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Why does smoke go faster up a taller chimney?

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vtsteam:
This may seem like a question even grade school science classes have been answering forever. Everybody knows that warm air is less dense than cool air.

But why wouldn't it slow down as it goes up a taller chimney and cools? Even an insulated chimney cools air progressively and at the very least you'd think it slows down slightly more, rather than speeds up as you increase height.

And even though I can understand that warm air in contact with cooler air would tend to rise, in a chimney it is in a tube, separated from the cold air. So how can its non-contact surroundings affect it except at the top, where it comes back into contact with cool air.

If the answer to that is there is a vacuum created at the top when it suddenly cools, why would that increase dramatically with small increases in chimney height?

And why do we assume that exhaust from a fire is less dense than air, anyway? It has a lot more in it, creosote, water vapor, and soot, to name a few things. I mean maybe it is but at the chimney top where it cools what is its density?

So altogether what is really happening in a chimney as you increase the height?

On the net a bunch of searches all yield the same non-answer that  warm air is less dense than cold air.

Yes, I register,

and........?

John Hill:
Wellllll...... I s'pose the chimney contains a column of air and gases that is less dense than the air around it, the chimney air at the very bottom is at almost the same pressure as the air around it but the chimney air halfway up is 'quite a bit' less dense than the air at that altitude outside the chimney (because a column of chimney air is less dense than a column of cold air) so the difference becomes greater the higher you go up the chimney. 

.....best I can do.... :coffee:

pjf134:
 It all comes down to draw. Hot air rises and cold air sinks and once you heat up the chimney it creates a draw as long as it keeps warm. If you have a 6" stove pipe and a bigger chimney it takes longer to heat the chimney pipe to create a draw and if the chimney is to big in diameter it might not work at all. I have a chimney that is for a 8" stove pipe but now I have a 6" stove pipe and now I have more of a down draft than before and it takes longer to create a draw. My chimney is at 20 ft. which is the min. height for draw and I will have to make the chimney higher for it to work right (I hope). Look at some install manuals for wood stoves and it will show how to create draw which there are many factors that causes problems like wind, trees too close ect. I hope this helps.
Paul

John Hill:
OK, but what is "draw"?

awemawson:
Imagine a wooden log 1 foot long floating  upright in water. It takes a certain force to push it under water. Now take a 2 foot log of the same diameter and push that under water. The 2 foot log takes twice the force to submerge. OK the log represents the column of warmer (hence less dense) flue gases. The water represents the colder hence more dense air. So the longer your log (column of warm gases) the more upthrust (draw) on your chimney. Simple Archimedes physics.

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