Flue vs. Liner

Two words, one passage

Think of a garden hose. The empty space inside is where the water flows. The rubber wall is what holds it in. In a chimney, the flue is the empty space and the liner is the wall.

People say "the flue is cracked" when they mean the liner tiles are cracked, and "we need a new flue" when they mean a new liner. That shorthand is common and usually harmless. It matters when you read an estimate, because "flue repair" and "relining" can describe very different jobs.

• Flue: the passage. It has a size (cross-sectional area), a height and a shape. Each appliance usually gets its own flue.
• Liner: the material lining that passage. It has a type, a condition and an age.
• Chimney: the whole structure, which may contain one or several flues.

What a liner actually does

The Chimney Safety Institute of America describes a liner as a clay, ceramic or metal conduit installed inside a chimney to contain combustion products, direct them outside and protect the chimney walls from heat and corrosion. In practical terms that breaks into three jobs:

1. Heat protection. Brick conducts heat. Without a liner, heat can travel through the chimney wall to wood framing that sits near or against it.
2. Corrosion protection. Wood smoke, oil exhaust and gas exhaust all carry moisture and acidic compounds. Over years they eat away mortar joints from the inside.
3. Correct sizing. A flue that is the right size for the appliance drafts better and stays warmer, which means less condensation and, with wood, less creosote.

CSIA also cites testing by the National Bureau of Standards (now NIST) in the mid-20th century that judged unlined chimneys to be seriously unsafe. That is the historical reason modern codes call for lined chimneys.

What "unlined" means

An unlined chimney has no separate liner. The smoke travels up a passage whose walls are the chimney's own brick and mortar. Many houses built before liners became standard practice were built this way.

A chimney can also be effectively unlined even though it was built with tiles. Liner tiles can crack, shift, lose the mortar between them, or partially collapse after decades of use or a chimney fire. A report may say the liner is "compromised" or "not continuous" to describe this.

You usually cannot tell from the fireplace whether a chimney is lined. Looking up from below shows only the first few feet. A camera run up the flue or an inspection from the top is how the liner is checked. See What a chimney camera inspection shows.

The main liner types

Clay tile is standardized under ASTM C315, which Brick Industry Association guidance cites for flue liners. Factory-made relining systems are tested and listed as a system, typically to UL 1777. For example, one stainless steel liner manufacturer's manual lists its product to UL 1777 and requires insulation wrapped around the liner for wood-burning use, a listed cap, and a cleaned, structurally sound chimney before installation. Each system has its own rules, so the installation manual for the specific product governs.

Metal liners also come in other alloys and grades for specific fuels. Which one is acceptable is set by the liner listing and the appliance manufacturer, not by general rules of thumb.

Choosing among them is a separate question, covered in Stainless, cast-in-place or clay liner?

Why flue size matters as much as liner type

A liner can be perfectly intact and still be the wrong size. Two common New Jersey examples:

• A furnace or water heater on a big old flue. When an oil or gas appliance vents into a flue much larger than it needs, the exhaust can cool before it reaches the top. Cool exhaust drafts weakly and condenses, which can wet and damage the inside of the chimney. This is part of the orphaned water heater problem.
• A stove or insert into a fireplace flue. Inserts and stoves usually need a liner sized to their outlet, connected directly to the appliance, rather than venting into the open fireplace flue around them.

Manufacturer instructions set the required size for each appliance. For open fireplaces, one liner maker's manual relates the minimum liner area to the size of the fireplace opening. Building codes also address flue sizing.

What liner damage looks like

Common findings on a clay tile liner include vertical or horizontal cracks, missing mortar at the joints, offset tiles, pieces of tile at the bottom of the flue, and heavy flaking. Stainless liners can show corrosion, crushed sections or a loose top connection. Cast-in-place liners can show cracking or gaps.

Signs you might notice from inside the house, such as tile shards on the smoke shelf or a persistent odor, are clues. They point to a closer look. They do not tell you which repair is needed. See What cracked flue tiles mean.

One chimney, several flues

Many New Jersey homes have a chimney with two or three tiles sticking out of the top. That usually means two or three flues sharing one masonry structure: one for the fireplace, one for the furnace or boiler, sometimes one for a second fireplace or a water heater.

Each of those flues has its own liner and its own condition. A clean, intact fireplace flue says nothing about the furnace flue next to it. Masonry guidance calls for a full layer of brick (a wythe) between adjacent flues in many configurations, so that a problem in one does not spill into the other. When liners crack or joints open, exhaust can leak between neighboring flues, which is one reason inspections look at every flue in the chimney, not only the one you use most.

If an appliance is removed or replaced, for example when an oil boiler is swapped for a high-efficiency unit that vents out the side wall, the flue it used may be left unused or may still serve a water heater. That change is worth having evaluated.

Where to go from here

If you are not sure whether your chimney is lined, or what condition the liner is in, the first step is an inspection with a look up the full length of the flue. When the flue allows it, we use a chimney camera and include photos in the findings we send before any work is scheduled.

If the liner needs work, the options range from repairing or resurfacing the existing flue to installing a new liner. See chimney liners and flue repair.

Questions homeowners ask

Sources

1. CSIA: About chimney liners
2. Brick Industry Association, Technical Note 19B: Residential Chimneys, Design and Construction (archived PDF copy)
3. Olympia Chimney Supply: Forever Flex and HybridLiner installation manual (PDF)
4. CSIA: Homeowner FAQs

This guide is general information. It can't diagnose a specific chimney; an inspection can.