Combustion & Flues Flashcards
6 cards from real ACS practice questions. Tap to flip, then mark Knew It or Still Learning — missed cards come back until you master them.
Read the first 6 Combustion & Flues flashcards as text
What is 'vitiation' of the combustion air supply?
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.
What is the purpose of a 'draught diverter' on an open-flued appliance?
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.
What is the Wobbe Index of a gas and why is it relevant to appliance operation?
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.
What is a 'fanned draught' (fan-assisted) flue system on a modern condensing boiler?
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.
What is the purpose of an over-heat thermostat (high limit thermostat) in a gas boiler?
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.
What does 'condensing' mean in the context of a condensing gas boiler?
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.