Flue — exhaust passage for chimneys and appliances

A flue is a passage for combustion exhaust that carries smoke, gases, and heat from an appliance or fireplace to the outdoors.

Flue diagram — labeled parts, dimensions, and installation context

What It Is

A flue is the internal passage that vents combustion byproducts from a fireplace, furnace, boiler, water heater, or stove. It may be part of a masonry chimney, a metal vent system, or a factory-built chimney assembly.

Types

Common flue systems include masonry chimney flues, metal vent pipes, Type B gas vent flues, pellet vent flues, and insulated stainless chimney liners. Fuel type determines what materials are acceptable.

Where It Is Used

Flues are used anywhere fuel-burning appliances need to exhaust safely. In homes that includes fireplaces, wood stoves, gas furnaces, boilers, and gas water heaters.

How to Identify One

The flue is often hidden inside a chimney or vent pipe, but you can identify the system by tracing the appliance exhaust path to the outdoors. Chimney tops and vent connectors are the main clues.

Replacement

Replace or reline a flue when it is cracked, undersized, corroded, blocked, or no longer compatible with the connected appliance. Venting problems are safety issues, not cosmetic ones.

Flue — FAQ

What does a flue do?

It carries smoke, gases, and heat from an appliance or fireplace to the outdoors. In a home, that helps the surrounding system work the way it should.

How can I tell if a flue needs attention?

Drafting problems, soot buildup, odors, corrosion, and crumbling liner material are common warning signs. Condensation and staining around the vent path also deserve attention.

Can I replace a flue myself?

Basic visual checks and cap cleaning may be within reach, but flue sizing and replacement are not simple guesswork jobs. Venting mistakes can create fire and carbon monoxide hazards.

How much does flue repair or replacement cost?

Costs range from minor connector repairs to major chimney relining projects. Access, appliance type, and whether a stainless liner is needed drive the final price.