Is a Stainless Steel Chimney Liner Worth It?

What a stainless liner is

A stainless steel liner is a metal pipe that runs inside the chimney from the appliance or fireplace to the top. At the bottom it connects to the appliance, either directly or through a tee with a cleanout cap. At the top it is fixed with a plate that seals the old flue around it, and a cap goes on. The old masonry flue stays in place and becomes a protective chase around the new liner.

Liners come in different alloys and wall thicknesses, and the liner maker lists which fuels and appliances each one is made for. That listing matters more than any sales claim: a grade made for gas may not be suited to wood or oil, and some fuels, such as oil and coal, are hard on lighter alloys because of the acid in their exhaust.

Why chimneys need relining

Most chimneys in older homes in the five boroughs and the inner suburbs were built with clay tile liners, or with no liner at all in the oldest houses. Clay works well while it is intact and correctly sized, but tiles crack from chimney fires and sudden heat, the mortar joints between them erode from acidic condensation, and many flues are far too big for the boilers and water heaters now venting into them.

A damaged or oversized flue causes trouble in two directions. Heat and combustion gases can reach the brick and the framing through cracks, and an oversized flue keeps exhaust cool, which weakens draft and floods the masonry with condensation. Relining with stainless addresses both at once.

When it is worth it

Stainless relining earns its keep in situations where the masonry flue cannot do the job safely on its own. These are the cases we see most, roughly in order of how often they come up.

• A new boiler, furnace or water heater needs a smaller flue than the old one
• Clay tiles are cracked, offset or missing, often after a chimney fire
• Condensation is damaging the masonry from the inside
• A wood stove or insert is being installed into a fireplace flue
• The chimney has no liner at all
• The flue bends or offsets, so rigid tile cannot be replaced without opening walls

When it is not

If the existing clay liner is intact, its joints are sound and it is the right size for the appliance, relining adds cost without adding much. The same is true when the appliance is moving to a sidewall vent and the chimney will serve nothing; then the flue only needs a cap and a seal. A good inspection says so plainly rather than recommending a liner by default.

There are also chimneys where a liner alone is not enough: masonry that is leaning, badly cracked or falling apart above the roof needs structural repair first, and a liner installed into it would only be as good as the chimney around it.

Flexible or rigid

Flexible stainless liners can follow bends in a flue, which is common in older brick houses where the chimney jogs to clear a roof line, a party wall or a neighboring flue. Rigid liners come in straight sections and suit straight flues. Rigid pipe has a smoother inside and is easier to sweep, while flexible liner is easier to install in a crooked chimney.

The right choice depends on the shape of the flue and the appliance. A technician decides by looking at the chimney from top to bottom and measuring it, not from a catalog.

Sizing the liner

Size is where relining succeeds or fails. For a gas or oil appliance, the diameter comes from the appliance maker's instructions and the venting tables in the fuel gas code, based on the appliance input, the chimney height and any offsets. For a wood stove or insert, the liner matches the appliance outlet as its manual requires. For an open fireplace, the flue has to be large enough for the fireplace opening or the fire will smoke into the room.

Bigger is not safer. An oversized liner recreates the cold, slow flue problem it was meant to fix. Smaller is not cheaper in the long run either, since an undersized liner can spill exhaust or smoke.

For a wood-burning fireplace or stove

Lining a wood-burning fireplace or stove takes more care than lining a gas flue. Wood fires run hot and make creosote, so the liner has to be a grade listed for wood, and many liner makers call for insulation around it. Insulation keeps the flue warmer, which improves the draft and cuts creosote, and it may be required to maintain safe clearance from framing where the chimney sits close to wood.

Open fireplaces are the hardest case, because a round liner small enough to fit the old flue may be too small for the opening. Sometimes the answer is an oval liner, a smaller opening with a smoke guard, or a cast-in-place liner instead.

What to expect from the work

The flue is cleaned first. Old tile may need to be broken out so the liner and its insulation fit, especially on wood installations. The liner goes in from the roof, is connected at the bottom and sealed at the top, and a cap goes on. The thimble where the connector enters the chimney is made tight, and the appliance is tested for draft once it is running.

Most relines take a day on a straightforward chimney, longer when tile has to be removed or the flue has several offsets. The appliance is off while the work is done, so a boiler reline is best scheduled on a mild day or outside the heating season. Ask whether the job includes the connector pipe and any repairs needed at the top, such as a cracked crown, so the new liner is not sealed into a chimney that still leaks.

Expect the alloy, diameter, insulation and scope to be written into the estimate before work starts. The finished liner still needs a yearly inspection and cleaning when deposits build up, like any flue.