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  • Writer's pictureSomesh De Swardt

Chimney Draft Issues?

Updated: Feb 10


Is your chimney sending smoke down the wrong way?

You are not getting enough draw?

There are a number of reasons why your chimney isn't working the way it should...

* Your chimney is not tall enough

* Incorrect cowling

* Negative pressure issues

* Not enough ventilation

* Blocked or clogged up

* Badly designed flue Issues



Negative Pressure Issues

Chimney Drafting Problems

In the photo above, there may be a few solutions to the customers draw issues. The first solution could be to replace the cowl for an anti downdraft cowl. A second possible solution is to cut the trees down to below the height of the chimney.

Badly Designed Flue Issues

In an open fireplace example. The size of the opening needs to be at least in the ratio of 7 to 1, in proportion to the size of the flue. If the fireplace is too large the flue will not be able to create enough suction to pull the products of combustion i/e/ smoke up the chimney. If the flue is too large, then it cannot heat up enough to create a draw in the first place.

If there are too many twists and turns in the flue, the flue will create excessive friction not allowing the smoke an easy passage through the system to the outside.


What are some common causes of downdraft issues?

Downdraught, which is a phenomenon often observed as puffs of smoke coming out of a stove or fireplace, is caused by certain factors related to the chimney and its surroundings. One primary cause is having a chimney that is not tall enough, which can hinder the upward flow of air and result in the reversal of airflow. Additionally, downdraught can be influenced by the presence of nearby high buildings or trees, which disrupt the natural wind currents around the chimney. It is important to note that downdraught may only occur under specific conditions, such as when the wind is blowing from a particular direction.


How can downdraught be identified?

Downdraught can be identified by observing the behaviour of smoke coming out of the chimney. When a chimney experiences downdraught, puffs of smoke will be blown back down into the stove or fireplace. It's important to note that if smoke continuously exits the stove or fireplace, it is not a case of downdraught, and other factors should be considered. Downdraught is typically caused by specific conditions such as a chimney that is not tall enough or the presence of tall buildings or trees nearby, which affect the wind patterns around the chimney. It is often noticed that downdraught occurs only on days when the wind blows from a particular direction, resulting in the stove or fireplace emitting smoke in puffs.


What is downdraught and what causes it?

Downdraught, in the context of smoking chimneys, refers to the occurrence of air being forced back down the chimney, resulting in the emission of smoke puffs from the stove or fireplace. This phenomenon is typically caused by specific conditions related to the chimney's design or environmental factors. The primary cause of downdraught is often attributed to low chimney height or the presence of tall buildings or trees that affect the wind patterns around the chimney. When a chimney is not sufficiently tall, it creates an unfavorable imbalance of air pressure. As a result, the wind is unable to create sufficient upward force within the chimney, causing the air to flow downward instead. Similarly, nearby tall structures or trees can disrupt the normal wind currents, diverting them towards the chimney and causing down-drafting. It's worth noting that downdraught is not a constant occurrence but is rather dependent on certain conditions. For instance, it may only happen on days when the wind is blowing from a particular direction, exacerbating the effects of downdraught. This intermittent nature of downdraught makes it apparent that external factors, such as wind direction, play a significant role in its manifestation. In summary, downdraught refers to the situation where air is blown back down the chimney, leading to puffs of smoke emanating from the stove or fireplace. This phenomenon is primarily caused by low chimney height or the presence of nearby tall buildings or trees, which disrupt the normal wind patterns around the chimney. Understanding the causes of downdraught can help identify potential solutions to mitigate its effects.

Can a rotorvent cowl become clogged up and hinder draw?

Yes, a rotorvent cowl has the potential to become clogged up with deposits if it is not cleaned on a regular basis, and this can indeed hinder the draw.

Rotorvent Cowl
Rotorvent Cowl

What is a Rotorvent cowl and how does it improve draw?



Rotorvent Cowl
Rotorvent Cowl

A Rotorvent cowl is a specific type of cowl designed to enhance the draw of air in a chimney system. Its purpose is to address downdraught issues, which occur when strong winds force air back down the chimney, causing smoke and fumes to enter the living space. Unlike traditional cowls, a Rotorvent cowl utilizes a wind-driven turbine mechanism that spins when subjected to wind, creating a positive pressure which aids in drawing air up the chimney. The improved draw of the Rotorvent cowl is primarily due to its wind-responsive design. When there is wind blowing, the spinning turbine increases the air pressure within the chimney, creating a vacuum effect that enhances the updraft. This increased updraft helps to alleviate downdraught by effectively preventing the wind from pushing air and smoke down the flue. However, it is worth noting that while the Rotorvent cowl is designed to optimize draw under windy conditions, there are other anti-downdraught cowls available. In fact, the MAD anti-downdraught cowl is often preferred over the Rotorvent cowl due to its consistent effectiveness in eliminating downdraught. Additionally, unlike the Rotorvent cowl, the MAD cowl is not prone to clogging with deposits, which could obstruct air flow and impede the overall draw of the chimney. In conclusion, a Rotorvent cowl is a specially designed cowl that improves the draw in a chimney by utilizing a wind-driven turbine, which creates positive pressure to counteract downdraught. While it can be effective in enhancing draw, other options such as the MAD cowl may provide more consistent downdraught prevention and without the risk of clogging.


What is an anti downdraught cowl and how can it help?

An anti downdraught cowl is a device designed to address the issue of downdraught in a chimney or flue system. It serves as a solution to this problem by effectively preventing or reducing downdraught. This cowl is specifically designed to counteract the downward air current that can occur in chimneys, especially during windy weather conditions. By installing an anti downdraught cowl, you not only mitigate downdraught but also enhance the overall functioning of the chimney or flue system. This device works by redirecting the airflow, creating a barrier that prevents downdraught from entering the chimney. It accomplishes this through various design features like baffles, vents, or cowling elements. The anti downdraught cowl serves as a quick and simple solution to the issue of downdraught. Its installation is relatively easy, and it provides immediate relief from the negative effects of downdraught. By effectively countering downdraught, the cowl ensures that the chimney or flue system functions optimally, allowing smoke, gases, and other byproducts to be properly vented. One of the advantages of an anti downdraught cowl is that it enhances the overall safety of the chimney or flue system. By preventing downdraught, the risk of smoke, fumes, or even sparks being pushed back into the living space is significantly minimized. This not only protects the occupants of a building but also helps to prevent potential chimney fires. In summary, an anti downdraught cowl is a device specifically designed to address and alleviate the issue of downdraught in chimneys or flue systems. By redirecting air currents and creating a barrier, it effectively prevents or reduces downdraught, improving the functionality and safety of the entire system.


Anti downdraught cowl
Anti downdraught cowl

What are the options for increasing the height of a masonry chimney?

For masonry chimneys, the height can be increased by either extending the brick/block courses or by adding a taller chimney pot.


What are the options for increasing the height of a chimney made from twin wall flue?

For chimneys made from twin wall flue, the height can be increased by attaching another length of flue.


How can the height of the chimney be increased to address downdraught?

The height of the chimney can be increased by either attaching another length of flue for chimneys made from twin wall flue or by extending the brick/block courses or adding a taller chimney pot for masonry chimneys.


Contact me to help you resolve your negative drafting and poor performance of your chimney and stove. There hasn't been a problem I havn't been able to solve yet.

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