What Is a Fireplace Smoke Chamber?

Where it sits in the fireplace

Smoke passes through several distinct parts on its way out of a masonry fireplace, each with its own job:

• Firebox: the lined box where the fire burns
• Throat and damper: the narrow slot above the opening that you open and close
• Smoke shelf: the ledge just behind the damper
• Smoke chamber: the inverted funnel rising above the damper
• Flue: the lined passage that carries smoke to the outlet

What the chamber and shelf actually do

A typical firebox opening is far wider than the flue above it. The smoke chamber bridges that difference. Its side walls slope inward and its front wall leans back, so smoke and hot gases speed up as they are squeezed toward the flue and keep moving rather than swirling. A chamber with even, smooth slopes lets the draft build steadily; one with abrupt corners and ledges creates turbulence, and turbulent smoke cools, slows and lingers.

The smoke shelf sits at the bottom of the chamber, behind the damper. Cold air dropping down the flue strikes the shelf and turns back upward with the rising smoke instead of pouring into the room. The shelf also collects whatever falls down the chimney: soot flakes, crumbs of mortar, leaves, nesting material and rain. Clearing that debris is part of every proper sweep.

Why older smoke chambers cause trouble

In many older fireplaces, including those in brownstones, rowhouses and brick colonials across the region, the chamber was built by corbeling: each course of brick stepped inward a little past the one below until the walls met the flue. The result is a staircase, not a slope. Smoke tumbles over each step, creosote collects on every ledge, and the mortar in those stepped joints was often never smoothed or sealed.

Decades of heat, acidic flue deposits and moisture from rain or from a boiler sharing the stack wear those joints open. That matters because the smoke chamber sits close to the house's structure. Floor joists, headers and wall framing are often only a brick's width away, and an open joint is a path for heat and hot gases into those spaces. In attached houses, the chimney breast may also share masonry with the neighboring home.

Signs of a smoke chamber problem

Chamber trouble often gets mistaken for a draft problem, so the symptoms are worth knowing:

Some of this you can check yourself with the fireplace cold. Open the damper, put on safety glasses, and shine a bright flashlight up past the damper plate. You will not see the whole chamber, but you can often see whether the lower walls look smooth or stepped, whether there is a crust of black, shiny deposit, and whether loose debris is resting on the shelf. Note what you see and pass it on at the inspection.

• Smoke rolling out of the top of the opening after the fire is well established
• Thick, crusty or glazed creosote visible on the walls just above the damper
• Chips of brick or mortar lying on the damper plate or the smoke shelf
• A smoke smell in rooms or closets next to the chimney breast
• Dark stains or warm spots on the wall or ceiling near the chimney during a fire
• Open joints or missing mortar visible when looking up past the damper

How a technician inspects it

The chamber is one of the hardest parts of a fireplace to see. From below, the damper plate and frame block much of the view, so a thorough look starts with sweeping the chamber clean and then working a strong light and a mirror above the damper, sometimes with the damper plate removed. From above, a light lowered down the flue shows the upper slope and where the chamber meets the first flue tile.

The technician looks at the shape of the walls, the condition of every joint, whether creosote has glazed on the steps, and whether the transition into the flue liner is tight. They also check the outside of the chimney breast and the framing around it from the attic or basement where it is accessible, because heat damage there confirms what the joints suggest.

Parging and rebuilding

The usual repair is parging. After a full cleaning, including removal of any glazed creosote that would stop new material from bonding, the inside of the chamber is coated with a heat-resistant refractory mix. The coating fills open joints and builds the stepped walls into a continuous, sloped surface. Smoke moves through more cleanly, there are fewer ledges for creosote to cling to, and the chamber becomes a sealed passage again.

Parging is not the answer to everything. Bricks that are loose, cracked through or missing, a front wall that has slumped, or a chamber that was built far out of shape may need sections rebuilt before any coating goes on. The product's curing instructions govern how soon the fireplace can be used again, and a few small fires usually come before a full one.

Smoke chambers, liners and inserts

When a fireplace flue is relined, the new liner normally begins at the top of the smoke chamber. If the chamber below is rough or open at the joints, smoke still passes through a defective section before it ever reaches the new liner, which is why the two jobs are often planned together. Relining a chimney and ignoring the chamber leaves the weakest part untouched.

Inserts work differently. An insert's own liner runs from the unit up through the damper area and the chamber, and the space around it is usually closed off with a sealing plate so room air and heat do not escape around the liner. Even so, the chamber has to be sound and clean before the insert goes in, because once the unit is installed that space becomes very hard to reach.

Keeping the chamber in good shape

Burning dry, seasoned wood in moderate fires keeps creosote thin and easy to brush away, while smoldering, damp wood coats the steps with heavy deposits. A chimney cap keeps rain from soaking the chamber and the shelf, and a sweep clears debris before it piles up. NFPA 211 calls for chimneys and fireplaces to be inspected every year and again after a chimney fire, and the chamber belongs in both of those checks. We look at it from below with a strong light and tell you plainly, with a free written estimate, whether it needs work.