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What Is A Fireplace Flue?

By Lawrence Macmillan August 15, 2021

If you are lucky enough to live in a home where you have a chimney and a working fireplace, then you know how wonderful it is to have a warm fire burning in your home during those cold winter months.

However, a chimney is not something that you can just leave there to do its job, and it is not simply just a hole in the wall. There are different parts, different areas of it that do different jobs, and you need to understand how the whole system works in order to properly maintain your fireplace. 

One of the main parts of your chimney and fireplace is the flue. If you have just started thinking about lighting your fireplace, and you have come across the term ‘flue’, you might be wondering what on earth it is. Well, let us explain this for you and help you to understand what it does, if you have a flue, and what the difference is between a chimney and a flue. 

What is a flue? 

In the simplest of terms, a flue is simply a passage for the exhaust gasses from your fire to the outdoors. This may be a duct, a pipe, a vent, or a chimney. An unlined chimney is technically a flue, even though an unlined chimney is, by all rights, a hazard. This may cause a bit of confusion, as many people will view flues as a passage for venting, but also a safe passage for venting. 

Another way to define this is that a flue is the lining in a masonry chimney. This is often a clay, ceramic, or metal conduit that is installed inside of a chimney, intended to contain the products of combustion and direct them outside of the home, thus also protecting the walls of the chimney from heat and corrosion. 

Although building codes will differ depending on where you live, the installation of a flue lining has been recommended for quite a while, and almost all fire codes will now mandate liners. 

A flue liner will serve multiple purposes, and all of these will keep your fire burning and your home safe. No matter what type of liner your system has, it will protect the house from any heat transfer, and it will protect the chimney from any corrosive byproducts, also creating a correctly-sized flue for optimal efficiency.

Some older chimneys would have had clay tiles that lined them, however, over time these will have broken away into pieces that you may find inside your fireplace. If you find these pieces in the fireplace, then your flue will need to be relined or repaired immediately. 

How exactly does your chimney work? 

How exactly does your chimney work?

So, thinking about flues, how does your chimney work? 

Let's think about the chimney system. The whole system must be complete and properly maintained in order for it to work properly. Every part of the system is imperative for it to function properly. Even though new technologies may make your chimney work better, older chimneys can work fine as they are, as long as they are complete and in place. 

Your chimney works in tandem with your fireplace. Without any fire there is no heat to carry up the chimney, and without a chimney your fire will not burn safely. 

For any fire to burn properly it needs heat, oxygen and fuel. To fuel a fire you may use wood, gas, or pellets. The heat you provide may be ignition, or a pilot mechanism, or it could be kindling and a match. Finally, you need oxygen, and this is where the flue is imperative. 

Your fire will need airflow to be able to both vent and burn, an open fireplace, or a stove will draw this from your home and then vent through a chimney system. A direct vent will not need to have a flue or a chimney at all, as it would draw air in from outside and vent it through the same opening it came in through. 

A flue can be opened and closed by a damper. Smoke will rise from the fire in the firebox, rise through the throat, go through the open damper, and into the flue, it will then escape the flue through the cap and outside. 

What is the difference between a chimney and a flue? 

There is a lot of confusion between chimneys and flues. Many people may think that masonry chimneys are just brick, however this is not true. The uneven and porous surface of brick would make a great space for the accumulation of combustion byproducts, which is a major fire hazard in every way. 

Instead of allowing homes to be a major fire hazard in this way, chimneys are lined, so that the smoke will pass over a smooth surface that helps to prevent any accumulation of the combustion byproducts. This therefore minimizes the chance that there could be a fire in the chimney. Chimney liners are often also called chimney flue liners 

So you could say, flues actually make chimneys safe.

Does your setup have a flue? 

If your fire is a wood-burning fire, then no matter what type of fireplace or appliance it is, it requires a flue to be in place. It may be inside your chimney, or it may just be a simplistic flue pipe. But nonetheless, the flue is the duct that the smoke and waste gases will exit your home. 

The duct needs to be properly sized to match the capacity of your fire system, it should also be smooth to allow for optimal airflow and no hazards. It also needs to be adequately lined in order to prevent any high heat or toxic fumes from getting into your home. 

If your chimney is a wood burner, then a flue is required for it to work properly. Perhaps you have moved into a new home, if this is the case, call a professional to inspect your chimney to make sure it is safe for you to use. 


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