What is the Correct Flue Length for a Wood Burning Stove?

If it wasn’t for the flue, there would be nowhere for waste gasses from a chimney to escape. They’re integral to any setup but what is the correct flue length for a wood burning stove?

In short, the correct flue length is 12-15ft and the correct flue diameter is 25% larger than the stovepipe diameter. Your flue should also begin approximately 3 feet from its opening.

This might sound confusing or overly complex, but we’ll explain the crucial details below.

What is a Flue and why do Stoves have them?

A flue is nothing. No, really, a flue is quite literally nothing but empty air.

Many people confuse the “stovepipe” (i.e. the pipe attached to and rising out of the stove body) for the flue but both are different yet connected. The flue is the empty space, beyond the stovepipe but between the fire and the outside. The chimney is at the top of the flue, and around the flue is the flue wall.

But as we’ve said, the actual “flue” isn’t technically a thing, but the air inside the flue walls. A space for smoke and waste gasses to disappear. You could think of it as the “internal” chimney within the home.

Despite being literally nothing, the flue is vital for any stove (or fireplace). If it wasn’t for the flue, all the waste gasses would have nowhere to escape. They would collect within your stove, steal the oxygen, and cause the fire to die.

Alternatively, they might also leak into the room and naturally this can cause health issues as it contains carbon dioxide and sometimes carbon monoxide.

Wood Burning Stoves and Fireplaces Explained

To understand a flue, it might help to first understand how stoves and fireplaces actually work.

In most fireplaces and wood stoves, you’ll have what’s called a damper just above the flame. This allows the user to control how much oxygen the fire is fed. If you want a bigger flame, you can add more oxygen. For a smaller flame, just close the valve, which closes the damper, which reduces the amount of oxygen.

Some smoke might settle on the smoke shelf. This prevents too much build-up of ash.

However, most smoke exits the fireplace via the flue. Hence, when you walk past a home with a traditional fireplace, smoke coming from the chimney shows that the fire is on.

What is the Correct Flue Length for a Wood Burning Stove?

Some people might opt to put their wood burning stove directly underneath the flue.

If you live in a house with a flue, this is sensible as there is already a place for waste gas to escape.

If your home doesn’t have a flue, you will need to create and install a hole in the wall (or ceiling) for the stove pipe to exit out of.

The general consensus around how long a flue should be is approximately 12-15ft, and the width or diameter should be approximately 25% larger than the stovepipe diameter. In other words, think of two circles, one bigger and the smaller circle sitting inside the larger circle. The larger circle is the flue, and the smaller circle is the stovepipe.

In another example, let’s say you have a stove. The stovepipe has a diameter of 6 inches. 6 inches increased by 25% is 8 inches.

Chimney Flue vs Stove Pipe

People who live in older homes will likely have a chimney flue. This is built-in as part of the structure of the house. Generally, chimney flues are larger, allowing more oxygen to come in and allowing more waste gas to escape.

A stovepipe is different and is part of your wood burning stove. As a result, you actually need both the stove pipe and the flue. It’s not a case of either or.

In most homes, the stove will either be fitted into an existing traditional fireplace or it will be built into a new area of the home and an external flue system will be installed in lieu of a traditional brick chimney.

Can a Flue be too Long?

Yes, a flue can be too long.

If your flue is too long, it’s more likely that heat from the fire will dissipate through the walls. This is heat that will thus not be used to keep you warm.

On the flip side, if the smoke has too long a journey to escape the house, it will cool down and sink back into the stove. From here, it will bellow into the house.

Hot gasses rise, and cold gasses sink. If the waste gas cools before it escapes, it will fall, and the pressure will cause it to go back into the house.

What if a Flue is too Short?

However, it’s also crucial that a flue isn’t too short.

If the flue is too short, too much air will be able to enter the fire. Of course, oxygen is necessary for the fire to survive, but too much and the flame will burn out too quickly thus ruining the classic long, slow burn a wood burning stove can provide over long periods of the day or overnight.

In this case, with the flame burning out quicker that normal, you will need to use more wood to stay heated. As a result increasing both your workload and your fuel costs.

Also, if the fire becomes too hot, there is even a possibility that the roof may become damaged from constant exposure to excess heat.

As you can see, the flue is an integral part of the wood burning stove (and fireplace) design.

We hope this helps, and for everything else home heating stick with CosyWarmer.com