By Daniel Watkins, Noble Chimney Sweepers.
Look at the top of many chimneys in Shorewood and you will see more than one flue tile sticking out of the crown. One might serve the fireplace, another the furnace or water heater, and sometimes a third serves a basement fireplace that nobody has used in years. Sharing one masonry structure is normal and it works well when it is built right. When it is not, smoke and exhaust from one flue can find their way into another, and the symptoms show up in rooms that seem to have nothing to do with the problem.
In 14 years of sweeping chimneys around Will County, multi-flue chimneys produce some of the most confusing service calls I get, so here is how they work and what goes wrong.
What a multi-flue chimney is
A multi-flue chimney is one masonry stack that contains two or more separate flues, each venting a different appliance. Each flue should be its own sealed passage from the appliance connection to the top, with no path into its neighbor.
Builders did this because one chimney is cheaper than two, and it keeps the roofline clean. In many homes built through the middle of the last century, the fireplace flue and the furnace flue sit side by side in the same stack, separated only by a partition of brick called a wythe.
How smoke crosses from one flue to another
Crossover happens through gaps in the partition between flues or at the top where exhaust from one flue drifts into the next. Inside the stack, mortar in the wythe can erode over decades of acidic exhaust and freeze-thaw cycles. Once there are open joints, a flue under negative pressure will pull air from the flue beside it.
At the top, flues that end at the same height are the other common path. When the fireplace is not running, its flue is cold and often pulls air downward. Warm furnace exhaust leaving the flue next door can be drawn right back down the fireplace side. That is why the long-standing practice is to stagger adjacent flue tops, typically by a few inches of height, or to separate them with a wythe that extends above the crown.
Symptoms that point to crossover
The classic sign is a smoky or exhaust smell at the fireplace on cold days when the fireplace has not been used, while the furnace is running. Others include soot or black streaking on the fireplace damper or firebox in summer, a water heater that spills at its draft hood, or a carbon monoxide alarm that sounds on the level where an unused fireplace sits. Any of these is a reason to stop and call, because furnace and water heater exhaust carry carbon monoxide.
What we check on a shared chimney
On a chimney inspection we identify every flue and trace which appliance it serves. Then we look at the condition of the partitions from inside with a camera, the flue top heights and crown from the roof, and whether each appliance is venting into the right flue at the right size. The standard we work to is NFPA 211, which calls for an annual inspection and a Level 2 inspection whenever the fuel, the appliance, or the flue lining changes. Replacing a furnace in a shared chimney counts, and it is one of the most common times crossover problems start.
Fixes range from raising one flue tile or adding a separating cap, to repointing a failed wythe, to relining the flue with a stainless liner so it becomes a sealed passage again. We explain the options and quote the price up front before any work starts. Our pricing page lists published rates.
We serve Shorewood and the surrounding towns, and we are certified, bonded and insured, owner-operated. Call (708) 432-5747, 7 days a week, 7 AM to 7 PM.
Frequently asked questions
How do I know if my chimney has more than one flue?
From the ground or with binoculars, count the flue tiles or pipes that stick up out of the top of the chimney. Each opening is usually a separate flue. An inspection confirms which appliance each one serves.
Can two appliances share the same flue?
A wood-burning fireplace or stove should always have its own dedicated flue. Some gas appliances can share a properly sized flue under specific conditions, but that has to be designed and verified, never assumed.
Why do I smell smoke at the fireplace when it is not in use?
On a shared chimney, a common cause is exhaust from the furnace or water heater flue crossing into the cold fireplace flue at the top or through gaps inside the stack. Weather-driven downdraft and stale creosote odor are other possible causes.
Is exhaust crossover dangerous?
It can be. Furnace and water heater exhaust contain carbon monoxide, so any sign of it entering the living space should be checked promptly. Working carbon monoxide alarms on every level are essential.
What does staggering flue heights accomplish?
When adjacent flue tops end at different heights, exhaust leaving one flue is less likely to be drawn straight down into the neighboring flue. It is a simple, long-standing way to reduce crossover at the top.
Does a new furnace affect my chimney?
Often, yes. A newer furnace may produce cooler, wetter exhaust or vent differently, which can leave the old flue oversized or orphan a water heater. A Level 2 inspection is called for when the appliance changes.
Can a relined flue fix crossover?
Yes, in most cases. A properly sized stainless liner creates a sealed passage from the appliance to the top, so gaps in the old masonry partition no longer matter for that flue.
Will one chimney cap cover all the flues?
Multi-flue caps exist and can cover the whole crown, but they need to be the right height and style for each appliance. A cap that is too low can make crossover worse, so the choice should follow an inspection.
Published October 15, 2026