ACS Combustion & Flues 2 — Questions and Answers
Question 1: What is 'vitiation' of the combustion air supply?
- The deliberate introduction of additional air to improve combustion
- The depletion of oxygen in the room air available to an open-flued appliance, caused by prolonged combustion or poor ventilation, leading to incomplete combustion and CO production (Correct answer)
- The contamination of the gas supply with air
- Condensation on the flue terminal in cold weather
Correct answer: The depletion of oxygen in the room air available to an open-flued appliance, caused by prolonged combustion or poor ventilation, leading to incomplete combustion and CO production
Vitiation occurs when an open-flued appliance depletes the oxygen in its surrounding air; as O2 falls, combustion becomes incomplete, CO rises, and the risk to occupants increases — ventilation prevents vitiation.
Question 2: What is the purpose of a 'draught diverter' on an open-flued appliance?
- To increase the draught in the flue to improve combustion
- To protect the burner from downdraught, preventing wind-driven reversals of flue flow from extinguishing the flame or causing combustion products to spill into the room (Correct answer)
- To divert excess combustion air away from the burner
- To connect two separate flues to one appliance
Correct answer: To protect the burner from downdraught, preventing wind-driven reversals of flue flow from extinguishing the flame or causing combustion products to spill into the room
A draught diverter introduces dilution air between the heat exchanger and flue, neutralising the effect of negative or positive pressure fluctuations in the flue and preventing them from disrupting combustion.
Question 3: What is the Wobbe Index of a gas and why is it relevant to appliance operation?
- A measurement of the gas temperature at the burner
- A measure of gas interchangeability — it combines calorific value and specific gravity to indicate whether a gas will deliver the correct heat input through a given orifice (Correct answer)
- The concentration of CO in the gas supply
- The flow rate of gas through the meter
Correct answer: A measure of gas interchangeability — it combines calorific value and specific gravity to indicate whether a gas will deliver the correct heat input through a given orifice
The Wobbe Index determines whether a gas will provide the correct heat input through a burner jet — appliances are designed for a specific Wobbe Index range, and gases outside that range will cause overfiring or underfiring.
Question 4: What is a 'fanned draught' (fan-assisted) flue system on a modern condensing boiler?
- A natural draught system using a large chimney
- A flue system that uses a fan to force or draw combustion gases through the flue, enabling horizontal runs, longer flue lengths, and flexible terminal placement (Correct answer)
- A system that uses the kitchen extract fan to vent the boiler
- A shared flue serving multiple appliances in an apartment block
Correct answer: A flue system that uses a fan to force or draw combustion gases through the flue, enabling horizontal runs, longer flue lengths, and flexible terminal placement
Fan-assisted flues use an integral fan to overcome resistance in the flue system, allowing flexible routing that would not be possible with natural draught alone — most modern condensing boilers use this system.
Question 5: What is the purpose of an over-heat thermostat (high limit thermostat) in a gas boiler?
- To regulate the room temperature
- To prevent the boiler water temperature from exceeding a safe maximum by cutting off the gas supply if the primary temperature rises too high (Correct answer)
- To control the domestic hot water temperature at the tap
- To initiate the boiler self-diagnostic programme
Correct answer: To prevent the boiler water temperature from exceeding a safe maximum by cutting off the gas supply if the primary temperature rises too high
The high limit thermostat is a safety device that cuts gas supply if the boiler water overheats — it prevents damage to the heat exchanger and potentially dangerous pressure build-up.
Question 6: What does 'condensing' mean in the context of a condensing gas boiler?
- The boiler condenses the gas supply to increase pressure
- The boiler recovers latent heat from the water vapour in the flue gases by condensing it, achieving higher efficiency than a standard boiler (Correct answer)
- The boiler uses condensed water as a secondary heat source
- Condensing refers to the reduction of NOx emissions only
Correct answer: The boiler recovers latent heat from the water vapour in the flue gases by condensing it, achieving higher efficiency than a standard boiler
A condensing boiler operates its secondary heat exchanger at sufficiently low temperatures that water vapour in the flue gases condenses, releasing latent heat that would otherwise be wasted up the flue.
ACS Combustion & Flues 2
What is 'vitiation' of the combustion air supply?