ACS Combustion & Flues 2

Question 1

What is 'vitiation' of the combustion air supply?

Accepted 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

Answer

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?

Accepted 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

Answer

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?

Accepted 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

Answer

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?

Accepted 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

Answer

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?

Accepted 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

Answer

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.

Gas Safe ACS Practice Test

Gas Safe ACS Practice Test

ACS Combustion & Flues 2