceeds safe limits. Question 53. The term “condensing furnace” refers to a system that: A) Uses a water-cooled heat exchanger to increase efficiency B) Operates without a vent pipe C) Requires only primary air for combustion D) Has a built-in humidifier Answer: A
Explanation: Condensing furnaces extract latent heat from water vapor, using a secondary heat exchanger cooled by condensate. Question 54. When measuring the voltage across a thermocouple in a standing-pilot furnace, a reading of 0 mV indicates: A) Proper pilot flame B) Open circuit or failed thermocouple C) Excessive gas pressure D) Correct manifold pressure Answer: B Explanation: A thermocouple must generate millivolts when heated; 0 mV means it is not producing voltage, likely due to a break or being cold. Question 55. Which component in a furnace’s control board provides the “spark” for an intermittent pilot? A) Relay coil B) Ignition transformer C) Capacitor bank D) Flame sensor Answer: B Explanation: The ignition transformer steps up voltage to create a high-voltage spark for the pilot. Question 56. The primary purpose of a “draft hood” on a Category I vent is to: A) Increase gas pressure B) Prevent rain ingress C) Reduce turbulence and improve draft stability D) Provide a mounting point for the pressure switch Answer: C Explanation: Draft hoods streamline flow, minimizing turbulence and helping maintain a stable draft.
C) Open limit switch D) Defective igniter Answer: B Explanation: A shorted or stuck “G” (fan) terminal will keep the blower energized continuously. Question 61. The “inducer motor” in a furnace is typically a: A) Single-phase shaded-pole motor B) Three-phase induction motor C) Permanent-split-capacitor (PSC) motor D) Direct-drive brushless DC motor Answer: C Explanation: Most residential furnace inducer motors are PSC motors, providing reliable start-up torque. Question 62. In a furnace vent system, a “condensate trap” is used to: A) Prevent water from entering the vent pipe B) Collect and drain condensate safely C) Increase draft pressure D) Filter out dust particles Answer: B Explanation: The trap captures condensate from a condensing furnace and routes it to a drain. Question 63. The acceptable range for indoor carbon monoxide concentration for a healthy adult is: A) 0–9 ppm B) 10–35 ppm C) 40–70 ppm D) 100–150 ppm Answer: A
Explanation: The EPA’s recommended safe level is 0–9 ppm for continuous exposure. Question 64. When a furnace’s pressure switch fails to close, the most common cause is: A) Faulty thermostat B) Blocked inducer air intake C) Shorted gas valve D) Bad flame sensor Answer: B Explanation: If the inducer cannot create sufficient draft due to a blockage, the pressure switch remains open. Question 65. A “dual-fuel” system typically combines which two heating sources? A) Natural gas and oil B) Propane and electricity C) Wood and solar D) Geothermal and electric heat strip Answer: A Explanation: Dual-fuel systems often use natural gas and oil, switching based on efficiency or fuel availability. Question 66. In a furnace, the “secondary air” is introduced: A) At the burner tip during ignition B) Through the vent pipe C) At the heat exchanger inlet D) Inside the blower motor housing Answer: A Explanation: Secondary air is added at the point of combustion to complete the fuel-air mixture.
