Understanding Flues for Wood Burning Stoves

Date: 21 May 2015

When buying a wood-burning stove it can be tempting to focus all your energy on selecting the perfect stove – without any consideration for the flue. However, the reality of the situation is that a properly-fitted flue should be considered just as important as the stove itself. Indeed, even the best stove in the world will underperform if it is not attached to an appropriately-fitted flue.

But what are the important considerations you should be aware of?

According to HETAS there are four key factors that determine the effectiveness of a flue system for stoves. These are the differences in air pressure and temperature between the inside of the flue and the exterior atmosphere, the quality of insulation in the chimney and the route that the chimney takes.

The perfect flue arrangement for wood-burning stoves takes all four of these factors into consideration, allowing your stove to function at its optimum rate and to efficiently heat your home.

Differences in Air Pressure between the Flue and the Building Exterior

It is a fact of physics that air pressure naturally moves from extremes to the average. In this case from higher to lower.

Imagine blowing up a balloon and then holding the end open. The air inside, which is at a higher pressure, rushes out to join the rest of the air. The same also happens when you use an aerosol can – the higher pressure gas inside migrates to the lower pressure outside.

This principle occurs whenever you light your stove. For smoke and gases to be effectively removed from your stove they are sucked up the chimney to join the air outside. This happens as a result of the lower air pressure that is 30 feet above us.

The greater this pressure differential, the faster and more efficiently the exhaust gases from your wood-burning stove will be evacuated.

The key factor that affects the air pressure within your chimney is the length of the chimney itself. The greater the height difference between the start and finish of the flue, the greater the differential becomes.

The headline here is, therefore, that (in general) the taller your chimney is, the better your wood-burning stove will perform.

Differences in Temperature between Smoke and the Building Exterior

As every schoolchild knows, hot air rises while cool air drops. This is known as ‘buoyancy’. From the perspective of the stove owner, this means that the hotter the gases from your wood-burning stove, the faster they will travel up your chimney.

This is important for two reasons. Firstly, of course, it helps your stove to burn more efficiently.

In addition, however, the exhaust gases from wood-burning stoves contain an assortment of impurities and resins. Over time these particles can settle on the inside of your chimney in the form of creosote or soot. Rather like plaque building up in the arteries, over time these build-ups can constrict the space in your chimney through which exhaust gases can escape.

The hotter the exhaust gases leaving your stove, the faster they will travel up your chimney the less time they have to make deposits. Hotter flue gases are also a sign of more complete combustion, which is better for you and the environment. And, as a result, your chimney will remain cleaner.

Insulation of the Chimney

If hotter smoke from your wood-burning stove means fewer deposits of creosote and soot, then it is essential to keep the exhaust gases from your stove as hot as possible on their journey up your chimney.

This can be accomplished through chimney lining. An assortment of materials can be used for chimney lining, including stainless steel, pumice and ceramics. All of them have the same goal, however, which is to insulate your chimney from the cold air outside and to keep the smoke from your stove moving as quickly as possible.

Route of the Chimney

Hot smoke typically travels straight upward. Expecting smoke to travel around bends and corners will slow down its journey and increase deposits within your flue. When fitting a wood-burning stove it therefore becomes important to correctly attach your stove to the flue in order to minimize the number and angle of any bends. Ideally, the angle of your stove pipe will be kept below 45º for best results.

As you can see, there are many factors that must be considered when fitting a stove to your property. Here at Dorking Stoves, we have decades of experience in helping homeowners to successfully site their stove for maximum effectiveness.

If you’re based in the Dorking or Guildford area and are considering a new stove we’d encourage you to pay us a visit. Not only can you inspect one of the largest ranges of stoves in the south of England, but our highly-experienced team can also provide all the advice you need regarding proper flue design.