OFTEC Combustion Efficiency Testing 2 — Questions and Answers
Question 1: What is the minimum acceptable combustion efficiency for a new oil-fired boiler at commissioning?
- 85% (Correct answer)
- 70%
- 60%
- 95%
Correct answer: 85%
New oil-fired boilers should achieve a minimum combustion efficiency of 85% at commissioning. Modern condensing oil boilers typically achieve 90-95%. Lower readings indicate a problem requiring investigation.
Question 2: What does excess air in combustion mean for boiler efficiency?
- It reduces efficiency as heat is wasted heating unused air that goes up the flue (Correct answer)
- It increases efficiency by providing more oxygen
- It has no effect on efficiency
- It improves safety but has no efficiency impact
Correct answer: It reduces efficiency as heat is wasted heating unused air that goes up the flue
Excess air beyond what is needed for combustion absorbs heat as it passes through the boiler and carries that heat up the flue. While some excess air is necessary for safe combustion, too much wastes energy.
Question 3: What is the correct position to take a flue gas sample from an oil-fired appliance?
- In the flue pipe, at the test point closest to the appliance outlet (Correct answer)
- At the top of the chimney
- In the combustion chamber
- At the oil tank
Correct answer: In the flue pipe, at the test point closest to the appliance outlet
The flue gas sample should be taken from the test point in the flue pipe as close to the appliance flue outlet as practical. This gives the most accurate reading before the gases are diluted by any air ingress further up the flue.
Question 4: How long should an oil burner run before taking combustion readings?
- At least 10-15 minutes to reach stable operating conditions (Correct answer)
- Immediately after ignition
- At least 2 hours
- Only when the burner is cold
Correct answer: At least 10-15 minutes to reach stable operating conditions
The burner should run for at least 10-15 minutes to reach stable operating conditions before taking readings. Cold-start readings are unreliable as the combustion chamber temperature affects the combustion process.
Question 5: What does a high oxygen (O2) reading in the flue gas indicate?
- Too much excess air is entering the combustion process (Correct answer)
- The fuel supply is too high
- The flue is blocked
- The burner is operating perfectly
Correct answer: Too much excess air is entering the combustion process
High O2 readings (above 6-7% for kerosene) indicate excessive air entering the combustion zone. This lowers CO2, reduces efficiency, and may cause flame instability. The air band or damper needs adjusting.
Question 6: What is the relationship between CO2 and O2 readings in flue gas analysis?
- As CO2 increases, O2 decreases (they are inversely related) (Correct answer)
- Both increase together
- Both decrease together
- There is no relationship between them
Correct answer: As CO2 increases, O2 decreases (they are inversely related)
CO2 and O2 readings are inversely related. As more oxygen is consumed in combustion, CO2 production increases. Higher excess air dilutes the CO2 and increases O2. The ideal is to maximise CO2 while keeping O2 at a safe minimum.
OFTEC Combustion Efficiency Testing 2
What is the minimum acceptable combustion efficiency for a new oil-fired boiler at commissioning?