The Weight That Lifts the Smoke

Ask why a chimney works and almost everyone says the same thing: hot air rises. True — but it hides the real machine. A chimney draws because the cold air outside is heavier than the hot gas inside, and the outside column, pressing down harder, pushes the light column up and out. The pull you feel at the fire is a weight difference. Below, you can weigh the two columns yourself.

Light a fire and smoke goes up the flue — obviously. But watch what happens on a cold morning when the chimney is stone-cold: the fire sulks, smoke curls back into the room, and only once the flue warms through does it suddenly catch and start to draw. If the smoke rose simply because it was hot, that lag wouldn't happen. Something else has to be built first.

That something is a column of warm, thin gas standing in a column of cold, dense air. Air has weight — about 1.2 kilograms in every cubic metre at the door, less when it's hot because the molecules spread apart. So the tall column of cold air outside the house weighs more than the equally tall column of warm flue gas inside. At the bottom, where they meet across the fire, the heavier outside column wins: it presses in and up, and the light column has nowhere to go but out the top. That upward net push is the draft — and it is a pressure, measured in pascals, that you can compute.

Why a taller chimney draws harder

Slide the height up and the draft climbs in a straight line: double the chimney, double the pull, at the very same temperatures. That's the part the "hot air rises" story can't explain — the gas isn't any hotter, yet a 10-metre flue draws twice as hard as a 5-metre one. It has to, because the weight difference between two columns grows with how tall they are. This is why tall industrial stacks and old kitchen chimneys draw so fiercely, and why a low stove-pipe struggles. Draft is bought by the metre.

Why the cold chimney backfires

Now drag the flue temperature down to meet the outside air. As the two columns approach the same density the draft fades to zero — and if the flue is colder than the air outdoors (a cold masonry chimney in a warm house, a flue chilled overnight), the sign flips. The heavier column is now the one inside, and it sinks: air pours down the chimney and smoke spills into the room. That's the cold-start backdraft, and it's why the old trick is to burn a twist of newspaper up the flue first — not to "start the smoke rising," but to make the inside column lighter than the outside one so the draft can turn on.