Natural Gas
Core concepts
Natural gas is predominantly methane (typically 85–95%) with ethane, propane, butane, nitrogen and CO2, plus odorant (usually mercaptan) for leak detection. Candidates must know it is lighter than air (relative density about 0.55–0.65), so leaks rise and disperse, unlike LPG. Know the flammable range (roughly 5–15% in air), ignition temperature, and the concept of Wobbe index - the key parameter that tells you whether a gas will deliver the same heat input through a given injector. Two gases with the same Wobbe index are interchangeable without re-jetting; different Wobbe index means re-jetting is required. Understand the combustion equation for methane: CH4 + 2O2 → CO2 + 2H2O, and the excess-air requirement. Know the components of a domestic/commercial installation: meter, emergency control valve (ECV), regulator, pipework, appliance isolation valve, and the requirement for ventilation and flueing. Understand gas pressure tiers (low pressure, medium pressure) and the principle of pressure drop along the pipe run.
Traps
A common trap is assuming natural gas is heavier than air (it is lighter). Another is confusing Wobbe index with calorific value - CV alone does not determine injector sizing. Questions also test the ECV location and operation, and the difference between a let-by test and a tightness test. Watch for items about flue categories and room-sealed versus open-flue appliances.
Worked rule of thumb
For methane combustion, each 1 volume of CH4 needs 2 volumes of O2, i.e. about 9.5 volumes of air for complete combustion; practical appliances run with excess air (often 20–50%) to ensure CO is minimised.