Gas Properties, Combustion and Ventilation
Gas properties you must know
Candidates need a working grasp of relative density, calorific value, Wobbe index, flame speed, and the difference between natural gas and LPG. Relative density determines where a leak accumulates: natural gas (lighter than air) rises and collects at high points, while LPG (heavier than air) sinks into low points, pits, and drains. This single fact drives many ventilation and leak-detection questions. Know also that gas odorants are added for detection, and that odor fade can occur in some situations.
Combustion fundamentals
Complete combustion of a hydrocarbon produces carbon dioxide and water vapour; incomplete combustion produces carbon monoxide. The stoichiometric air requirement differs by gas, and excess air is needed for clean burning. Traps include confusing the colour of a flame with combustion quality, and forgetting that yellow tipping or lifting can indicate incorrect gas pressure, blocked ports, or wrong injector sizing.
Ventilation
Ventilation must supply combustion air and remove products of combustion. Distinguish permanent ventilation (fixed openings) from secondary or adventitious air, and understand that a room's volume and the appliance's input determine the required opening size. A frequent trap is treating an openable window as permanent ventilation, which it is not.
Worked example
An appliance is installed in a small enclosed space. First calculate the appliance input and compare it to the ventilation table for that space volume. If the space is too small for the input without dedicated ventilation, permanent openings sized to the table are required. Do not credit doors, windows, or gaps as permanent ventilation unless the standard expressly permits it.