CCN1 Combustion Principles & Flue Systems 2 — Questions and Answers
Question 1: What is a Type C appliance (room-sealed appliance) and what is its key safety advantage?
- An appliance that draws combustion air from and discharges flue gases to outside the building — it cannot draw vitiated or contaminated room air (Correct answer)
- An appliance with a sealed burner that cannot be accessed by the user
- A high-efficiency condensing boiler
- An appliance that automatically shuts down in the event of incomplete combustion
Correct answer: An appliance that draws combustion air from and discharges flue gases to outside the building — it cannot draw vitiated or contaminated room air
A Type C (room-sealed) appliance draws combustion air directly from outside and discharges combustion products outside, through a balanced flue. It is completely isolated from room air, eliminating vitiation risk.
Question 2: What is a Type B appliance and what is its primary flue requirement?
- An open-flued appliance that draws combustion air from the room and discharges to outside via a flue — must have adequate room ventilation (Correct answer)
- An appliance with a balanced flue that draws air from outside only
- A condensing appliance that discharges via a plastic flue
- Any appliance installed in a boiler house or plant room
Correct answer: An open-flued appliance that draws combustion air from the room and discharges to outside via a flue — must have adequate room ventilation
Type B appliances are open-flued — they draw combustion air from the room and discharge combustion products to outside via a flue. They require adequate permanent ventilation to the room to prevent vitiation.
Question 3: What is 'spillage' in the context of an open-flued gas appliance?
- The leakage of flue gases from the appliance or flue back into the room, potentially introducing CO into the living space (Correct answer)
- Leakage of condensate from a condensing boiler
- An overflow of gas beyond the appliance's burner
- The discharge of unused gas to atmosphere during start-up
Correct answer: The leakage of flue gases from the appliance or flue back into the room, potentially introducing CO into the living space
Spillage occurs when flue gases escape from the draught diverter or flue into the room rather than discharging to outside. This can introduce CO and other combustion products into the living space, creating a serious health risk.
Question 4: What is 'back-draughting' in a flue system and what causes it?
- Reverse flow of flue gases down the flue and into the room — caused by negative pressure in the building, stack effect, or external wind conditions (Correct answer)
- Forward flow of flue gases being blocked by a bird's nest in the flue
- Condensation forming in the flue and dripping back into the appliance
- Excessive draught causing the pilot light to blow out
Correct answer: Reverse flow of flue gases down the flue and into the room — caused by negative pressure in the building, stack effect, or external wind conditions
Back-draughting occurs when the pressure differential reverses, causing combustion products to flow down the flue into the room rather than up and out. It can be caused by extract fans, stack effect, or external wind conditions.
Question 5: What is a draught diverter and what is its function?
- A component on the flue of an open-flued appliance that prevents down-draughts from extinguishing the burner, while allowing room air to dilute flue gases (Correct answer)
- A device that increases the draught to improve combustion
- A valve that prevents back-flow of gas from the flue into the burner
- A device that controls the rate of air supply to the burner
Correct answer: A component on the flue of an open-flued appliance that prevents down-draughts from extinguishing the burner, while allowing room air to dilute flue gases
A draught diverter is fitted between the appliance flue outlet and the main flue. It breaks the direct draught connection, preventing down-draughts from extinguishing the burner, and allows room air to dilute the flue gases.
Question 6: What is the function of a flue gas analyser when commissioned or servicing a gas appliance?
- It measures the composition of flue gases (CO, CO2, O2) to confirm complete combustion and identify potential problems (Correct answer)
- It measures the gas pressure in the flue
- It tests the mechanical strength of the flue components
- It detects gas leaks in the flue pipework
Correct answer: It measures the composition of flue gases (CO, CO2, O2) to confirm complete combustion and identify potential problems
A flue gas analyser measures the percentage of CO, CO2, and O2 in the flue gases. The results confirm whether combustion is complete and efficient. Abnormal readings indicate combustion problems requiring investigation.
CCN1 Combustion Principles & Flue Systems 2
What is a Type C appliance (room-sealed appliance) and what is its key safety advantage?