Combustion basics · for gas engineers in training
What air does to a flame
Natural gas needs about ten times its own volume of air to burn completely. Given enough, it burns to carbon dioxide and water vapour. Starved of air, some of its carbon only gets as far as carbon monoxide, a poison you cannot see, smell or taste. Turn the burner's air collar and the chimney's damper and watch what the analyser reads.
Where you'll meet it
Why the flame picture is not a safety test
Try “Yellow, yet clean” and then “Not enough secondary air”. The colour of a flame tells you how much air was mixed with the gas before it burned. Whether it makes carbon monoxide depends on how much air the flame gets in total, and whether it can finish burning before it touches anything cold. These two things are separate, so a flame can look wrong and be safe, or look right and be dangerous.
- Living flame effect (LFE and ILFE) and decorative fuel effect (DFE) gas fires are yellow by design. They burn the gas with little or no primary air so the flames look like a coal or log fire: tall, lazy, yellow and flickering. On these fires a yellow flame is normal, so its colour can't warn you of anything.
- What makes such a fire unsafe usually can't be seen in the flame. Coals, logs or pebbles laid in the wrong place or in the wrong number, a broken or non-original coal, dust and debris in the fuel bed, or a blocked or unsuitable flue or chimney. If the flames hit the fuel bed and are chilled, they make soot and carbon monoxide, and the fire looks much the same.
- Their flue gases are very dilute. An open-fronted fire draws a lot of room air up the flue with its products. A study for HSE (Advantica, 2002) found DFE and ILFE fires difficult to measure and too few in its survey to set a CO/CO₂ action level for them. So these fires are commissioned and serviced by the manufacturer's instructions: the fuel bed laid exactly as shown, the flue checked, a spillage test, and the room's ventilation checked.
- A blue flame is not proof either. A burner with its primary air set right still makes carbon monoxide if its secondary air is restricted, if the air it gets is already full of combustion products (vitiated), or if the flame touches a cold surface. Boilers, cookers and gas fires can all do this behind a normal-looking flame.
The tests that count are the ones that measure: a calibrated flue gas analyser read against the BS 7967 action levels or the manufacturer's figures, flue flow and spillage tests, and a carbon monoxide alarm in the room. Since 1 October 2022, landlords in England must fit a CO alarm in any room used as living accommodation that has a fixed combustion appliance (gas cookers excepted).
How the burner works
Gas leaves a small jet, the injector, in the foot of the burner fast enough to pull air in through the ports in the barrel: the primary air. The collar turns over the ports to set how much. A Bunsen with its ports open premixes about two-thirds of the air the gas needs.
That mixture burns at the sharp inner cone, which glows blue-green. What is left, mostly carbon monoxide and hydrogen, burns in the faint outer mantle with the secondary air the flame finds round it. Here it comes in through the slots at the foot of the chimney; in a real appliance it comes from the room, through the ventilation, into the combustion chamber.
Shut the ports and nothing is premixed. The gas burns only where it meets air, so the flame grows long and soft, and tiny carbon particles glow yellow as they heat. With enough secondary air they burn away before the tip.
What the analyser reads
The probe draws a sample from the flue spigot. The analyser dries it (the water trap on the hose catches the condensate) and measures oxygen, carbon monoxide and carbon dioxide. Natural gas burned with exactly the air it needs gives about 11.7% CO₂ in the dry sample; spare air shows up as oxygen and dilutes the CO₂.
Spare air dilutes the carbon monoxide too, so a low CO figure can just mean a diluted sample. The CO/CO₂ ratio cancels the dilution: both fall together as air is added, so the ratio describes the flame itself. BS 7967 uses 0.008 as the action level for most flued appliances, unless the manufacturer gives another figure.
Carbon monoxide takes the place of oxygen in the blood: it binds to haemoglobin more than 200 times as readily. Early symptoms (headache, dizziness, nausea, tiredness) are easy to mistake for flu.
The burner, chimney and analyser are drawn for teaching and are not to scale. The readings come from a simplified model of methane burning in air (model.mjs): exact chemistry for the oxygen, carbon dioxide and water vapour, a water-gas shift split when short of air, and a fitted allowance for gas that has not mixed with its air in time. They show the trends truly; they are not measurements of any real appliance. Sources: Advantica report R5205 for HSE, Combustion performance safety action levels for domestic gas appliances (https://www.hse.gov.uk/GAS/assets/docs/safetyactionlevels.pdf) (2002); BS 7967:2015, Carbon monoxide in dwellings and the combustion performance of gas-fired appliances; BS 5871-2 and BS 5871-3, installation of inset live fuel effect and decorative fuel effect gas appliances; The Smoke and Carbon Monoxide Alarm (Amendment) Regulations 2022 (England).