Why Does My Chimney Have Two Flues?

One appliance, one flue

A flue is a single passage inside the chimney, running from one fireplace or appliance to the top. A chimney can hold several flues side by side, separated by thin brick walls, and from the street you see each one as its own clay tile or metal pipe sticking up through the crown.

The basic rule of good chimney design is that a wood-burning fireplace gets its own flue. Two fires sharing a flue can push smoke out of the other opening, because each fire's draft interferes with the other. A fireplace sharing a flue with a gas or oil boiler can upset the boiler's venting and send its exhaust into the house. Some gas appliances, such as a boiler and a water heater, can share one flue when the appliance instructions and venting tables allow it and the flue is sized for both.

Common two- and three-flue setups

Brownstones, rowhouses and older brick houses in Brooklyn, Jersey City and Philadelphia were often built with a fireplace in several rooms, one above the other. Every one of those fireplaces had its own flue, so a single stack on the roof can hold three, four or more. Later, when central heating went in, one of those flues was usually taken over by the boiler.

Colonials and center-hall houses often have one chimney with a living room fireplace flue and a separate boiler flue, and sometimes a third for a basement fireplace or a long-gone kitchen stove.

• Fireplace plus a boiler or water heater flue
• Parlor floor fireplace plus an upstairs fireplace
• A fireplace, a boiler and a second fireplace above
• One working flue and one or more old flues no longer in use
• A basement fireplace or stove flue beside the boiler flue

What a double flue chimney is

A double flue chimney is one masonry stack with two separate passages inside. The brick wall between them is called a withe or wythe. When it is sound, smoke and gases stay in their own flue. When the mortar in that wall wears away, or the flue tiles crack, smoke can cross from one flue into the other.

That matters most when one of the two flues serves a boiler. Exhaust from the boiler getting into a fireplace flue can come out in a room upstairs, especially when the fireplace damper is open or missing. Keep CO alarms on every floor, and if one sounds or someone feels unwell, go outside and phone 911 or your gas utility.

Telling the flues apart

On the roof, flue tiles are often set at different heights so the smoke from one does not get drawn down the one next to it. From the ground, count the openings and see whether each has its own cap, or whether one wide cap covers them all. A metal pipe sticking out of one tile usually means that flue has been relined, most often for a boiler or water heater.

Inside, the boiler's connector pipe enters the chimney through a hole in the brick called a thimble, usually in the basement. Fireplaces each connect at their own throat. A chimney inspection maps which flue goes where, which is useful before any repair or new appliance.

• Wood fireplace flue: clay tile or a liner sized to the opening, with creosote and heat as the main concerns
• Oil boiler flue: clay tile or stainless sized to the boiler, where soot, acidic scale and base cleaning matter
• Gas boiler or water heater flue: a stainless or listed liner sized to the appliance, to control condensation and keep the draft
• Unused fireplace flue: left in place, but capped, ventilated and kept from taking on water

Problems that come from shared stacks

Because the flues share masonry, problems in one flue affect the others. Condensation from a gas boiler flue can soak the wythe and the outer brick around a fireplace flue. A chimney fire in one flue can crack tiles in the next. And a cap on one flue but not the others lets rain into the stack anyway.

Downdraft between flues is another common complaint: smoke or exhaust leaving one flue top drifts over and is pulled down an adjacent open flue, often a fireplace flue with no cap or damper. Raising one flue above the other, or a cap design that separates the outlets, usually cures it.

Liners for each flue

Each flue is lined to suit what it vents. A wood fireplace needs a liner that handles high heat and is big enough for the fireplace opening. A gas boiler's flue needs one sized to its exhaust so moisture does not condense and soak the brick. When a boiler is replaced or switched from oil to gas, its flue often needs a new liner to match, while the fireplace flue next to it stays as it is.

Relining one flue also restores separation where the wythe has deteriorated, because the new liner gives that appliance its own sealed path.

Flues that are no longer used

Flues that are no longer used should still be capped at the top to keep out rain and animals, while letting a little air move so moisture does not build up inside. Their openings inside the house, such as an old thimble in the basement or a fireplace throat, are closed properly. Before closing any flue, confirm that nothing, including a neighbor's appliance in a shared stack, still vents into it.

An unused flue can also be put to work later, for example as the path for a liner serving a new appliance, so keeping it capped and dry preserves that option.

What an inspection of a multi-flue chimney covers

Each flue is checked on its own: what it serves, whether it is lined, the condition of the liner, deposits, and the cap. The wythes are checked where visible for cracks, and smoke from one flue may be watched to see whether it escapes into another. The result is a simple map of the stack, flue by flue, with any repair tied to the flue it affects.

It helps to bring what you know: which appliances run on gas or oil, when each was installed, and which fireplaces have been closed. Older houses often have flues nobody has looked at in decades, and finding out that a closed fireplace upstairs shares a wythe with the boiler flue is exactly the kind of detail that changes the repair plan.