Chimneys with More Than One Flue

Start by matching flues to appliances

From the ground, count the clay tiles or metal liners sticking up above the crown. Then count everything in the house that connects to that chimney: fireplaces on each floor, a furnace or boiler, a water heater, perhaps a stove or insert. The numbers should line up, with each appliance using its own flue.

If they do not, there is a story to find out. A flue may be abandoned, perhaps after a furnace was replaced with a unit vented through the wall. Two appliances may share one flue by design, such as a furnace and water heater joined before the chimney. Or an appliance may have been connected to the wrong flue. Knowing which flue is which is the first thing a technician works out on a multi-flue chimney, and it is useful information to keep with your house records.

The table below is a simple record you can fill in after your next inspection.

What separates one flue from the next

Inside the chimney, each flue has its own liner, and between the liners there should be masonry. The Brick Industry Association's chimney design note says separate flues may share one chimney as long as minimum wall thickness is met and a full wythe of brick is laid between them and bonded to the chimney walls. A wythe is a single vertical layer of masonry, one brick thick.

Model code text has been similar. The 2003 International Residential Code, as reproduced by Buckley Rumford Fireplaces, required masonry wythes at least 4 inches thick between adjacent flue linings, bonded into the chimney walls. It allowed an exception for two flues serving one appliance, which could adjoin with only their liners between them, provided the liner joints were staggered at least 4 inches. New Jersey enforces a later code edition, so the local construction office has the current wording.

The point of that wall is containment. If a liner cracks, the wythe behind it still keeps hot gases and sparks from reaching the next flue, and keeps a chimney fire in one flue from involving the others.

Smoke crossover between flues

A common complaint in houses with two fireplaces on one chimney is smoke or odor coming out of the fireplace that is not in use. CSIA's homeowner FAQ describes the mechanism: the fireplace in use exhausts household air until the house is under negative pressure, and the unused flue becomes an easy path for outside air, carrying smoke from the active flue's outlet back down into the house. CSIA suggests providing makeup air, or installing a top-mounted damper on the least-used fireplace.

Crossover can also happen inside the chimney when the masonry between flues has deteriorated, letting exhaust leak from one flue into another. The two causes call for very different fixes, which is why the inside of each flue is examined before deciding.

Why the flue tops are sometimes at different heights

You may notice that the flue tiles on top of a chimney end at different heights. Builders sometimes finish adjacent flues this way with the aim of keeping exhaust from one flue drifting into the next. We did not find a primary source that sets a required height difference for residential chimneys, so treat it as one detail an inspector looks at alongside the cap design, the wind exposure and the condition of the flues, not as a rule your chimney has to meet.

Different heights do affect caps. A single cap covering the whole top has to clear the tallest tile with enough room for the others to vent, and tiles that are uneven or damaged may need to be trimmed or repaired before a full-width cap fits properly.

Capping several flues

There are two basic approaches. A trade article in Masonry Magazine, published by the Mason Contractors Association of America, describes single-flue caps that mount on a flue tile projecting above the crown, and crown-mounted caps used to cover several flues at once or when no tile projects. It notes that single-flue caps need enough space between them for their lids, and that codes call for clearance between the top of the chimney and the underside of the cap lid.

• Individual caps work when flues are spread far enough apart and each tile is sound enough to hold one.
• A full-width cap covers every flue and part of the crown. It must be sized so each flue has enough open area and must suit every appliance underneath, including gas or oil appliances whose exhaust is hard on some metals and screens.

For the differences between cap designs, see chimney cap types compared.

When one flue changes, look at the others

Work on one flue can affect its neighbors.

• Relining one flue for a new stove, insert or furnace is common. The other flues still need their own evaluation; a new liner in one does not seal cracks in the next.
• Removing an appliance leaves a flue unused. An open, unused flue next to an active one is a ready path for crossover and for rain. It may need to be capped, sealed at the top or relined if it will be reused.
• Upgrading a furnace often changes exhaust temperature. DuraVent's relining guide describes how an efficient gas furnace discharged into a large masonry flue can produce poor draft and condensation, and how a water heater left on a shared flue can be affected; our orphaned water heater guide explains that case.
• A chimney fire in one flue calls for checking the masonry around the adjacent flues as well.

How a multi-flue chimney is inspected

Each flue is a separate inspection job. In an assessment we identify which appliance uses which flue, look at the top of each from the roof when it is safe, and run a chimney camera down each flue the camera can pass through. We look at the liners, the joints and, where visible, the condition of the separation between flues, and you get the findings and photos flue by flue before any repair is scheduled.

If smoke crossover, odors or unexplained soot are part of the reason for the visit, tell us which fireplace or appliance was running at the time. That detail helps narrow down whether house pressure or the chimney itself is the more likely cause. See what a chimney camera inspection shows and chimney inspection.

Questions homeowners ask

Sources

1. Brick Industry Association, Technical Note 19B: Residential Chimneys, Design and Construction (archived PDF copy)
2. 2003 International Residential Code, Chapter 10, Chimneys and Fireplaces (text reproduced by Buckley Rumford Fireplaces)
3. CSIA: Homeowner Resources and FAQ
4. Masonry Magazine (MCAA): Installing the Correct Chimney Cap
5. DuraVent: Relining Chimneys for Gas Appliances (technical guide, PDF)

This guide is general information. It can't diagnose a specific chimney; an inspection can.