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Bad Smell Coming Down Chimney?


To answer this question, initially we need to understand how chimneys work. 

A chimney functions because the air inside the chimney is lighter than the air outside of the chimney. This is based on temperature differentials. 

As it’s based on differentials, this means it’s all relative. 

If the temperature inside the chimney is 15C, but the air outside is 22C, your chimney probably won’t work properly. 

On another day when the temperature is 6C outside, if the chimney is 15C it’ll probably work. 

Air pressure causes the cool, less buoyant air to push down around the building. Simultaneously the warm, buoyant air rises replacing the cool air. 

The longer the chimney, the bigger the difference in temperature and pressure, resulting in the air moving at a greater velocity – if the conditions are right. 

This is similar to the reason the temperature of a room will be higher at the ceiling than near the floor. 


The volume of air moo traversing the flue is not only determined by the difference in temperature or pressure. 

A major factor is how permeable the house or room is – whether or not there is enough air getting into the room to allow the chimney to keep allowing air to escape through it. 

If there is not enough air getting into the room, the chimney won’t work. But whether or not there is enough air getting into the room is a variable factor. 

Things like externally vented tumble dryers and extractor fans need plenty of air to function. If there isn’t enough ventilation into the house, when they’re running they will pull air down the chimney. 

There should be enough ventilation into the house to simultaneously supply all appliances with sufficient air; tumble dryers, extractors and fireplaces/chimneys. 

But how does that explain smell?

Chimneys are innately smelly. They carry acrid Smoke. They house damp, fusty bird nesting material. They are dusty. They are often coated in soot. 

Nearly every chimney is smelly. Due to the way a chimney functions though, you can’t smell it. 

When a chimney is not working properly and the air comes down instead of going up, the smell will be noticeable. Suddenly all the smells that are usually escaping through the chimney pot start escaping through the fireplace instead. 

Cold chimney smelly chimney 

If a chimney is colder than the air outside it’ll blow down and bring smells with it. This is a particular problem with chimneys situated externally. 

Dirty chimneys get colder. Soot gets damp, as does dust and bird nesting material. The dampness lowers the temperature and causes the air to blow down. 

This can be a particular problem in the summer. Though the chimney won’t be ‘cold’, on especially hot days the temperature outside will be higher than in the chimney and in the house. This will make the air come down, once again bringing the smells with it. 

The only way to stop this is to increase the temperature in the chimney. Having the chimney cleaned will help raise the temperature as it’ll reduce the likelihood of damp. Lighting a fire will help too. 

Air tight doesn’t smell right 

If there’s a lack of ventilation, a chimney will act as an air vent for the house. That means it will bring air into the property rather than taking it out. 

When air is coming into a house through the chimney, it’ll bring with it particles from substances in the chimney creating an odor. 

If extractor fans are turned on, it will make the smell even worse. The air will come down the chimney faster, disrupting the substances in the chimney further creating a more powerful scent. 

This problem is fixed by removing as many of the smelly substances as possible through sweeping and installing more ventilation into the property to supply all the appliances that need it.

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