What Is a Chimney Flue?

You may have heard the term “flue” being thrown around a lot when working with chimney technicians, but what are your sweeps referring to when they say that? Is a chimney the same thing as a flue? Are there different types of chimney flues? What’s the most accurate way to refer to your chimney?

Well, we’re hoping to clear up some confusion, so let’s start with the big question…

Is a Flue Different from a Chimney?

Ok, so a flue is a hole that smoke and gases can vent through. So…it’s a chimney? Technically no. A flue is different from a chimney, although the two are closely related.

• The chimney is the structure as a whole, that houses the flue and other components
• The flue, on the other hand, is the internal passageway within the chimney that guides smoke and gases upward and out of the home.

Think of it like this…

Your chimney is like a tall, skinny cup, and the straw inside is the flue. The cup provides structure and support, while the straw creates a direct channel to carry the liquid (or in the chimney’s case, smoke and gases) from one point to another.

Or think of it like a pencil! The wood (or plastic for a mechanical pencil) encases and protects the graphite core – just like the chimney houses the flue. The graphite (flue) is the part that actually carries out the writing (venting), while the outer structure (chimney) supports and protects it.

Is a chimney flue different from a chimney liner?

Yes! Although, we see how it gets confusing. A chimney flue is the passageway, while the chimney liner is the metal (or, in some cases, clay tile or ceramic) lining that safeguards that passageway.

How Does My Chimney & Fireplace Work?

So, since we’re already diving into it, let’s break down your chimney’s anatomy:

• Firebox: The firebox is the main chamber where the fire burns. It’s typically lined with firebrick or other heat-resistant materials designed to withstand high temperatures.
• Damper: The damper is a metal plate that sits just above the firebox and can be opened or closed with a lever. When open, it allows smoke and gases to flow up the chimney. When closed, it seals off the chimney to prevent heat loss when the fireplace isn’t in use.
• Smoke Shelf: Located above the firebox, the smoke shelf is a flat surface that helps prevent downdrafts and directs smoke into the flue. It also catches debris and prevents it from falling back down into the firebox.
• Smoke Chamber: The smoke chamber is a funnel-shaped area above the smoke shelf where the chimney narrows before reaching the flue. Its sloped walls help to compress and direct smoke smoothly into the flue.
• Flue: The flue is the inner passage within the chimney that directs smoke, gases, and other byproducts of combustion safely out of the home.
• Chimney Liner: The chimney liner (often made of clay tiles, stainless steel, or other heat-resistant materials) lines the flue to protect the chimney walls from heat and corrosion.
• Chimney Cap: A chimney cap is installed at the very top of the chimney and serves as a protective covering. It prevents rain, snow, animals, and debris from entering the chimney, while still allowing smoke and gases to escape.
• Chimney Crown: The chimney crown is the masonry slab at the top of the chimney that seals the chimney structure from the outside. It’s designed to prevent water from entering and damaging the brick or mortar, which can cause long-term issues if not properly maintained.

How do all of these parts work together?

When you light a fire in the firebox, the heat generated creates hot air and gases. Because hot air rises, it moves up from the firebox and into the smoke chamber, where it’s funneled into the flue. Opening the damper allows smoke to travel up the flue and out of the chimney – which is protected by the chimney liner.

Meanwhile, the chimney cap keeps rain, debris, and animals out while still allowing air and smoke to escape, and the crown sends water out and away from the sides of the chimney.