By Johnathan Mason, Ace Chimney Service.
Short answer: two heating appliances can sometimes share one chimney, but almost never one flue — and a gas furnace or water heater should never be vented into the same flue as a wood-burning fireplace. Older homes around McHenry County were often piped that way before anyone thought twice about it, and it is one of the more common problems we flag on inspections after 29 years of climbing these roofs.
A chimney and a flue are not the same thing
A chimney is the masonry structure; a flue is the individual passage inside it.
One brick stack can hold two or three separate clay flues running side by side, each serving its own appliance, and that arrangement is perfectly normal. The trouble starts when one flue has two things connected to it — a fireplace and a furnace, or a furnace and a water heater on a shared thimble. From the outside those two situations look identical. You cannot tell them apart from the driveway, which is exactly why so many homeowners in Oakwood Hills are surprised by what a camera finds.
Why gas and wood cannot share
They produce different exhaust at different temperatures, and mixing them damages the flue and puts fumes where they do not belong.
Wood exhaust is hot and dirty; it deposits creosote. Gas exhaust is cooler and carries a great deal of water vapor, which condenses on flue walls and attacks mortar and clay from the inside. Run both up one passage and each makes the other worse: creosote deposits collect on a surface that is now chronically damp, and the acidic condensate eats joints that a wood fire alone would have kept dry. Beyond the material damage, a fire in the fireplace can pressurize the shared flue and push combustion gases back down the furnace connector into the house. Codes have prohibited the combination for decades, but the plumbing in a house that was remodeled piecemeal does not always reflect that.
What we actually find in older houses
The usual version is a furnace connector cut into the fireplace flue during a heating upgrade.
Somebody replaced an old gravity furnace, needed somewhere to vent it, and the fireplace flue was right there. The second version is two gas appliances sized wrong for one flue — a high-efficiency furnace vented out a sidewall leaves an oversized flue serving only the water heater, and that flue is now far too large to warm up and draft properly. Cold, oversized flues condense. Condensing flues deteriorate. It is a slow failure that shows up as a crumbling liner or a rusted-through connector years later.
How the problem gets found
A camera run of the flue plus a look at every appliance connector in the basement settles it in one visit.
We are looking for extra thimbles, abandoned openings that were never sealed, connectors that pitch downward instead of up, and clay joints that have washed out. A standard chimney inspection covers this, and it is worth doing before you buy a house or after any heating equipment change. If the flue turns out to be shared or oversized, the fix is usually a properly sized chimney liner for one appliance and a separate vent path for the other — not a patch. Our full chimney services page lays out what each of those involves, and pricing is published up front so nothing is a surprise on the day.
What to check yourself before you call
You can rule a few things in or out from the basement without any tools.
Follow the metal pipe from your furnace and your water heater and see where each one enters the masonry. If both disappear into the same opening, or if one enters anywhere near the fireplace flue, that is worth a professional look. Check that connectors rise toward the chimney rather than sag, and that no abandoned pipe hole sits open in the brick. Simple seasonal checks like these are collected on our DIY chimney care page.
Ace Chimney Service is insured and bonded, gives you the price before the work starts, and answers seven days a week from 7 AM to 7 PM at (815) 320-0883. We serve Crystal Lake and McHenry County.
Frequently asked questions
Can two fireplaces share one chimney?
They can share the masonry structure, but each fireplace needs its own flue inside it. Two fireboxes connected to a single flue will interfere with each other’s draft and can push smoke from one room into another.
Can a furnace and a water heater share a flue?
Sometimes, if both are gas, both are the same category of appliance, and the flue is correctly sized for the combined input. That sizing calculation is the whole question, and an oversized flue is as much a problem as an undersized one.
How do I know if my chimney has more than one flue?
Count the clay tiles poking above the crown, if there is more than one. But a stack can also be built with hidden divisions, so counting from the ground is a starting point rather than an answer. A camera run confirms it.
My furnace vents out the side wall now. Does the old flue still matter?
Yes, especially if the water heater still uses it. A flue sized for a furnace plus a water heater is far too large for the water heater alone, and it will run cold and condense. That is one of the most common relining calls we get.
What does a shared-flue problem look like from inside the house?
Often nothing at all, which is the risk. Occasionally you get a faint exhaust smell near the furnace when the fireplace is burning, or rust and staining on the connector pipe where condensate has been running back.
Is this something a home inspection would catch?
A general home inspection may note it, but flues are not usually scoped from the inside during one. If a report mentions the chimney at all, it is worth following up with a dedicated chimney inspection before closing.
Does relining fix a shared flue?
Relining gives one appliance a correctly sized, sealed path of its own. The other appliance still needs somewhere legitimate to vent, which may mean a second liner in another flue or a different vent route entirely.
How often should a shared or converted setup be checked?
Once a year, like any working chimney, and again any time heating equipment is replaced. Equipment changes are what create these problems in the first place.
Published October 17, 2026.