NATE Gas Heating 1 — Questions and Answers
Question 1: What is the primary safety device that shuts off the gas valve if the pilot flame is extinguished on a standing pilot furnace?
- Pressure switch
- Thermocouple (Correct answer)
- Limit switch
- Flame sensor rod
Correct answer: Thermocouple
The thermocouple generates a small millivolt signal that holds the gas valve open; when the pilot goes out the signal drops and the valve closes.
Question 2: What is the purpose of the heat exchanger in a gas furnace?
- Mix combustion gases with supply air
- Transfer heat from combustion gases to the circulating air without mixing them (Correct answer)
- Remove moisture from combustion products
- Regulate gas pressure to the burners
Correct answer: Transfer heat from combustion gases to the circulating air without mixing them
The heat exchanger keeps combustion products separated from conditioned air while allowing heat transfer through the metal walls.
Question 3: What component in an induced-draft furnace proves that combustion airflow is established before the gas valve opens?
- Thermocouple
- Pressure switch (Correct answer)
- Limit switch
- Rollout switch
Correct answer: Pressure switch
The pressure switch senses negative pressure created by the inducer motor to confirm draft is established before allowing ignition.
Question 4: What type of ignition system uses a glowing ceramic element to ignite the gas burners in modern furnaces?
- Spark ignition
- Hot surface igniter (HSI) (Correct answer)
- Standing pilot
- Direct spark ignition
Correct answer: Hot surface igniter (HSI)
Hot surface igniters heat to approximately 1800–2500°F using electricity to ignite the gas burner, replacing standing pilots.
Question 5: What is the function of the rollout switch on a gas furnace?
- Monitor supply air temperature
- Detect flame rollout and shut down the furnace if heat exceeds safe limits near the burner compartment (Correct answer)
- Measure gas manifold pressure
- Control the inducer motor speed
Correct answer: Detect flame rollout and shut down the furnace if heat exceeds safe limits near the burner compartment
Rollout switches are manual-reset thermal devices that trip if flames exit the burner compartment, indicating a serious combustion problem.
Question 6: What is the standard gas manifold pressure for natural gas residential furnaces in the US?
- 7–14 inches WC
- 3.2–3.8 inches WC (Correct answer)
- 0.5–1.0 PSI
- 10–12 inches WC
Correct answer: 3.2–3.8 inches WC
Natural gas furnace manifold pressure is typically set at 3.2–3.8 inches water column (around 3.5 inches WC nominal).
NATE Gas Heating 1
What is the primary safety device that shuts off the gas valve if the pilot flame is extinguished on a standing pilot furnace?