By Daniel Watkins, Noble Chimney Sweepers.
Most homeowners never think about the chimney flue their furnace vents into, because nothing about it is visible from inside the house. It is also the part of the system where a quiet, gradual failure has the most serious consequences, and where a high-efficiency furnace upgrade can create a problem that did not previously exist.
A liner is what keeps combustion gases inside the chimney
The flue liner is the interior surface of the chimney, usually clay tile in older masonry construction or a stainless steel pipe in newer and relined systems. Its job is threefold: contain combustion gases and route them outside, protect the surrounding masonry from heat and acidic condensation, and provide a correctly sized passage so the appliance drafts as designed.
When a liner cracks or its joints open, gases can enter the chimney structure and from there find their way into living space. Because furnace exhaust is invisible and largely odorless, there is no obvious signal that it is happening.
The high-efficiency upgrade problem
This one catches a lot of households and it is worth understanding. An older furnace sent hot exhaust up the chimney, and that heat kept the flue warm enough that moisture stayed as vapor and left the building. Replace it with a modern high-efficiency unit and most of that heat now goes into the house instead of up the flue, which is the entire point of the upgrade.
The consequence is that the chimney now runs cooler. Exhaust condenses on the flue walls instead of exiting as vapor, and that condensate is acidic. It attacks clay tile and mortar joints steadily. Meanwhile the chimney is frequently now oversized for whatever is still venting into it, typically a water heater left on its own, which weakens draft further and worsens the condensation.
Housing in Mokena includes plenty of homes where the furnace has been upgraded once or twice while the original masonry flue was never reassessed. That combination is the classic case for relining.
What tells you a liner is failing
Tile fragments in the cleanout
Pieces of clay in the base of the chimney are shed liner material. There is no other source for them.
Staining or dampness on the chimney breast
Discoloration on interior masonry or adjacent drywall points to moisture and condensate moving through a compromised liner.
A draft that has changed
An appliance that used to vent without complaint and now spills, backdrafts or triggers a safety shutdown may be venting into a flue that is no longer correctly sized or sealed.
White crusting around the chimney exterior
Efflorescence on the outside of the chimney tells you water is moving through the masonry, which frequently accompanies liner failure.
How relining works
Chimney relining installs a continuous stainless steel liner sized to the appliance it serves, insulated where required, running the full height of the chimney with no joints for gas to escape through. It restores containment, it corrects the sizing problem created by a modern furnace, and stainless resists the acidic condensate that destroyed the clay.
The sizing point deserves emphasis. Relining is not simply putting a pipe in a hole. A liner too large for the appliance drafts poorly and condenses; too small and it cannot carry the volume. Correct sizing comes from the appliance specifications rather than from what happens to fit.
The way to know whether you need this is a camera inspection of the flue. A chimney inspection with a video scan shows the liner condition directly rather than inferring it, and it is the difference between a documented recommendation and a guess. If you have replaced a furnace in the last decade without having the flue assessed, that scan is overdue.
We are certified, bonded and insured, fourteen years at this, and you get the price up front with everything published on the pricing page. Call (708) 432-5747, seven days a week, 7 AM to 7 PM.
Frequently asked questions
How do I know if my chimney needs relining?
A camera inspection is the reliable answer. Cracked or spalling tiles, open joints, a liner damaged by a previous chimney fire, or a flue mismatched to a newly installed appliance are the usual findings that lead to relining.
Why would a new furnace cause a chimney problem?
High-efficiency furnaces send far less heat up the flue, so the chimney runs cooler and exhaust condenses inside it. That acidic condensate deteriorates clay tile and mortar, and the flue is often oversized for what remains venting into it.
How long does a stainless liner last?
A quality stainless liner, correctly sized and installed, is a long-term component and is commonly sold with an extended or lifetime warranty. Longevity depends on matching the alloy to the fuel it serves.
Can a cracked clay tile be repaired instead of relined?
There are resurfacing systems for certain conditions, and they can be appropriate where damage is limited. Where cracking is widespread or joints have opened along the run, a full stainless liner is the more dependable outcome.
Does a water heater venting alone into an old chimney matter?
It often does. When a furnace is removed from a shared flue, the water heater alone may not generate enough heat to establish draft in a large masonry chimney, leading to condensation and poor venting.
How long does relining take?
Many residential relines are completed in a day. Chimneys with offsets, unusual height, or difficult access take longer, and we set expectations before the work is scheduled.
Do I need a permit?
Requirements vary by municipality, and some local jurisdictions require permitting or inspection for liner work. We handle what is required for the town the work is in.
Is relining something I can defer a year?
If an inspection has identified a breached liner on an appliance in active use, deferring means running a heating system whose exhaust may be entering the structure. That is the one category of chimney work we do not suggest postponing.
Published August 15, 2026