ACS Combustion & Flues 3

Question 1

What is the minimum distance that a flue terminal must be positioned above an openable window or door, as required by BS 5440 Part 1?

Accepted Answer

300mm

Answer

BS 5440 Part 1 specifies minimum separation distances between flue terminals and openings such as windows and doors to prevent flue gases being drawn back into the building — typically 300mm minimum.

Question 2

What causes 'sooting' of a gas appliance heat exchanger or burner?

Accepted Answer

Incomplete combustion due to insufficient air, a blocked burner, or a restricted flue, producing soot (carbon) deposits instead of clean combustion products

Answer

Sooting is a symptom of incomplete combustion — insufficient oxygen or a faulty burner prevents complete conversion of carbon to CO2, depositing carbon as soot and almost certainly producing CO.

Question 3

What is a 'plume management kit' used for on a condensing boiler?

Accepted Answer

To direct the visible water vapour plume from the flue terminal away from windows, doors, and neighbouring properties

Answer

A plume management kit redirects the highly visible water vapour plume produced by a condensing boiler flue terminal, preventing it from obscuring windows or causing nuisance to neighbours.

Question 4

What does 'excess air' mean in combustion analysis?

Accepted Answer

The amount of air supplied above the stoichiometric (theoretical) minimum needed for complete combustion, measured as a percentage above the theoretical requirement

Answer

Excess air is the additional air beyond the stoichiometric minimum required to ensure complete combustion; some excess is needed to ensure complete burning, but too much reduces efficiency by carrying away heat.

Question 5

Which condition in a flue system can cause 'backdraught' and allow combustion gases to enter a room?

Accepted Answer

Negative pressure in the building from extract fans, trickle vents, or sealed rooms that reverses the natural draught in the flue of an open-flued appliance

Answer

Modern buildings are often very airtight; powerful extract fans or extractor hoods can create negative internal pressure that reverses the draught in an open flue, causing combustion gases including CO to spill into the room.

Gas Safe ACS Practice Test

Gas Safe ACS Practice Test

ACS Combustion & Flues 3