CCN1 Flue Systems and Chimneys — Questions and Answers
Question 1: What is the minimum vertical distance a room-sealed balanced flue terminal must be from ground level or a surface people can stand on?
- 150 mm
- 300 mm (Correct answer)
- 600 mm
- 1000 mm
Correct answer: 300 mm
A room-sealed (balanced) flue terminal must be sited at least 300 mm above ground level or any surface that people can stand on (such as a balcony). This clearance prevents the terminal from being blocked by debris, snow, or vegetation, and keeps the hot flue gases away from passers-by.
Question 2: What type of flue system draws combustion air from the room and discharges products of combustion to the outside?
- Room-sealed flue (Type C)
- Open flue (Type B) (Correct answer)
- Flueless (Type A)
- Forced draught flue (Type D)
Correct answer: Open flue (Type B)
An open flue (Type B) system draws its combustion air from the room in which the appliance is installed and discharges the products of combustion to the outside through a flue pipe and terminal. Because it takes air from the room, adequate permanent ventilation must be provided to replace the air consumed.
Question 3: What is the purpose of a draught diverter (draught break) fitted to an open-flued gas appliance?
- To increase the flue draught to maximum
- To prevent excessive draught (down-draught or up-draught) affecting the burner and to allow dilution air to enter the flue (Correct answer)
- To filter particles from the combustion products
- To regulate the gas pressure to the burner
Correct answer: To prevent excessive draught (down-draught or up-draught) affecting the burner and to allow dilution air to enter the flue
A draught diverter stabilises the draught conditions at the appliance by preventing excessive variations in flue pull from affecting the burner. It allows dilution air from the room to enter the flue, which helps to maintain steady combustion conditions. It also provides a point at which spillage of combustion products can be detected during a spillage test.
Question 4: When carrying out a flue flow test (spillage test) on an open-flued appliance, where should the smoke match be held?
- At the flue terminal on the outside of the building
- At the base of the draught diverter or canopy (Correct answer)
- At the gas meter
- At the ventilation grille
Correct answer: At the base of the draught diverter or canopy
The smoke match or smoke pellet should be held at the base of the draught diverter or canopy of the appliance. The smoke should be drawn upwards into the flue if the flue is operating correctly. If the smoke spills outward into the room, it indicates that the flue is not drawing properly and combustion products may be entering the living space.
Question 5: What is the minimum internal diameter of a flue liner suitable for a standard domestic gas boiler?
- 75 mm
- 100 mm
- 125 mm (Correct answer)
- 150 mm
Correct answer: 125 mm
The minimum internal diameter for a flue liner serving a standard domestic gas boiler is 125 mm (approximately 5 inches). This applies to both rigid and flexible flue liners installed in masonry chimneys. The liner must be suitable for gas and must be installed in accordance with the appliance manufacturer's instructions and BS 5440-1.
Question 6: A fanned draught flue system operates by using what method to discharge combustion products?
- Natural convection (buoyancy of hot gases)
- An electrically powered fan to push or pull combustion products through the flue (Correct answer)
- Compressed air injection
- A venturi effect created by wind passing over the terminal
Correct answer: An electrically powered fan to push or pull combustion products through the flue
A fanned draught (fan-flued) system uses an electrically powered fan to move combustion products through the flue to the terminal. This allows more flexible flue routing (including horizontal runs and longer distances) compared to natural draught systems. Most modern condensing boilers use fanned draught flue systems.
CCN1 Flue Systems and Chimneys
What is the minimum vertical distance a room-sealed balanced flue terminal must be from ground level or a surface people can stand on?