FC FC Combustion & Fuel Systems 2 — Questions and Answers
Question 1: What instrument is used to measure gas manifold pressure on a furnace?
- Ammeter
- Manometer (Correct answer)
- Flue gas analyzer
- Anemometer
Correct answer: Manometer
A manometer (or magnahelic gauge) measures the low pressure in inches of water column at the furnace manifold.
Question 2: What is the typical supply gas pressure (upstream of the gas valve) for a natural gas furnace?
- 1–2 inches W.C.
- 3.5–5 inches W.C.
- 5–7 inches W.C. (Correct answer)
- 10–14 inches W.C.
Correct answer: 5–7 inches W.C.
Natural gas utility supply pressure is typically 5–7 inches W.C. at the meter and furnace inlet.
Question 3: A cracked heat exchanger is most likely to allow which dangerous gas to enter the living space?
- Propane
- Carbon monoxide (Correct answer)
- Nitrogen dioxide
- Sulfur dioxide
Correct answer: Carbon monoxide
A cracked heat exchanger allows combustion byproducts including deadly carbon monoxide to leak into the circulating air.
Question 4: What is the primary purpose of a draft inducer motor on a high-efficiency furnace?
- To circulate warm air through the ducts
- To pull combustion gases through the heat exchanger and exhaust them outdoors (Correct answer)
- To pre-heat incoming combustion air
- To control burner ignition timing
Correct answer: To pull combustion gases through the heat exchanger and exhaust them outdoors
The draft inducer creates a negative pressure to draw flue gases through the heat exchanger and out the venting system.
Question 5: Which type of ignition system uses a resistance element heated to approximately 1,800°F to ignite the gas?
- Standing pilot
- Intermittent pilot
- Hot surface igniter (Correct answer)
- Spark igniter
Correct answer: Hot surface igniter
A hot surface igniter (HSI) uses silicon carbide or silicon nitride that glows red-hot to directly ignite the gas-air mixture.
Question 6: What causes delayed ignition (a loud boom or puff) when a furnace starts up?
- Excessive gas pressure
- Gas accumulating in the combustion chamber before igniting (Correct answer)
- A faulty limit switch
- Low blower speed
Correct answer: Gas accumulating in the combustion chamber before igniting
Delayed ignition occurs when gas builds up before the igniter lights it, causing a small explosion when combustion finally starts.
FC FC Combustion & Fuel Systems 2
What instrument is used to measure gas manifold pressure on a furnace?