What Happens If a Chimney Flue Tile Is Cracked?

How a clay tile liner is built

Most chimneys built from the early twentieth century on were lined with clay flue tile: square, rectangular or round sections roughly two feet long, stacked one on top of the next inside the brick. The joints between sections were set with mortar, and ideally a small air space was left between the tile and the surrounding brick so each could expand without pushing on the other.

The tile is the part that faces the fire. It holds smoke and hot gases, resists the acids in exhaust, and gives the flue a smooth, sized path so the draft runs steadily. The brick around it is there to support and enclose. When a tile cracks, that one section stops protecting the brick and anything that touches the outside of the chimney, such as joists, studs and the lath behind plaster.

What usually cracks a tile

More than one cause is often at work in the same flue. Ranked roughly by how often each one turns up in pre-war homes on both sides of the Hudson and along the Delaware:

• A chimney fire, often a quiet one nobody noticed, which heats the inside of the tile far faster than the outside
• Water: rain down an uncapped flue or through a cracked crown, which soaks the tile and then freezes on cold nights
• Acidic condensation from oil and gas boilers or water heaters, which softens joints and flakes the tile face
• Thermal shock from a hot fire in a cold flue, which tends to leave vertical cracks
• Settling of the house or the chimney, which shifts tiles out of line at the joints
• Original workmanship: tiles packed tight to the brick with no room to expand, or joints left unfilled

The risk on a wood-burning fireplace

In a fireplace flue the main concern is fire. Creosote collects inside cracks and open joints, where a brush passes over it rather than into it. If that creosote ignites, flames and very hot gases can reach through the gap to the brick and, in many older houses, to wood framing that was built close to the chimney.

Even without a chimney fire, repeated heat passing through a crack can slowly char nearby wood over many seasons. That kind of damage gives little warning. Under NFPA 211, any chimney fire, or other event that could have harmed the chimney, is grounds for an inspection, and cracked tiles are one of the main things that inspection looks for.

The risk on a heating appliance flue

On a heating flue the bigger worry is exhaust. Oil and gas appliances run for hours a day all winter, and their exhaust contains carbon monoxide and a lot of water vapor. A crack lets that exhaust into the masonry and from there into the walls, the cellar or rooms on upper floors, especially in rowhouses where the flue runs up through the middle of the building.

Broken pieces of tile also fall to the base of the flue and pile up. Enough debris narrows the opening, slows the draft and can push exhaust back out at the appliance. Make sure the CO alarms in the house work. An alarm means leaving the house at once and calling 911 or the utility from the sidewalk.

Types of damage and how each is usually handled

Not every crack means a full relining. Location, size and the appliance on the flue all change the answer.

Signs you can look for without climbing

Most cracks are invisible from a room. A few clues show up in places you can reach safely.

• Curved, flat pieces of reddish or buff clay in the firebox, on the smoke shelf or in the cleanout
• A smoke or exhaust smell in rooms next to the chimney while the fireplace or heat runs
• Stains, damp patches or peeling paint on the chimney breast or nearby walls
• A cracked or missing top tile visible from the ground with binoculars
• Draft problems that start suddenly, or a boiler that soots up or trips more than usual
• Any recent chimney fire, oil burner puff back or lightning strike near the chimney

Repair or reline: the decision points

A single cracked tile at the top, where it sits above the crown, can often be replaced on its own. A few hairline cracks or eroded joints in an otherwise sound flue may suit a heat-resistant resurfacing coating applied inside the tile. Once cracks run through several sections, tiles have shifted, or the face is flaking from acid, patching becomes a guess and relining is the dependable answer.

The appliance matters too. If a new boiler or water heater is going in, its venting instructions may call for a different flue size anyway, which usually settles the question in favor of a properly sized liner. For a wood fireplace that will see regular fires, the choice is often a stainless steel liner or a poured cast-in-place liner, depending on the flue's shape and condition.

Keeping a sound liner from cracking

Most of what cracks clay tile can be slowed down. A cap on every working flue keeps rain from soaking the tile from above, and a crown that sheds water away from the flue keeps it from running down between the tile and the brick. Those two parts do more for a clay liner in a freeze-thaw climate than anything inside the house.

On a wood fireplace, burning split, seasoned wood in fires that burn bright rather than smolder keeps creosote thin, which removes the fuel for the chimney fires that crack tile from the inside. Bringing a cold flue up to temperature with a modest kindling fire, rather than a large load all at once, reduces the thermal shock a cold tile takes.

On a heating flue, the risk shifts to condensation. When an old boiler is swapped for a smaller or more efficient one, the exhaust runs cooler and the existing flue may now be too large for it, so water forms on the tile walls. Having the flue size checked against the new appliance's instructions at the time of the swap prevents a slow, hidden kind of damage.

What to expect from the inspection

The flue is examined over its full height, from the appliance or firebox to the top, because the damage that matters is often halfway up where no one can see it from a room. The technician notes which sections are cracked, how wide the gaps are, whether tiles have shifted, and how much debris sits at the base, then explains what that means for your fireplace or appliance.

You get the findings and the repair options in writing before any work, and until then the fireplace or appliance on that flue should stay off. If a heating appliance is the only source of heat in winter, say so when you call so the job can be planned around keeping the house warm.