CCN1 Flue Systems and Chimneys 2 — Questions and Answers
Question 1: What is the minimum distance a balanced flue terminal must be from an opening window, air vent, or other opening into a building?
- 100 mm
- 200 mm
- 300 mm (Correct answer)
- 600 mm
Correct answer: 300 mm
A balanced flue terminal must be positioned at least 300 mm from any opening into a building, including openable windows, air vents, and doorways. This separation prevents combustion products from being drawn back into the building. Specific distances may vary depending on the appliance rating and flue type — always refer to the manufacturer's instructions and BS 5440-1.
Question 2: What material is commonly used for flexible flue liners in gas installations within masonry chimneys?
- Plastic PVC
- Aluminium alloy
- 316 grade stainless steel (Correct answer)
- Galvanised mild steel
Correct answer: 316 grade stainless steel
316 grade stainless steel is the standard material for flexible flue liners used in gas installations within masonry chimneys. It provides excellent resistance to the corrosive condensates produced by gas combustion and has the durability required for the expected lifespan of the installation. It must conform to BS EN 1856-2.
Question 3: When installing a vertical flue from a gas boiler through a flat roof, what is the minimum height the flue terminal must extend above the roof surface?
- 150 mm
- 300 mm
- 600 mm (Correct answer)
- At least 250 mm above the highest point of the roof penetration
Correct answer: 600 mm
A vertical flue terminal passing through a flat roof must extend at least 600 mm above the roof surface. This height ensures adequate dispersal of combustion products, prevents the terminal from being blocked by debris or ponding water, and reduces the risk of flue gases being drawn back into the building through nearby openings.
Question 4: What is the purpose of a flue gas analysis test carried out during commissioning of a gas boiler?
- To check the gas supply pressure
- To measure the carbon monoxide, carbon dioxide, and flue gas temperature to verify safe and efficient combustion (Correct answer)
- To test the condensate drainage system
- To verify the electrical connections are correct
Correct answer: To measure the carbon monoxide, carbon dioxide, and flue gas temperature to verify safe and efficient combustion
A flue gas analysis (FGA) measures the levels of CO, COâ‚‚, Oâ‚‚, and flue gas temperature in the products of combustion. This verifies that the appliance is burning efficiently and safely, with combustion products within acceptable limits. The CO/COâ‚‚ ratio must be within safe limits (maximum 0.004 for most boilers).
Question 5: When a chimney serves an open-flued gas fire, what test must be carried out to confirm the chimney is safe to use?
- A pressure test at 200 mbar
- A smoke test (flue integrity test) and spillage test (Correct answer)
- An electrical continuity test
- A water flow test
Correct answer: A smoke test (flue integrity test) and spillage test
Before connecting an open-flued gas fire to a chimney, a smoke test (flue integrity test) must be carried out to confirm the chimney is structurally sound and does not leak combustion products. A spillage test must also be performed with the appliance operating to ensure the chimney provides adequate draught and products of combustion are safely evacuated.
Question 6: What component must be fitted where a gas flue pipe passes through a combustible material such as a timber floor or wall?
- A gas isolation valve
- A fire-resistant sleeve with a minimum 25 mm air gap (Correct answer)
- A condensate trap
- A pressure relief valve
Correct answer: A fire-resistant sleeve with a minimum 25 mm air gap
Where a gas flue pipe passes through combustible material (such as a timber floor, ceiling, or wall), a non-combustible fire-resistant sleeve must be fitted. There must be a minimum 25 mm air gap between the flue pipe and the combustible material, filled with non-combustible material if necessary. This prevents heat transfer from the flue igniting the surrounding structure.
CCN1 Flue Systems and Chimneys 2
What is the minimum distance a balanced flue terminal must be from an opening window, air vent, or other opening into a building?