OFTEC Combustion Efficiency Testing 4 — Questions and Answers
Question 1: What is the significance of the dew point temperature in condensing oil boilers?
- It is the temperature at which water vapour in flue gases condenses, recovering latent heat (Correct answer)
- It is the maximum safe operating temperature
- It is the temperature at which oil ignites
- It is the minimum flue gas temperature allowed by regulations
Correct answer: It is the temperature at which water vapour in flue gases condenses, recovering latent heat
In condensing boilers, when flue gas temperature drops below the dew point (approximately 47°C for kerosene), water vapour condenses and releases latent heat. This recovered heat boosts efficiency above the theoretical 100% (gross) figure.
Question 2: What is a CO/CO2 ratio and what does it indicate?
- The proportion of CO to CO2 — a high ratio indicates dangerous incomplete combustion (Correct answer)
- The cost of carbon credits
- The age of the boiler
- The fuel consumption rate
Correct answer: The proportion of CO to CO2 — a high ratio indicates dangerous incomplete combustion
The CO/CO2 ratio compares carbon monoxide (incomplete combustion) to carbon dioxide (complete combustion). A ratio above 0.004 indicates poor combustion that could be dangerous. It is a key safety indicator during testing.
Question 3: What reading on a flue gas analyser would indicate a potentially dangerous appliance requiring immediate action?
- CO reading above 200 ppm in the undiluted flue gas (Correct answer)
- CO2 reading of 10%
- O2 reading of 5%
- Flue gas temperature of 200°C
Correct answer: CO reading above 200 ppm in the undiluted flue gas
CO readings above 200 ppm indicate severely incomplete combustion and an immediate danger. The appliance should be turned off, the customer warned not to use it, and a 'Do Not Use' label applied until repairs are completed.
Question 4: Why must combustion readings be taken with all boiler covers and panels in place?
- Removing covers changes the air supply and gives false readings that don't represent normal operation (Correct answer)
- It is only for safety during the test
- Covers have no effect on readings
- It prevents dust entering the analyser
Correct answer: Removing covers changes the air supply and gives false readings that don't represent normal operation
Removing boiler covers changes the air flow to the burner, giving readings that do not represent how the boiler actually operates when the covers are fitted. Tests must replicate real operating conditions for accurate results.
Question 5: What does a CO2 reading below 8% on a kerosene pressure jet burner typically indicate?
- Excessive air is entering the combustion zone (Correct answer)
- The burner is operating at peak efficiency
- The flue is blocked
- The oil supply pressure is too high
Correct answer: Excessive air is entering the combustion zone
A CO2 reading below 8% for kerosene indicates significantly too much excess air. This wastes heat up the flue, reduces efficiency, and may cause flame instability. The air band settings need adjusting to increase CO2 towards 10-12%.
Question 6: What additional test should be performed if there is any suspicion of combustion gas spillage into the room?
- A spillage test using smoke pellets or matches at the draught diverter (Correct answer)
- A water pressure test
- An electrical continuity test
- A sound level measurement
Correct answer: A spillage test using smoke pellets or matches at the draught diverter
A spillage test uses smoke pellets or a match/taper at the draught diverter or flue break to check that combustion products are being drawn up the flue. If smoke is pushed into the room, there is a dangerous spillage condition.
OFTEC Combustion Efficiency Testing 4
What is the significance of the dew point temperature in condensing oil boilers?