Can a Chimney Liner Be Repaired?

Three questions that decide it

Whether a liner can be saved comes down to what it is made of, how much of it is damaged, and what burns below it. A single open joint halfway up an otherwise solid clay flue is a repair. Cracks running through many tiles, a liner that is the wrong size for the appliance, or metal that has corroded through usually are not.

The decision needs the full height of the flue in view. A look up from the firebox or the cleanout shows a few feet at most, and the worst damage in older chimneys often sits higher up: near the roofline where freeze-thaw is harshest, or at an offset where the flue bends around a floor beam. A technician lowers lights and an inspection tool down the flue, or works from both ends, before saying repair or reline.

What breaks a liner in the first place

Knowing the cause matters as much as knowing the damage, because a repair that leaves the cause in place fails again. In the brick chimneys of the metro area, from Queens rowhouses to South Jersey colonials, the causes run roughly in this order.

• Water from a cracked crown or missing cap, then years of freeze-thaw opening the joints
• Acidic condensation from gas or oil boilers venting into a flue too big and too cold for them
• A chimney fire, which heats the tiles unevenly and splits them, often in vertical cracks
• Mortar joints that were never fully filled when the chimney was built
• A metal liner of the wrong alloy or grade for the fuel it serves

Damage and the usual verdict

Every flue is judged on its own, but these are the patterns a technician sees again and again, and where each one usually lands.

How a clay flue repair is done

Clay tile is the liner most often repaired, simply because so many older chimneys have it. The flue is swept first, and hard deposits are removed, since nothing bonds to soot or glazed creosote. For open joints, a heat-resistant sealing material is pushed into the gaps by a tool drawn up or down the flue, filling the joint without coating the whole tile. For wider surface wear and fine cracks, a resurfacing compound is applied in layers over the tile face.

These products are made for particular fuels and flue conditions, and the maker's instructions set how the flue is prepared, how thick the material goes on and how long it cures before the first fire. A top tile that has cracked above the crown is a separate repair: it is cut out from above, replaced, and sealed into a rebuilt or repaired crown so water cannot follow the same path.

Where a clay repair stops

A coating needs a sound surface to hold on to. It cannot bridge a gap where a tile has slipped out of line, and it cannot rebuild a piece that has fallen away. Replacing tiles deep inside a chimney means cutting into the chimney walls at that height. In a brownstone or rowhouse, where the chimney runs inside a party wall, that can mean opening finished plaster in your own rooms or a neighbour's, which is rarely practical.

When the damage is that widespread, the usual fix is a new liner run inside the old flue: a stainless steel liner sized to the appliance, or a cast-in-place lining poured around a form. Either one gives a continuous new flue without taking the chimney apart.

Metal liners: part, patch or replace

A flexible stainless liner is installed as one continuous piece, so a damaged flexible liner is replaced rather than patched; a patch inside a flue cannot be reached, sealed or checked properly. A rigid liner built from sections sometimes allows one section to be swapped, depending on where it sits and how it was joined.

Plenty of metal liner problems are not the liner at all. A loose joint at the bottom tee, a top plate that has lifted, or a cap that blew off in a nor'easter can all let water or exhaust go where it should not, and each is a small repair. Corrosion is different. Pitting or holes from the wrong alloy for the fuel mean the replacement has to be a different product, not the same one again.

Urgent now or fine until spring

Some damage stops the flue from being used today. Pieces of tile falling into the cleanout, a gap where a tile is missing, cracks found after a chimney fire, or any sign of exhaust spilling at a boiler or water heater all mean the appliance should stay off until the flue is fixed. A flue that leaks exhaust can let carbon monoxide into the house; CO alarms on every floor are the backstop, and if one sounds, everyone goes outside first and the call to 911 or the utility is made from there.

Other findings can be scheduled. Open joints near the top of a flue that serves only an occasional fireplace, or light surface wear on otherwise sound tile, can usually wait for a dry stretch of weather, provided the fireplace stays cold in the meantime and the cause, often a cap or crown, is sorted first. Coatings and crown work both go better above freezing, which is another reason not to leave the decision until January.

Cost drivers and fair comparisons

What a flue repair costs depends on how much of the flue is affected and at what height, the chimney's height and roof access, the method, and whatever caused the damage, such as a broken crown that also needs work. A top-of-flue repair and a full reline are very different jobs, so ask each contractor to answer the same questions in writing.

• Exactly what is damaged, and at what height in the flue?
• Is the flue the correct size for the appliance or fireplace it serves?
• What caused the damage, and is that being fixed too?
• Is the repair product made for this fuel and this kind of flue?
• What should the next yearly inspection find if the repair has held?