Flue Requirements & Pressure Relief Flashcards

What is the minimum horizontal distance a room-sealed flue terminal must be from an openable window or door?

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.

What is the minimum height above a flat roof that a flue terminal must be positioned?

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.

What colour must the outer casing of a flue pipe be to indicate it carries flue gases from gas appliances?

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.

What is the purpose of a flue draught diverter on an open-flued appliance?

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.

What is the maximum length of 100 mm diameter horizontal flue permitted on most domestic gas boilers without additional checks?

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.

What is a concentric flue system?

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.