Clay, stainless, aluminum or cast-in-place: choosing a flue liner

What the code accepts as a liner

Under FBC Residential R1003.11, masonry chimneys must be lined, and the section lists what counts: clay flue lining meeting ASTM C315, listed chimney lining systems meeting UL 1777, listed factory-built chimney units made to go inside a masonry chimney, and other approved materials able to withstand corrosion, erosion, softening and cracking from flue gases and condensate. The lining has to suit the appliance connected, following that appliance's listing and instructions.

The liner is what separates hot, corrosive flue gases from the masonry and the framing around it. A cracked or missing liner therefore idles the fireplace until it is put right. The types of chimney liners below all do that one job in different ways.

Clay tile

Clay flue tile is the lining built into a masonry chimney as it is laid. Under R1003.12 the tiles are set in refractory mortar, starting 8 inches or more under the lowest inlet (for a fireplace, at the smoke chamber's top) and running up past the enclosing walls. Clay handles the heat of an open wood fire and stands up to the acids in wood smoke.

Its weak points are the joints and sudden heat. Tile expands as it warms, and a fast, hot fire or a chimney fire can crack it. The mortar joints between tiles erode, leaving gaps where heat and gases reach the masonry. Clay can also be a poor match for a gas appliance, whose cooler, wetter exhaust can condense in a large tile flue and attack the joints.

When clay tile cracks, the options are replacing tiles near the top, a listed resurfacing system, or relining with one of the types below. The inspection decides which of those fits your flue.

Stainless steel

Stainless steel liners are listed to UL 1777 and come rigid, in straight sections, or flexible, as a continuous corrugated tube that can pass offsets. The alloy is chosen for the fuel. Some grades are listed for wood, oil and gas, and others for narrower uses, so the listing on the liner, not the word stainless, decides what it can serve.

Stainless is the broadest relining option. It is cut to length, sized for the appliance, wrapped when its listing requires, and capped with a top plate. Its life is limited by corrosion from condensate, by damage from a chimney fire and by skipped upkeep, and liner warranties can depend on correct installation and yearly inspection under the maker's terms.

Aluminum

Aluminum liners are light, easy to feed down a flue, and listed under UL 1777 for certain gas appliances only. Where the listing matches the appliance, they do that one job well. They are not a wood-burning liner, and not a liner for any appliance outside their listing. High heat and the wrong exhaust degrade aluminum, so an aluminum liner found serving a fireplace or an unlisted appliance is a finding to fix before the next use.

Cast-in-place

A cast-in-place liner is a cement-based mix poured into the old flue around an inflatable or removable form, using a listed system. When the form comes out, it leaves a smooth, continuous flue with no joints, backed by the old masonry.

It adds some strength to a tired chimney and suits a flue whose shape rules out a round liner. It is also the most involved to install: the flue is prepared, the form is set, the mix is poured and then cured. Like the others, it has to be listed for the appliance it will serve.

Factory-built chimney units inside a masonry chimney

R1003.11 also accepts a factory-built chimney, listed for installation inside a masonry chimney, as the lining. That means a listed metal chimney system run down the old flue and installed to that system's instructions. This route appears when a new appliance comes with a specified venting system. The appliance's manual, not the old masonry, decides what goes in.

Clay or stainless for an open fireplace

For an open wood-burning fireplace with sound tile, the clay flue it was built with is doing its job, and nothing is gained by relining for its own sake. The comparison only comes up once the tile has failed.

At that point, stainless is the quicker route and can fit a rectangular flue when an oval liner is used, while cast-in-place suits a flue whose walls also need stiffening. Replacing the clay tile itself means taking down the masonry around it, which makes it part of a rebuild rather than a relining job.

Matching the liner to the appliance

The appliance chooses the liner, and the pairings run like this:

• Open wood-burning masonry fireplace: sound clay tile, a stainless liner sized to the fireplace opening, or a cast-in-place liner listed for the use.
• Wood insert in a masonry fireplace: a stainless liner sized to the insert's outlet, per the insert's instructions and the liner's listing.
• Gas fireplace insert: the liners named in the insert's manual, which can be a pair, exhaust in one and combustion air in the other.
• Other gas appliances: stainless or aluminum, only where the liner's listing covers that appliance.
• Any appliance: the flue size follows the appliance, not the size of the old flue.

What limits a liner's life

Every liner wears, and the causes are shared across the types. Heat cycles crack tile and fatigue metal. Condensate, which forms when flue gases cool below their dew point, carries acids that eat mortar joints and corrode metal. Water from above, through an open flue top or a cracked crown, adds to the damage, and a chimney fire can crack tile or warp metal in a single event.

Upkeep slows all of it. A cap, a sound crown, a flue sized to its appliance and a yearly check under NFPA 211 let a liner reach the service its material allows. A fair answer to how long a liner lasts depends on those conditions more than on a number.

Sizing is the quiet factor. A liner too large for its appliance lets exhaust cool and condense on the way up, and one too small chokes the draft. Both shorten a liner's service, and the size therefore comes from the appliance listing, not from the old flue.

How to tell which liner you have

Open the damper and look up the flue with a flashlight. Clay tile shows square or rectangular sections with joints between them. Stainless shows a round metal tube, ribbed or smooth. Cast-in-place shows a smooth, continuous surface with no joints.

Outside, a round pipe sealed to the crown by a flat top plate points to a metal liner. The top of the flue is best left to someone on the roof, where the liner's condition can be judged along with the crown and cap.