LOPCO Blogs

What Is A Flue?

A while back, I messaged one of our masons that we had a client who needed to have a flute on their chimney fixed.

Thankfully, although it would have been the perfect opportunity to do a deep dive into chop-busting, our mason chose otherwise and was relatively tame with the snarky remarks that ensued and which I very much deserved.

“A flute? LOL, you mean a flue?”

Yikes! I immediately felt like an idiot.

So, what exactly is a flue (or in my case in this instance a ‘flute’!)?


A flue is an integral component of a chimney system and plays a crucial role in safely expelling combustion gases from a home’s heating appliance, such as a fireplace, wood stove, or furnace, to the outside atmosphere. Consisting of a passage or duct that runs through the chimney, the flue serves as a conduit for the removal of smoke, gases, and other byproducts generated during this combustion process.

Structurally, the flue is typically constructed of fire-resistant materials, such as clay tile, metal, or concrete, and is designed to withstand high temperatures and corrosive elements. It is strategically positioned within the chimney to ensure efficient ventilation and prevent the buildup of dangerous gases, such as carbon monoxide, inside the home.

In New England, the flues we run into most often on the residential side of things are made out of clay tile.

One of the primary functions of the flue is to create a draft, or upward flow of air, which pulls combustion gases out of the heating appliance and directs them safely outside. This draft is generated by the temperature differential between the air inside the flue and the cooler air outside, as well as by the buoyancy effect of hot gases rising. Proper drafting is essential for the efficient operation of the heating appliance and for minimizing the risk of backdrafting, which occurs when combustion gases are forced back into the home.

To facilitate effective draft and prevent obstruction, the flue must be properly sized and maintained. The size of the flue is determined by factors such as the type and size of the heating appliance, the height and diameter of the chimney, and the local building codes and regulations. A flue that is too small can impede airflow and lead to poor combustion, while one that is too large may result in excessive cooling of the gases and reduced draft efficiency.

Regular maintenance of the flue is essential to ensure its continued functionality and safety. Over time, creosote, a highly flammable substance formed by the condensation of combustion byproducts, can accumulate on the walls of the flue. If not removed through chimney sweeping, creosote buildup can pose a serious fire hazard and impair draft performance. Additionally, debris such as leaves, bird nests, and other obstructions may block the flue, further hindering airflow and increasing the risk of carbon monoxide poisoning – hence the value of the stainless-steel chimney caps that we often see protecting the flues above the chimneys in many homes.

In addition to facilitating ventilation, the flue also serves as a barrier to prevent heat transfer and protect the surrounding structure from fire damage. By containing the intense heat and flames produced by the heating appliance, the flue helps to prevent the ignition of combustible materials within the chimney and adjacent walls.

A flue – not ‘flute’! – is a critical component of a chimney system that functions to safely expel combustion gases from a home’s heating appliance to the outside atmosphere. By facilitating efficient draft, preventing obstruction, and protecting against fire hazards, the flue plays a vital role in ensuring the safe and effective operation of the heating system. Regular inspection, maintenance, and cleaning of the flue are essential to its continued functionality and the safety of the occupants.

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