OFTEC Combustion Efficiency Testing 3 — Questions and Answers
Question 1: What is the purpose of a draught gauge (manometer) in oil burner testing?
- To measure the negative pressure (suction) in the flue system (Correct answer)
- To measure the oil pressure at the pump
- To measure water pressure in the heating system
- To check gas tightness
Correct answer: To measure the negative pressure (suction) in the flue system
A draught gauge measures the negative pressure in the flue that draws combustion gases away from the burner. Correct draught (typically -0.02 to -0.05 mbar for oil) is essential for proper combustion and safe operation.
Question 2: What effect does a soot-covered heat exchanger have on combustion efficiency?
- It acts as insulation, reducing heat transfer to the water and increasing flue gas temperature (Correct answer)
- It improves heat transfer
- It reduces flue gas temperature
- It has no measurable effect
Correct answer: It acts as insulation, reducing heat transfer to the water and increasing flue gas temperature
Soot on the heat exchanger surfaces acts as an insulator, reducing heat transfer from combustion gases to the water. This causes higher flue gas temperatures and lower efficiency. Just 1mm of soot can reduce efficiency by 5%.
Question 3: When should a Bacharach smoke test be carried out during a service?
- After the burner has been running for at least 5 minutes at stable conditions (Correct answer)
- Before the burner is started
- Only if the customer reports a problem
- Only during the initial commissioning
Correct answer: After the burner has been running for at least 5 minutes at stable conditions
The smoke test should be performed once the burner has reached stable operating conditions (at least 5 minutes running). This gives a representative reading. It is a standard part of every annual service, not just commissioning.
Question 4: What could cause a sudden increase in smoke number compared to previous service records?
- A worn nozzle, dirty air intake, or contaminated fuel supply (Correct answer)
- A new ignition transformer
- The boiler water being too hot
- A new oil delivery
Correct answer: A worn nozzle, dirty air intake, or contaminated fuel supply
Increased smoke numbers indicate deteriorating combustion. Common causes include a worn or partially blocked nozzle, restricted air supply (dirty filter or blocked air inlet), contaminated fuel, or incorrect pump pressure.
Question 5: What is the net flue gas temperature and why is it important?
- Flue gas temperature minus room temperature — it indicates how much heat is being lost (Correct answer)
- The highest temperature the flue can reach
- The temperature at which the flue will crack
- The temperature of the oil before combustion
Correct answer: Flue gas temperature minus room temperature — it indicates how much heat is being lost
Net flue gas temperature is calculated by subtracting the ambient room temperature from the measured flue gas temperature. This figure is used with the CO2 reading to calculate combustion efficiency and assess heat losses.
Question 6: How often should a flue gas analyser be calibrated?
- At least annually, with fresh air zeroing before each use (Correct answer)
- Only when purchased
- Every five years
- Never — they are factory-sealed
Correct answer: At least annually, with fresh air zeroing before each use
Flue gas analysers must be professionally calibrated at least annually to maintain accuracy. Before each use, the instrument should be zero-calibrated in fresh air. Many instruments display a calibration due date.
OFTEC Combustion Efficiency Testing 3
What is the purpose of a draught gauge (manometer) in oil burner testing?