Gas Properties, Combustion and Ventilation
What the exam tests
Gas properties, combustion chemistry and ventilation. You need to know why natural gas and LPG behave differently in a room, what complete and incomplete combustion produce, and how ventilation supplies combustion air and removes heat and products.
Core rules
- Natural gas is mainly methane, relative density about 0.6 (lighter than air). LPG (propane/butane) is heavier than air, with propane relative density about 1.5. Leaks therefore accumulate at different levels.
- Complete combustion of methane: CH4 + 2O2 → CO2 + 2H2O. Air supply is much larger than the stoichiometric volume because excess air is needed for stable combustion.
- Incomplete combustion produces carbon monoxide (CO), a colourless, odourless toxic gas. A healthy NG flame is stable and blue; lifting, yellowing or sooting suggests a problem.
- Ventilation has two jobs: provide combustion air and dilute/discharge heat and products. I.S. 813 and the appliance manufacturer set the required free area; do not substitute a rule of thumb from another country.
- Adventitious air is not a designed ventilation source and must not be counted as the sole provision.
Worked example
A 10 kW open-flued appliance is to be fitted in a room with a permanent vent. The manufacturer's manual and I.S. 813 require a free area based on appliance input. Suppose the specified figure is 5 cm² per kW: 10 × 5 = 50 cm² free area. A louvred grille has only 50% free area, so the grille opening must be 100 cm². The reasoning: calculate from the actual input and the Irish/manufacturer figure, then correct for the grille's free-area ratio.
Marks-losing traps
Using 21 mbar as the Irish natural gas nominal pressure without checking the appliance data; assuming LPG is lighter than air; counting an open window as permanent ventilation; ignoring a second appliance drawing air from the same room.