ACS Combustion & Flues 3 — Questions and Answers
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?
- 150mm
- 300mm (Correct answer)
- 600mm
- 1000mm
Correct answer: 300mm
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?
- Excessive gas pressure at the burner
- Incomplete combustion due to insufficient air, a blocked burner, or a restricted flue, producing soot (carbon) deposits instead of clean combustion products (Correct answer)
- Condensation from a high-efficiency appliance
- Normal product of complete combustion at high temperatures
Correct answer: Incomplete combustion due to insufficient air, a blocked burner, or a restricted flue, producing soot (carbon) deposits instead of clean combustion products
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?
- To increase boiler efficiency by recycling condensate
- To direct the visible water vapour plume from the flue terminal away from windows, doors, and neighbouring properties (Correct answer)
- To measure flue gas concentrations
- To prevent freezing of the condensate pipe
Correct answer: To direct the visible water vapour plume from the flue terminal away from windows, doors, and neighbouring properties
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?
- Air that enters the room when the boiler is operating
- The amount of air supplied above the stoichiometric (theoretical) minimum needed for complete combustion, measured as a percentage above the theoretical requirement (Correct answer)
- The air leaking from the combustion chamber into the heat exchanger
- The air that is expelled as flue gases along with combustion products
Correct answer: The amount of air supplied above the stoichiometric (theoretical) minimum needed for complete combustion, measured as a percentage above the theoretical requirement
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?
- The flue is too tall for the appliance
- 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 (Correct answer)
- The condensate pipe being too long
- The boiler being on the ground floor
Correct 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
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.
Question 6: What is the 'stoichiometric air-to-fuel ratio' for natural gas combustion?
- Approximately 3.7:1 (air:fuel) by volume
- Approximately 10:1 (air:fuel) by volume — approximately 10 cubic metres of air are needed to completely combust 1 cubic metre of natural gas (Correct answer)
- Approximately 1:1 (air:fuel) by volume
- Approximately 25:1 (air:fuel) by volume
Correct answer: Approximately 10:1 (air:fuel) by volume — approximately 10 cubic metres of air are needed to completely combust 1 cubic metre of natural gas
Methane requires approximately 10 volumes of air (roughly 2 volumes of oxygen with 8 volumes of nitrogen) for complete combustion — insufficient air causes incomplete combustion and CO production.
ACS Combustion & Flues 3
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?