Why is smoke coming down the chimney?

An open fire is increasingly becoming something of a rare luxury in the modern world. What was once the only way of heating your home has become an unwieldy, inefficient way to keep warm in an age of central heating and triple glazed windows. Still, nothing beats a real fire for the aesthetic appeal to our core beings. Something that calls out to us from our evolutionary pasts, when huddling around a fire at night meant the difference between life and death.

Except, that is, when smoke starts billowing back down the chimney!

Nothing puts a cramp in your enjoyment of the soothing ambience of an open fire like a lung-full of smoke, so let’s talk a little about why this might happen. And, of course, how you can put a stop to it or prevent it from happening in the first place!

How Chimneys Work

The basic principle of how a chimney works is incredibly simple. Hot air rises, and the regular air in our atmosphere (when it’s not inside ovens or various industrial applications) rarely gets hotter than the air around a burning fire. So the natural inclination of the hot hair—and smoke—is to head upwards.

Chimney’s take advantage of this fundamental law of the universe by providing those gasses a clear path to outside your home; straight up.

However, as you might have guessed, there is more going on here than just providing smoke and a clear run to the sky.

The differentials in pressure, and the escaping air heading up the chimney, cause what is called a “draught”. Air pressure will always seek to even itself out, this is why gas released into a vacuum will expand explosively. 

Fortunately, the air movement in your home is more tame than that, and the drought is little more than a barely perceptible movement through your home.

However, this barely perceptible movement serves an important purpose. Firstly, it replaces the air that has left your home through the chimney, making sure you have plenty of fresh, non-burnt air to breath. But secondly, it allows for fresh air to be pulled towards the fire, ensuring that the fire stays healthy and strong. Your fire needs oxygen to burn, just as you do to breathe, and without a supply of fresh air, it will gutter out.

What is Chimney “Downdraft”?

A downdraft is when smoke is blown back down the chimney, coming out at the point where it should be leaving—your fireplace. It will usually come out in puffs of smoke, rather than a continuous stream of it. This is important because a continuous stream of smoke coming out of your chimney is often an indication that there is something else wrong, and not a downdraught.

It is also important to note that, in some situations, you could find yourself with smoke blowing back into your home through the chimney when your fire isn’t even lit. This is often caused by inconvenient wind direction (or interference from neighbouring buildings) in homes that share a chimney stack with another property. In some cases, the wind can blow the smoke coming out of one chimney straight back into the adjacent chimney.

Why Smoke Comes Down the Chimney

So, what could be causing smoke to blow back down your chimney? There are a few potential culprits, and, unfortunately, they’re not always easy to identify with the right tools and experience. Still, we’re going to do our best to help you figure out what’s going on

Chimney Blockage
If you find that smoke is billowing out constantly, you may have a blockage in your chimney. This will prevent the smoke from exiting the chimney flue, causing it to back up until, eventually, it starts escaping out from the fireplace.

Blockages can occur because a soot door has been left closed, or because something has fallen down the stack. If the fire has not been lit for some time, it can even be because birds have built a nest in the chimney!

Naturally, the only way to resolve this issue is to get the blockage removed. It may surprise you to learn that chimney sweeps are still a thing, but the trade has moved on from sending children up there with a brush! Looking for a local roofing expert?

Wind-Induced Downdraught

We’ve touched on this above, but when two properties share a chimney stack, the flues are typically right next to each other, meaning the chimney itself, where it emerges from the property, will be right next to its counterpart from next door.

In situations where the wind is aligned in the right way, the smoke coming out of one chimney can be blown back down the other chimney. This can happen whether the second chimney’s fire is lit or not.

Unfortunately, there is nothing you can do about the wind, and it probably isn’t an option to ask your neighbour to stop lighting their fire. But there are vents and speciality chimney cowls that can help prevent this from happening.

Dynamic Wind Loading

Going back to our description of how the pressure differentials lead smoke to head up the chimney, pulling fresh into the house from vents, windows, and other points of entry, dynamic wind loading is a way in which this process can reverse.

Typically, this will happen when windows are open on a windy day, but it can happen in other situations. Essentially, a negative pressure differential is created in the house independent of the fire. If this negative pressure differential is strong enough, it can pull air in from the chimney, which will include any smoke that happens to be trying to make its way out through there as well.

If this rarely happens, it may just be a case of closing some windows (or opening them) when it occurs. However, if it happens all the time, you may need to think about getting your home evaluated to see what can be done, such as adding new vents.

When Does Smoke Coming Down the Chimney Usually Happen?

The timing of when you will find smoke coming down your chimney will depend on which of the potential causes you are dealing with. If you are dealing with a blocked chimney, it will happen any time you light your fire. The smoke will build up in your chimney until it reaches the top of your fireplace, and then spill out. It might help to think of the smoke as water and your chimney as an upside down container.

The water accumulates in the bottom of the container, filling it up, and when it reaches the top (your fireplace) it spills over the side.

If, on the other hand, you are dealing with one of the wind-related causes, your smoke problem will manifest whenever the wind is right. It should be noted that this may not mean any time it is windy. For example, wind-induced downdraught will likely only occur when the wind is blowing from a specific direction. If the problem is dynamic wind loading, you may find it only happens when certain windows or doors are open

What are the Effects of Smoke Coming Down the Chimney?

The first and most obvious effect of smoke coming down your chimney is the unpleasant reality of having smoke in your immediate vicinity. It’s not just unpleasant to inhale, it’s dangerous, and breathing too much of it could seriously affect your health.

In the longer term, smoke can and will leave sooty deposits on anything it touches, meaning a lot more cleaning if you want to keep your home spotless. This is also a problem on the smell front, since anyone who’s been near a fire or anything with a distinct odour that has been burning will be able to tell you—those smells are hard to get rid of.

How do you Prevent Smoke Coming Down the Chimney?

For chimneys that are blocked, it’s time to call a chimney sweep to get that blockage removed. For properties that are suffering from wind-induced downdraught, there are special chimney cowls that can mitigate the problem, as well as soot doors that can be closed during problem times (and when your own fire is not lit, of course) blocking any smoke from being blown all the way back down your chimney.

For dynamic wind loading, there is little for it but to experiment with your doors and windows, and see if you can find a combination of things being open and closed that doesn’t result in smoke finding its way back down your chimney.

Final Thoughts

Smoke coming back down your chimney is understandably concerning, but it can often be fixed with relatively little fuss. Just remember that downdraught problems will typically manifest in puffs of smoke, rather a continuous stream of it. This should help you apply a little initial diagnosis to the situation and understand what is happening.

If the problem is beyond something that closing a window can fix, we certainly recommend getting a professional on the case. Anything requires messing around with the insides of the chimney or climbing on the roof is dangerous, and best left to the experts.

Get a quote from an expert today!

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