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Why Do Smoke And Hot Air Go Up A Chimney?

By Lawrence Macmillan October 25, 2021

A good chimney is definitely one of those things that we take for granted, and that’s largely because they do their job so well. After all, the only reason we’d have to really acknowledge it is if something went wrong.

It all seems so commonplace and naturalized to us. We light a fire, the nasty stuff goes up and away, and we get cozy in the living room and throw the Disney Channel on, but when you stop to think about it, there’s really nothing commonplace about it.

Why do smoke and hot air travel up through the chimney rather than out into the room? There’s nothing stopping them, so what’s really going on here?

Why Does Hot Air Go Up The Chimney - A Matter Of Molecules

First, let’s discuss why it is exactly that hot air rises. It helps to kick this topic off by thinking about water in a kettle.

When you flick your kettle on in the morning and prepare yourself for that first glorious cup of Joe of the day, the water is being heated.

You can tell the water is being heated because you can hear and see the formation and movement of bubbles within it. It’s as if the water is becoming irritable and can’t contain its energy, and that’s actually what’s happening.

The bubbles are caused by the excitement of molecules, which in scientific terms is what heating something is. The molecules start dancing and jumping around, rather than flowing languidly as they were when the water was cold. 

During this molecular shindig, the molecules are bouncing further and further away from one another, reducing the density of the water.

The molecules that are really partying hard draw so far away from one another, they become gaseous, rise in the form of bubbles, and pop on the water’s surface.

The thing to remember here is that if something (a bubble, in our case), is less dense than whatever is surrounding it (water), it naturally shoots upwards.

It’s harder to visualize due to air’s intangibility, but this is exactly what’s happening to air when it gets heated up in your fireplace. 

Picture a hot air balloon. Have you ever wondered how they work? It’s the exact same principle.

The pilot turns on the burner, which heats up the air molecules. The air molecules zoom upward before being caught by, and filling up, the balloon, which rises into the air due to their collective force.

Just as they do in the hot air balloon, when you light a fire, the air molecules kick off their invisible dance party, and as the surrounding air is much cooler, they start their ascension up the chimney. 

As these air molecules travel upwards, the space beneath them becomes depressurized, creating a vacuum, or in Layman’s terms, a space devoid of matter.

Consequently, more air is drawn from the room to fill the vacuum in the firebox created by the rising molecules. Then the air beneath those molecules is drawn upward, and the molecules after them too, then again, and again, perpetually — pretty neat, huh?

As air pressure in the room decreases due to the draw of the fireplace, more air is drawn in from outside, causing a river-like flow that only travels in one direction...up your chimney.

Why Does Smoke Go Up The Chimney - Riding The Draft

This constant molecular rise and fluctuating pressure creates something known as a draft, which you can basically think of as a wave that the smoke and all other combustion gases surf up the chimney and out of the building.

The reason smoke doesn’t come out of the fireplace and fill the room is that there are no forces within the room strong enough to pull it from the “current” of the draft.

In this respect, draft is a little bit like the event horizon of a black hole — nothing is powerful enough to prevent the smoke from traveling further in towards the opening of your chimney on your roof.

Well...that’s not strictly true. A strong enough breeze would draw some smoke out into the room, but you get the idea.

When Will Smoke Not Travel Up A Chimney?

Sometimes, smoke doesn’t seem like it’s in the mood for a draft surfing session, deciding instead to venture out into the room, which can be incredibly dangerous, but why the sudden change of heart?

Wet Wood

Did you know it takes most wood a year of storage in a dry place before it’s properly seasoned for burning in a fireplace? That seems like a long time, I know, but it’s worth the wait.

When we burn wet wood, it releases 3–4 times the amount of smoke as properly seasoned wood. This smoke is full of harmful by-products, and as there’s so much of it, it’s easy for some plumes to escape the draft and pollute the air in your room.

Closed or Blocked Flue/Damper

The flue is the main section of your chimney that the smoke travels up. The damper is like a little drawbridge that can be closed when the chimney’s not in use to prevent cold air finding its way into your house.

When the flue becomes blocked or the damper is closed, the draft is disrupted, and smoke will billow out into the room. That’s why it’s so important to hire someone to clean your chimney or do it yourself with a chimney cleaning brush.

Negative Air Pressure

Negative air pressure occurs when a home is too airtight. As the molecules rise and create the vacuum in the chimney, no new air can be drawn in from outside to continue feeding the fire and create the flowing air system that takes the smoke up the chimney.

But the external air doesn’t give up there, oh no; with the pressure outside exceeding the pressure inside, the air has no choice but to continue looking for an entry point, and the chimney is just that.

The air from outside will travel down the chimney, reversing the draft, and forcing the smoke out into the room.

Why Do Smoke and Hot Air Go up a Chimney - Summing Up

There you have it, folks; hot air is less dense than cool air, which causes it to rise through the chimney. This creates a draft that carries smoke and other combustion gasses along for the ride.

I hope that’s cleared things up for you!


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