By James Rodriguez, Central Chimney Service.
Most people think of a chimney as one thing. On a large share of the older homes we work in it is actually two or three separate vertical passages stacked inside a single brick structure, each serving a different appliance, each with its own condition and its own failure modes. Understanding that is what explains why a chimney can pass on the fireplace side and still be a problem on the heating side.
One chimney, multiple flues
A flue is the passage the exhaust travels through. A chimney is the structure that contains it, and one structure commonly holds more than one flue.
Look at the top of the chimney from the ground: if you see two or three separate clay tiles standing proud of the crown, you have that many flues. Typically one serves the fireplace, and a second, usually smaller, serves the furnace and often the water heater as well. They run side by side, share the same masonry shell, and never connect — which is the whole point, since combining them is a code violation and a serious safety problem.
Why the heating flue is a different animal
The two flues fail differently because they carry different exhaust. A wood fireplace produces hot, dry, particle-laden smoke, and its characteristic problem is creosote. A gas furnace produces cooler, water-vapor-heavy exhaust, and its characteristic problem is condensation.
That water vapor is corrosive. When it condenses inside a masonry flue it carries acidic compounds that attack mortar joints and clay tile from the inside, and over years it deteriorates the liner while the exterior brick still looks perfectly sound. The damage is invisible from outside and invisible from inside the house.
The furnace replacement trap
The most common defect we find on the heating side is a flue sized for an appliance that was removed decades ago. An old, inefficient furnace sent a large volume of very hot exhaust up the chimney, and the flue was sized for exactly that.
Replace it with a modern higher-efficiency unit and the exhaust becomes much cooler and much smaller in volume. In the same oversized flue, it cools too fast, never establishes proper draft, and condenses on the way up. The chimney did not change; the appliance did, and now they are mismatched. In the older housing across Joliet chimney sweep calls, this pairing shows up regularly, usually because nobody evaluated the chimney at the time the furnace was swapped.
The standard fix is a correctly sized liner installed inside the existing flue, which restores proper draft and isolates the masonry from the exhaust. Our chimney relining page covers how that is done and when it applies.
Signs the heating flue needs looking at
White chalky staining on the exterior brick, especially on the heating side, indicates moisture moving through the masonry. Crumbling mortar joints partway up, spalling brick concentrated on one face, and pieces of clay tile appearing at the cleanout are all liner-deterioration signals.
Indoors, look for rust on the furnace’s vent connector, staining on the chimney breast in the basement or utility area, and a damp smell near the cleanout. Any of these on a heating flue deserves prompt attention, because a compromised liner on a gas appliance is a carbon monoxide question rather than a masonry question.
What this means for the annual inspection
Every flue in the structure gets inspected, not just the one attached to the fireplace. That sounds obvious and it is routinely missed — homeowners book a fireplace sweep, get a clean fireplace flue, and assume the chimney is fine while the heating flue has never been looked at.
Both flues are checked as part of our chimney inspection, and the shared components — crown, cap, flashing, exterior masonry — are checked once for the whole structure. If the masonry needs work, the chimney repair page covers what that involves. You get the price up front rather than after. Insured and bonded, 29 years, 7 days a week from 7 AM to 7 PM at (815) 305-5510.
Frequently asked questions about multiple chimney flues
How do I know how many flues my chimney has?
Look at the top from the ground or with binoculars. Each flue normally terminates in its own clay tile projecting above the crown, so counting the tiles counts the flues. Each should also have its own cap.
Can a furnace and a fireplace share one flue?
No. Separate appliances require separate flues, and connecting them creates cross-venting problems where one appliance’s exhaust is pulled into the other’s path. Finding a shared flue during an inspection is a serious defect requiring correction.
Why would a new furnace cause a chimney problem?
Because efficiency changed the exhaust. A modern unit vents cooler, lower-volume exhaust than the old one, and in a flue sized for the old appliance it cools too quickly, drafts poorly, and condenses. A correctly sized liner brings the flue back into match with the appliance.
Does the heating flue need sweeping like the fireplace flue?
It does not build creosote, but it does need inspection and sometimes cleaning — debris, nesting material, and spalled tile fragments accumulate and can restrict or block it. The concern is blockage and liner condition rather than deposits.
What does white staining on the outside of the chimney mean?
It is generally efflorescence, mineral salts carried to the surface as water moves through the masonry and evaporates. It confirms water is passing through the brick, and on a heating flue that often traces back to condensation inside a deteriorating liner.
My water heater vents into the chimney too. Is that a problem?
It is common and can be correct, since a water heater and furnace are sometimes permitted to share a properly sized flue. Whether yours is sized right depends on both appliances and the flue dimensions, which is exactly what the inspection determines — and it frequently changes when one appliance is replaced.
Can I see liner damage myself?
Essentially never. Clay tile deterioration happens inside the flue, out of sight from both the basement and the roof, and is found with a camera run. Pieces of tile turning up at the cleanout are the one visible clue, and by then the damage is well advanced.
Is a deteriorated heating flue urgent?
Treat it as urgent. A compromised liner on a gas appliance can allow combustion gases, including carbon monoxide, into the building rather than out the top. It should be assessed before the heating season and it is not a wait-until-spring item.
Published September 3, 2026 · Last updated August 27, 2026