Combustion and Ventilation
What you must know
The combustion equation and air requirement: methane needs roughly 2 volumes of oxygen (about 10 volumes of air) per volume of gas for complete combustion, plus excess air. Appliances need air for combustion and ventilation for the room (adventitious air is not reliable). Know open-flue vs room-sealed appliances and how each ventilates. Understand flue types (open, balanced, fan-assisted), flue termination rules (clearances from openings, terminals, boundaries), and the causes of incomplete combustion: poor air supply, blocked flue, wrong gas, dirty burner, downdraught. Recognise spillage testing and the signs of CO production.
Traps in questions
- Confusing combustion air with room ventilation - both may be required and sized differently.
- Forgetting that a blocked or restricted flue causes spillage and CO, not just poor performance.
- Assuming an open-flue appliance in a small, sealed room is fine - check ventilation and flue pull.
- Mixing up terminal clearance distances or applying domestic rules to commercial settings.
Worked example / rule of thumb
Rule of thumb: No air = no safe flame. If combustion is incomplete, suspect air starvation, flue restriction, or contamination - in that order - and treat CO production as ID until proven otherwise. Always confirm flue flow and check ventilation before condemning an appliance.
Key exam behaviours
Questions often describe symptoms (yellow flame, sooting, CO alarm) and ask for the cause or action. Link symptoms to air/flue faults and choose "make safe and investigate" over "adjust and leave".