13 Jul 2021

Is Your Chimney Tall Enough for a Fire?


There are so many factors that make a chimney work effectively, but there are two, in particular, that stands out: temperature difference and pressure difference.


In this blog, we’re going to look at the latter and reveal how it’s impacted by the height of your chimney.


The physics behind chimney height


You may not realise it but the height of your chimney is one of the most important elements that enable it to work effectively.


The best way to think about this is to consider that air has weight. At the bottom of your chimney, the pressure is greater, because there’s more air above. At the top of the chimney, the pressure is subsequently less. This is why the difference in pressure between the opening of a fireplace and the area at the top of your chimney encourages air to travel up the flue.


Chimney stack effect


It’s this effect that balances out the difference and why you should expect smoke to immediately evacuate upwards when you first light the fire.


There’s no ‘luck’ involved with chimney design. A great chimney that ‘pulls like a train’ will have followed the right principles of design to take account of physics and pressure differences.


So, is a taller chimney better?

tall chimney stacks


Not necessarily. Taller flues may be able to deal better with pressure, but they also suffer from greater heat loss. That subsequently lowers the temperature of the gases and cancels out any benefit of additional height.


On the other hand, if the flue is too short, the pressure difference won’t be enough. This will result in the air having a harder time travelling upwards and will cause the fire to smoke back during lighting. More worryingly, short flues can only evacuate smoke once enough heat has been generated, thus sending harmful fumes into your home in the meantime


It is, therefore, a balancing act.


What’s the perfect chimney height?


There’s no specific, industry-standard number for the perfect height of a chimney. You can follow best practices, though.


For instance, if your home is two or three storeys, providing the chimney terminates above the ridge of the roof and isn’t near any trees or adjacent buildings, the height should be perfect.


If your property is older, it’s likely that the chimney is the tallest point of the house. This isn’t by accident; the designers knew that a specific height was required, regardless of the roof ridge, in order to correctly balance the pressure.


If you’re having a new chimney installed, it’s unfortunately not uncommon for the importance of its height to be ignored. So, while planning your chimney, consider the following:

  • don’t get bogged down in aesthetics – it needs to look nice, sure, but that shouldn’t be at the expense of an incorrect height;
  • don’t accept advice from installers who suggest that the flue can be less than 4.5m (it can’t, in accordance with regulations) – it might need to be much taller than that; and
  • expect to pay more than you think – twin wall flues are expensive, and that often leads people to reduce the height of the chimney to save costs. Please don’t.


Do chimneys really shorten over time?

Leaning stack


Yes, they do! But not on their own.


There’s a good reason for this. Chimney stacks are exposed to the elements and consequently deteriorate over time. This can lead a lot of chimney repairers or builders to simply reduce its height.


This can happen continually over a number of years until it is effectively a ‘stump’. This might save you the expense of regular remedial work, but if the chimney gets shorter, it won’t work as effectively and could become dangerous.


Above, is a perfect example, the stack on the left (circled) has been reduced to a ‘stump’.  It was probably leaning like the neighbouring stack so it was reduced in height. If there is no intention to use the fireplace then there is no problem. However, if anyone intends to use the fireplace in the future, regardless of the fuel type, the stack will need to be rebuilt to the original height. This is a very common scenario and will usually be misdiagnosed as a blocked chimney as smoke or fumes will not evacuate up the flue.


What to do?

If you’re worried about the height of your chimney, the best thing you can do is get a qualified chimney sweep to inspect it.


That’s right up our alley, as it happens! So, if you want to talk chimney height with people who understand its importance, just get in touch with the SweepSmart team.