FC FC Combustion & Fuel Systems 1 — Questions and Answers
Question 1: What is the stoichiometric air-to-fuel ratio for natural gas combustion?
- 10:1
- 15.4:1 (Correct answer)
- 12:1
- 20:1
Correct answer: 15.4:1
Natural gas requires approximately 15.4 parts air to 1 part fuel for complete stoichiometric combustion.
Question 2: Which color of a burner flame indicates incomplete combustion?
- Blue
- Yellow or orange (Correct answer)
- Blue with yellow tips
- White
Correct answer: Yellow or orange
A yellow or orange flame indicates incomplete combustion due to insufficient air or a clogged burner.
Question 3: What device on a gas furnace controls the flow of gas to the main burner?
- Pressure regulator
- Gas valve (Correct answer)
- Orifice spud
- Manifold
Correct answer: Gas valve
The gas valve opens and closes to control gas flow to the main burner based on thermostat demand.
Question 4: What is the purpose of a burner orifice in a gas furnace?
- To ignite the gas
- To meter the correct amount of gas into the burner (Correct answer)
- To mix air with gas
- To sense flame presence
Correct answer: To meter the correct amount of gas into the burner
The orifice spud meters the precise amount of gas needed to produce the correct BTU output at the burner.
Question 5: When converting a furnace from natural gas to propane (LP), what must be replaced?
- The heat exchanger
- The orifice spud and pressure regulator spring (Correct answer)
- The blower motor
- The thermocouple
Correct answer: The orifice spud and pressure regulator spring
Propane has higher BTU content and different pressure requirements, so orifices must be downsized and the regulator spring changed.
Question 6: What is the typical manifold gas pressure for a natural gas furnace?
- 0.5 inches W.C.
- 3.5 inches W.C. (Correct answer)
- 7 inches W.C.
- 11 inches W.C.
Correct answer: 3.5 inches W.C.
Standard natural gas manifold pressure is 3.5 inches water column as specified by most furnace manufacturers.
FC FC Combustion & Fuel Systems 1
What is the stoichiometric air-to-fuel ratio for natural gas combustion?