Gas and Combustion · 4 lessons
Combustion and what a flame tells you
Most gas safety comes back to one question: is the gas burning properly, and where are the products going?
Our gas engineering path is in public beta: some lessons are still being checked. More sections are coming.
Fuel, air and ignition
Gas burns when fuel, oxygen from the air and a source of ignition come together, and only within a flammable range of mixtures. Appliances need their designed air supply: never cover a required vent.
Complete and incomplete
Complete combustion of methane makes carbon dioxide and water. Too little air, a chilled flame or a blocked burner can cause incomplete combustion, which makes carbon monoxide.
Reading a flame, carefully
A compact blue flame that turns lazy, yellow or sooty can warn of a problem. Some fires burn yellow by design, and a blue flame does not prove safety. If a flame changes unexpectedly, stop using the appliance and call a Gas Safe registered engineer.
Flues and spillage
A flue carries combustion products outside. Spillage is when they come back into the room instead. Breathed in, carbon monoxide binds to haemoglobin and cuts the oxygen the blood carries.
The lessons this guide comes from
- The fire triangle for gas
- Complete and incomplete combustion
- Flame pictures
- Products and CO
Keep learning
Sources
Sources checked 29 October 2026. The law and official pages can change.
- HSE: carbon monoxideCarbon monoxide symptoms and causes.