When the code requires a chimney liner, and when yours needs a new one

What section R1003.11 says

Is a chimney liner required? For a masonry chimney, the Florida Building Code, Residential, 8th edition (2023), is direct about it: masonry chimneys shall be lined, and the lining has to be appropriate for the type of appliance connected, following the appliance listing and the maker's instructions (R1003.11). The code then lists what counts as a lining for ordinary residential appliances (R1003.11.1), as follows.

• Clay flue lining meeting ASTM C315.
• Listed and labeled chimney lining systems meeting UL 1777.
• Factory-built chimneys or chimney units listed for installation inside masonry chimneys.
• Other approved materials that resist corrosion, erosion, softening or cracking from flue gases and condensate at temperatures up to 1,800 degrees Fahrenheit.

Why brick alone is not enough

Bare masonry is poor at three jobs a flue has to do. They pass heat to whatever touches the outside of the chimney, including framing that may sit only inches away. Their rough, oversized passages slow smoke down and let it cool. And the acids that condense out of flue gases eat into mortar joints over time.

The code already requires a 2-inch airspace between a masonry chimney inside the house and any wood framing (FBC R1003.18). That clearance is written for a chimney that is also lined (R1003.11). Without the lining, flue heat goes straight into the brick, and the brick passes it on toward that framing.

A liner separates the smoke from the masonry. It holds heat inside the flue, gives the gases a smooth passage of the right size, and takes the chemical wear in place of the mortar. That is the reason an unlined masonry chimney is treated as a defect and not as an old way of building.

When a liner is present but failing

A clay tile liner can be there and still fail. Tiles crack in a chimney fire, when they heat unevenly, or when the chimney settles. Mortar washes out of the joints and leaves gaps where heat and gases reach the brick behind. Tiles shift out of line and form ledges that catch creosote.

Small, local damage to clay tiles can sometimes be repaired in place. Damage along much of the flue, or cracks whose end you cannot see, point toward relining. An inspection of the full flue is what separates the two, and NFPA 211 names relining as a reason for a deeper look before the work starts.

Appliance changes that call for a new liner

The code ties the lining to the appliance, so changing the appliance can make a sound liner the wrong one. For gas appliances the code points to its fuel gas chapter, Chapter 24 (R1003.11.3), so a flue that serves gas is sized and lined by the gas rules and the appliance's instructions, not the fireplace rules.

NFPA 211 treats changes like the ones below as a change in how the chimney is used, and it calls for a fuller evaluation when appliances are added, removed or replaced with a different type or rating.

• A wood stove or insert set into a fireplace flue: the flue was sized for an open fire, and the appliance needs a liner sized to its outlet collar.
• Venting a gas appliance into a masonry chimney: the lining must suit gas, and its size comes from NFPA 54 and the appliance manual.
• A switch between fuels: a liner listed for one fuel may not be listed for another, and aluminum liners are listed only for certain gas appliances.
• An appliance added to, or removed from, a shared flue: the one left behind may now vent into a passage too large for it.

A masonry chimney with no liner at all

Some masonry chimneys were built with no liner. A chimney without a liner can look fine from the room while its mortar joints erode from the inside and heat reaches the framing around it. It needs a liner. In a standing chimney we use a UL 1777 listed stainless liner, insulated where its listing requires, since setting clay tile would mean taking the stack apart.

Liner makers want a structurally sound chimney first, and they require the flue and smoke chamber to be cleaned before installation. Where clearances to framing cannot be confirmed, their instructions call for an approved insulation around the liner.

Prefab fireplaces: the metal pipe is the flue

A factory-built fireplace comes with a matching metal chimney. The fireplace is tested to UL 127 and the chimney to UL 103, and the code requires both to be installed to their listing and the maker's instructions (FBC R1004.1 and R1005.1). The inner wall of that metal chimney is the flue, so there is no separate liner to add, and damaged sections are replaced with the maker's listed parts.

So the question of a liner belongs to masonry chimneys. For a factory-built unit, the question is whether the chimney sections are intact and correct for the model, which the listing label and the manual answer.

What relining involves, in outline

Relining starts with a sweep and a full look at the flue, because the old passage has to be clean and the masonry around it sound. The liner is then sized from the appliance or the fireplace opening, chosen for the fuel, wrapped or packed with insulation where its listing asks, lowered from the top, and connected at the bottom to the stove, insert or smoke chamber.

At the top, a plate carries the weight and a cap sheds rain and bars animals. Whether a permit is needed is the building department's call, and the work order should say who files it.

Which case is yours?

Seen from the yard, a clay-tiled flue shows the thick edge of the tile standing above the crown, while a relined flue shows a thin steel liner and a top plate. From below, with a flashlight above the open damper, you may see tile, steel or bare brick. A factory-built fireplace has a metal firebox or refractory panels, and a listing label on the unit where the manual says to find it.

A yearly inspection answers the rest: whether the liner is continuous, what it is made of, and whether its size suits the appliance. If it is wrong, the written estimate says why and lays out the options.