Replacing an old furnace with a high-efficiency unit leaves the original flue substantially oversized. The new exhaust is cooler, wetter and lower in volume, so instead of rising it loses heat, slows and condenses on the flue walls. That condensate is acidic and works on mortar from the inside for years before anything shows outside — which is why the damage is usually found long after the installation.
What changed
An old furnace sent a large volume of very hot exhaust up the flue. That heat kept the gases moving fast and kept the flue walls above the temperature at which moisture condenses.
A high-efficiency unit extracts far more heat into your house and sends much less up the chimney. The exhaust is cooler, wetter, and lower in volume. Put that into a flue sized for the old appliance and it behaves completely differently: it loses heat quickly, slows down, and condenses on the flue walls instead of rising and exiting.
Why the condensate is the problem
The moisture that condenses out of gas combustion products is acidic. It sits against mortar joints and masonry, cycle after cycle, every heating season. Mortar softens and erodes, clay tiles deteriorate, and the damage works from the inside outward — which is why it is usually years before anything shows on the outside of the chimney or on a wall inside the house.
By the time you see white efflorescence staining on the brick, damp on the chimney breast, or crumbling joints near the top, the process has been running for a long time.
Why West Chicago sees a lot of this
The older housing near the centre has been through the full sequence — built for coal, converted to oil, converted again to gas — and those chimneys were substantial to begin with because coal needed them to be. Each conversion left the flue more oversized than the last, and very few were ever relined to match.
If your house is from that era and the furnace has been replaced at any point in the last twenty years, the odds that the flue was correctly resized at the same time are not good.
The fix
A correctly sized stainless liner, sized from the appliance output rating, the chimney height and the manufacturer specification — not from what happens to fit. The masonry stays put. Where the appliance is a modern condensing furnace, the correct venting may not be the masonry chimney at all, and we will say so rather than sell you a liner you do not need.
The first step is a camera inspection to see the current condition.
Serving West Chicago and the Fox Valley. See what we do in West Chicago, or look at our published starting prices.
Furnace replaced, chimney untouched?
Worth a camera inspection before another heating season.
Related questions
How do I know if my flue is oversized?
It is calculated from the appliance output rating, the chimney height and the manufacturer specification. That calculation is part of the inspection.
What are the warning signs?
White chalky efflorescence on the outside brick, damp on the chimney breast, crumbling mortar near the top, and rust on the appliance connector or damper.
Should the heating installer have flagged this?
A thorough one would. It is a common gap because the flue is outside the scope of a straight appliance swap.
Is it dangerous or just damaging?
Mostly damaging, over years. It becomes a safety issue when deterioration opens joints that allow combustion gases into the structure.
Does this affect wood fireplaces too?
The same physics applies. An oversized flue on a wood appliance produces cooler gases and heavy creosote rather than acidic condensate.
Can this be fixed without rebuilding the chimney?
Almost always. A correctly sized stainless liner goes down the existing flue.
Written by Rick Lee, owner of Zenith Chimney Service, Geneva IL. Published September 21, 2026 · Last updated September 4, 2026.