ACS Combustion & Flues 1 — Questions and Answers
Question 1: What is the product of complete combustion of natural gas (methane)?
- Carbon monoxide and water vapour
- Carbon dioxide and water vapour (Correct answer)
- Carbon dioxide and nitrogen
- Carbon monoxide and carbon dioxide
Correct answer: Carbon dioxide and water vapour
Complete combustion of methane (CH4) produces carbon dioxide (CO2) and water vapour (H2O) — CO2 and water are the safe end products when sufficient oxygen is present.
Question 2: What is carbon monoxide (CO) and why is it particularly dangerous?
- A visible yellowish gas with a strong smell that alerts occupants to danger
- An odourless, colourless, tasteless gas that binds to haemoglobin with far greater affinity than oxygen, causing rapid incapacitation and death (Correct answer)
- A gas that causes respiratory irritation but is rarely fatal
- A combustion by-product that is lighter than air and disperses quickly
Correct answer: An odourless, colourless, tasteless gas that binds to haemoglobin with far greater affinity than oxygen, causing rapid incapacitation and death
CO has approximately 250 times the affinity for haemoglobin that oxygen does — it prevents oxygen being carried by the blood, causing oxygen starvation that can be rapidly fatal without any warning to the victim.
Question 3: What does a high CO/CO2 ratio (Wobbe index deviation) in flue gas analysis indicate?
- The boiler is operating at peak efficiency
- Incomplete combustion is occurring, likely due to insufficient air supply, a dirty burner, or flue restriction (Correct answer)
- The flue temperature is too high
- The gas pressure is above the required operating range
Correct answer: Incomplete combustion is occurring, likely due to insufficient air supply, a dirty burner, or flue restriction
A high CO:CO2 ratio indicates incomplete combustion — too much carbon monoxide relative to carbon dioxide is a sign that the combustion process is not burning the fuel fully, producing dangerous CO levels.
Question 4: What is the primary air/secondary air function in a gas burner?
- Primary air cools the burner; secondary air ignites the flame
- Primary air mixes with gas before the burner to support combustion; secondary air provides additional oxygen around the flame to complete combustion (Correct answer)
- Both primary and secondary air cool the heat exchanger
- Secondary air is only used in condensing boilers
Correct answer: Primary air mixes with gas before the burner to support combustion; secondary air provides additional oxygen around the flame to complete combustion
Primary air is entrained by the gas flow upstream of the burner to create a combustible mixture; secondary air enters around the flame to provide additional oxygen for complete combustion.
Question 5: What does a 'balanced flue' on a gas appliance mean?
- The flue is balanced between two buildings sharing a party wall
- The appliance draws its combustion air from outside and expels flue gases to outside through a co-axial terminal, making it room-sealed and unaffected by internal air quality (Correct answer)
- The flue passes through a balancing chamber before exhausting
- The flue is shared between two appliances
Correct answer: The appliance draws its combustion air from outside and expels flue gases to outside through a co-axial terminal, making it room-sealed and unaffected by internal air quality
A balanced (room-sealed) flue has a concentric terminal — combustion air is drawn from outside and flue gases are expelled outside, so the appliance is entirely sealed from the room and unaffected by negative pressure or lack of ventilation.
Question 6: What is the function of a flue gas analyser in gas engineering?
- It measures the gas pressure at the meter
- It measures the concentrations of CO, CO2, and O2 in the flue gases to verify that combustion is complete and the appliance is operating safely and efficiently (Correct answer)
- It detects gas leaks in the pipework
- It measures the temperature of the heat exchanger
Correct answer: It measures the concentrations of CO, CO2, and O2 in the flue gases to verify that combustion is complete and the appliance is operating safely and efficiently
A flue gas analyser measures O2, CO, CO2, and flue temperature to assess combustion quality, efficiency, and safety — high CO levels indicate incomplete combustion that must be investigated and corrected.
ACS Combustion & Flues 1
What is the product of complete combustion of natural gas (methane)?