Combustion Principles & Flue Systems Flashcards

What is a Type C appliance (room-sealed appliance) and what is its key safety advantage?

Answer: An appliance that draws combustion air from and discharges flue gases to outside the building — it cannot draw vitiated or contaminated room air

A Type C (room-sealed) appliance draws combustion air directly from outside and discharges combustion products outside, through a balanced flue. It is completely isolated from room air, eliminating vitiation risk.

What is a Type B appliance and what is its primary flue requirement?

Answer: An open-flued appliance that draws combustion air from the room and discharges to outside via a flue — must have adequate room ventilation

Type B appliances are open-flued — they draw combustion air from the room and discharge combustion products to outside via a flue. They require adequate permanent ventilation to the room to prevent vitiation.

What is 'spillage' in the context of an open-flued gas appliance?

Answer: The leakage of flue gases from the appliance or flue back into the room, potentially introducing CO into the living space

Spillage occurs when flue gases escape from the draught diverter or flue into the room rather than discharging to outside. This can introduce CO and other combustion products into the living space, creating a serious health risk.

What is 'back-draughting' in a flue system and what causes it?

Answer: Reverse flow of flue gases down the flue and into the room — caused by negative pressure in the building, stack effect, or external wind conditions

Back-draughting occurs when the pressure differential reverses, causing combustion products to flow down the flue into the room rather than up and out. It can be caused by extract fans, stack effect, or external wind conditions.

What is a draught diverter and what is its function?

Answer: A component on the flue of an open-flued appliance that prevents down-draughts from extinguishing the burner, while allowing room air to dilute flue gases

A draught diverter is fitted between the appliance flue outlet and the main flue. It breaks the direct draught connection, preventing down-draughts from extinguishing the burner, and allows room air to dilute the flue gases.

What is the function of a flue gas analyser when commissioned or servicing a gas appliance?

Answer: It measures the composition of flue gases (CO, CO2, O2) to confirm complete combustion and identify potential problems

A flue gas analyser measures the percentage of CO, CO2, and O2 in the flue gases. The results confirm whether combustion is complete and efficient. Abnormal readings indicate combustion problems requiring investigation.