Combustion and Ventilation Flashcards

What are the three elements required for combustion to take place, commonly known as the 'fire triangle'?

Answer: Fuel, oxygen, and ignition (heat)

The fire triangle consists of fuel, oxygen (from the air supply), and a source of ignition (heat). All three must be present simultaneously for combustion to occur. Removing any one element will extinguish the flame or prevent ignition.

What is the ideal CO₂ reading for natural gas when an appliance is correctly set up for complete combustion?

Answer: 9-10%

For natural gas (methane), the ideal CO₂ reading is approximately 9-10% when combustion is complete and the appliance is correctly adjusted. This indicates the correct air-to-gas ratio is being achieved. A CO₂ reading significantly below this range suggests excess air, while above it may indicate incomplete combustion.

What is the minimum free area of ventilation required for an open-flued gas appliance with a rated input over 7 kW in a room?

Answer: 5 cm² per kW of rated input above 7 kW

For open-flued appliances with a rated input exceeding 7 kW, additional ventilation of 5 cm² of free area per kW above 7 kW is required. The first 7 kW is considered to be served by normal air infiltration in the room. This is specified in BS 5440-2 and Approved Document J.

What dangerous gas is produced as a result of incomplete combustion of natural gas?

Answer: Carbon monoxide (CO)

Carbon monoxide (CO) is a highly toxic, colourless, and odourless gas produced when natural gas undergoes incomplete combustion due to insufficient oxygen supply. CO poisoning can be fatal. Signs of incomplete combustion include yellow or orange flames, sooting, and increased CO/CO₂ ratios.

A room-sealed gas appliance draws its combustion air from where?

Answer: Directly from outside the building through a duct or balanced flue

A room-sealed (Type C) appliance draws all its combustion air directly from outside the building through a purpose-designed duct or the outer part of a balanced flue terminal. This means the combustion circuit is completely sealed from the room, making it safer as it does not deplete oxygen levels in the occupied space.

What is the approximate air-to-gas ratio required for the complete combustion of natural gas?

Answer: 10:1

Natural gas requires approximately 10 volumes of air to 1 volume of gas for complete (stoichiometric) combustion. In practice, slightly more air is provided (excess air) to ensure complete combustion, typically giving an actual ratio of around 11-12:1 at the burner.