A backflow burner makes smoke run downhill. It looks like a trick and it is simply physics: the smoke from a hollow cone is cooled as it passes down through the burner, becomes denser than the surrounding air, and falls instead of rising.
Why it needs a specific cone
A backflow cone has a channel bored up through it. Smoke is drawn down that channel, through the hole in the burner plate, and into the body of the burner where it cools further. A solid cone has nowhere to send smoke but upward.
So the first check when nothing happens is the cone, not the burner. Turn it over: there should be a visible hole in the base.
The four reasons it fails
- Wrong cone. As above. By a distance the most common cause.
- A draught. The effect depends on still air. An open window, a door being opened, even someone walking past briskly will lift the smoke and break the fall. This is why it works beautifully in a photograph and disappointingly on a windowsill.
- A blocked hole. Residue builds up in the channel over time. A cocktail stick or a pin clears it.
- The cone is not fully lit. Same as any cone — you need an even glowing ring across the tip before blowing out, not a single spark.
Give it time
The smoke does not fall immediately. It takes thirty seconds or so for enough cool smoke to accumulate to start running down. People often blow the cone out and relight it during this window, which resets the whole thing.
Cleaning
Backflow burners stain. The smoke leaves a brown resinous film along the path it travels, and on a pale ceramic burner it shows quickly. Warm soapy water and a soft brush handles most of it; for stubborn residue, bicarbonate of soda paste works without scratching the glaze. Avoid anything abrasive on a painted or gilded burner.
The plate the cone sits on takes the worst of it. Some burners have a removable plate, which is worth having.