12
$\begingroup$

Sometimes, while lighting a fire in a wood stove in a basement, the smoke does not exit through the chimney like it normally does. Rather, a large amount of the smoke seems to get "pushed back" into the room instead of exiting through the chimney.

Why does this occur?

$\endgroup$
1
  • 1
    $\begingroup$ Get your chimney checked for obstructions... This is potentially quite dangerous... $\endgroup$ Dec 28, 2022 at 15:38

3 Answers 3

19
$\begingroup$

Assuming there's not a mechanical problem (debris in the way, damper partially or completely closed, chimney cap damaged/missing and allowing wind to blow down the chimney, etc) then the phenomenon is as follows:

At a given pressure, cold air is more dense than warm air, and tends to fall. Warm air is less dense than cold air and tends to rise. The same phenomenon on a much larger scale is the cause of wind. A chimney is too small for both to happen at once. Either your house will draw cold air down the chimney and vent the excess air through the slight drafts that all houses have, or it vent air out of the chimney and draw cold air through those drafts. The intended function is the latter, but it's hard to engineer a passive chimney that's open on both ends that only does one or the other.

You can feel this: with the stove safely off and cold, open the damper and put your hand or a piece of tissue paper near the flue. You will feel or see a breeze flowing either up or down the chimney.

If the hot air from the fire rises up the chimney, as intended, it will rapidly warm the whole air column in the chimney, which will then rise, carrying the smoke out of the house.

However, if the hot air from the fire escapes out into the room instead, the cold air outside will flow down to fill the low pressure region. Wind will blow following the pressure gradient, keeping the chimney cold and the hot air from the fire (plus smoke) blowing out into the room.

To prevent this from happening (again assuming there's not a mechanical problem), you can use a hand-held electric heater or use tongs to hold a bit of burning kindling inside the flue opening after opening the damper to pre-warm the air column in the chimney and start the airflow moving in the desired direction before lighting the fire.

$\endgroup$
4
  • $\begingroup$ So, the problem is basically the initial conditions: if the air was moving down, it's going to continue this unless forced to move up. Right? $\endgroup$
    – Ruslan
    Dec 28, 2022 at 14:12
  • $\begingroup$ I always found popping a lit candle in for 15 minutes worked. $\endgroup$ Dec 28, 2022 at 14:52
  • $\begingroup$ @Ruslan: Yes. Basically the system is bistable. (Or at least, it can be and often is.) One of the stable states is the one where cold air comes in through the room while hot air and smoke go out through the chimney; the other is where cold air comes in through the chimney while hot air and smoke go out through (and into) the room. $\endgroup$ Dec 28, 2022 at 14:59
  • $\begingroup$ @ruslan I think the down-flowing condition is unstable with the fire lit, as there's a lot of energy being pumped into the system with an element of randomness. I wouldn't want to run the experiment long enough to find out, though. $\endgroup$
    – g s
    Dec 28, 2022 at 16:06
2
$\begingroup$

Could be one of two things:

  1. Cool air being forced down the chimney taking smoke with it - that is, we know that cold air sinks (and hot air rises), and so a column of cold air sinking can force smoke back down into the room.

  2. It could also be simply a matter of winds blowing downward pushing smoke back inside.

$\endgroup$
1
$\begingroup$

Airflow around buildings and over walls can compress air at the top of the roof at higher pressure and bring low pressure at the sides of the house and the shielded zones.

If the chimney has very cold conditions at the top, and compressed air on the roof, the chimney can draft backwards, especially if the house has leaks towards the lee side of the wind, where the pressure is low.

https://www.youtube.com/watch?v=WZrg_uxGyaU

$\endgroup$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.