CENWAT Flue Requirements & Pressure Relief 1 — Questions and Answers
Question 1: What is the minimum horizontal distance a room-sealed flue terminal must be from an openable window or door?
- 300 mm (Correct answer)
- 100 mm
- 1000 mm
- 50 mm
Correct answer: 300 mm
Building Regulations and manufacturer guidance require a minimum 300 mm horizontal clearance between a room-sealed flue terminal and any openable window, door or air brick to prevent flue gases re-entering the building.
Question 2: What is the minimum height above a flat roof that a flue terminal must be positioned?
- 600 mm above the roof surface (Correct answer)
- 100 mm
- 300 mm
- 1500 mm
Correct answer: 600 mm above the roof surface
A flue terminal on a flat roof must terminate at least 600 mm above the roof surface to ensure adequate dispersal of flue gases and prevent downdraught.
Question 3: What colour must the outer casing of a flue pipe be to indicate it carries flue gases from gas appliances?
- There is no mandatory colour — identification is by labelling and documentation (Correct answer)
- Yellow
- Red
- Blue
Correct answer: There is no mandatory colour — identification is by labelling and documentation
There is no specific mandatory colour coding for domestic gas flue pipes in the UK; correct identification is through labelling, documentation and professional installation practice.
Question 4: What is the purpose of a flue draught diverter on an open-flued appliance?
- To prevent down-draughts from extinguishing the pilot or burner and to dilute flue gases with room air (Correct answer)
- To increase flue draught
- To reduce noise from the flue
- To seal the flue when the appliance is not in use
Correct answer: To prevent down-draughts from extinguishing the pilot or burner and to dilute flue gases with room air
A draught diverter prevents excessive or reversed draught from affecting combustion; it mixes room air with flue gases to reduce corrosive condensation and protect the heat exchanger.
Question 5: What is the maximum length of 100 mm diameter horizontal flue permitted on most domestic gas boilers without additional checks?
- Typically up to the manufacturer's maximum stated equivalent length (often 2–3 m for standard kits) (Correct answer)
- 10 m
- 50 m
- There is no limit
Correct answer: Typically up to the manufacturer's maximum stated equivalent length (often 2–3 m for standard kits)
Flue length limits depend entirely on the boiler manufacturer's specification; most standard horizontal flue kits are limited to the manufacturer's stated maximum equivalent length (typically 2–5 m), with reductions for each bend.
Question 6: What is a concentric flue system?
- A flue system where the combustion air duct surrounds the flue gas duct in the same fitting (Correct answer)
- A flue system with multiple separate pipes
- A flue system for condensing boilers only
- A flue system used exclusively for open-flued appliances
Correct answer: A flue system where the combustion air duct surrounds the flue gas duct in the same fitting
A concentric flue has two coaxial tubes: the inner carries exhaust flue gases out, while the outer annulus carries fresh combustion air in, used for room-sealed appliances.
CENWAT Flue Requirements & Pressure Relief 1
What is the minimum horizontal distance a room-sealed flue terminal must be from an openable window or door?