What a Chimney Flue Is—and Why It's Needed

House roof with chimney flue
By Eve Livesey / Getty Images

Your home's chimney is tied to the fireplace: The chimney is the conduit for smoke and other contaminants to escape your home. Understanding the function of chimney flues—and chimney flue liners—is essential to overall fireplace and chimney safety and maintenance.

What Chimney Flues Are

A chimney flue is a vertical passage or duct that runs from the firebox (where the fire burns) to the top of the chimney.

Technically, a flue is any open vertical space in a chimney that allows smoke to escape the home from the firebox. But because every chimney's flue must be lined, a chimney liner is usually referred to as the flue, as well.

Every wood-burning or gas fireplace has a flue. Gel and faux fireplaces do not have chimneys or flues.

Chimney flues have traditionally been made from fireclay flue tile. More recently, stainless steel flues are used in addition to clay tile.

Clay Tile Flue Liners
  • Inexpensive

  • Easy to obtain

  • Can crack or split

  • Difficult to repair

Steel Flue Liners
  • Safe

  • Durable

  • Will not corrode

  • More expensive than clay liners

Why Chimney Flues Are Needed

They Prevent Combustion in Surrounding Areas

Unlined chimney flues will allow smoke and gasses to pass through the bricks and mortar. Smoke and heat can eventually turn into a chimney fire and pass through to combustible materials such as wood studs and drywall, causing a house fire.

Chimney flue liners are solid, largely seamless surfaces that keep the smoke in the flue and direct it upward instead of sideways.

What Is a Chimney Fire?

A chimney fire is a terrifying event that cannot be easily reversed. The fire begins with loud gunshot-like pops and quickly progresses to a deep rumble often compared to the sound of a freight train. The flue may crack, permitting flames to reach the home's structure. Fire departments must direct water down into the house from the top to extinguish the fire, resulting in flooding.

Chimney Flues Aid in the Passage of Smoke and Gasses

Chimney flues are smooth; brick and mortar are not. Smooth inner flue surfaces help the smoke travel upward more freely.

They Protect Chimney Materials

Fires produce contaminants that are carried by the smoke. Clay and steel chimney flues protect the surrounding brick from creosote, acids, and other contaminants.

How to Re-Line Chimney Flues

If the chimney flue is damaged on the inside, it cannot be easily repaired. However, many flues can be re-lined—in other words, the liner itself can receive a second liner.

Flue liners are made either from rigid or flexible stainless steel. Rigid liners afford slightly more space within the flue, since they better conform to the flue's size, but rigid liners are more difficult to install since they must be assembled in sections as they are dropped down the flue.

When the flue extends more than 12 feet or if there are complications within the flue, choose a flexible flue liner.

Relining your chimney can improve the fireplace's draft. Smoke and gasses will rise more freely. And, most importantly, fireplace operations will be safer.

Chimney Flue Legal Requirements

Building codes vary by municipality. Generally, areas that adopt the International Residential Code (IRC), Chapter 10, may require (in part):

  • All masonry chimneys must be lined with a flue liner.
  • Flue walls (not liners) must be grouted smooth and solid, with walls no less than 4 inches.
  • Grout should not bond with the flue liner. This allows for movement of the flue liner during thermal expansion.
  • Concrete, metal, or stone caps should be added to masonry chimneys.

Tips For Cleaning and Maintaining Chimney Flues

Cleaning the chimney flue should be done annually, prior to fire use season. The flue should also be cleaned when you notice soot or oily creosote falling into the firebox during a fire. You'll especially want to do this if the creosote is more than 1/4 inch thick on the walls of the flue.

Clean the chimney more than once a year if you use your fireplace frequently or if you burn green or unseasoned firewood.

Some homeowners opt to clean their own chimney flues by purchasing chimney sweeping kits. First, the firebox is sealed with plastic to prevent debris from entering the home. Standing on the roof, the homeowner starts sweeping at top and progressively adds extension rods to a chimney brush until they reach the firebox.

Because cleaning the flue is messy and dangerous, most homeowners opt to hire a professional chimney sweep for the job.