Chimney Liner for a Wood Stove Insert

Why the fireplace flue does not suit an insert

An open fireplace pulls a large volume of room air up a large flue. The flue in most older brick homes was sized for exactly that: a wide opening, a big fire and a lot of exhaust. An insert changes the picture. It is a closed steel or cast iron firebox with a controlled air supply and an outlet that is a fraction of the size of the old flue.

Put that small stream of smoke into a big masonry flue and it spreads out, slows down and cools. Cool smoke rises weakly, so the draft suffers, and as it cools the tars and moisture in it condense on the flue walls as creosote. The result is a stove that smokes when the door is opened, glass that blackens quickly and a chimney that coats up much faster than it did with an open fire.

What a full liner installation includes

The liner is the heart of the job, but a correct insert installation is a system of parts that have to work together. Leaving one out is how older jobs end up smoking or leaking heat.

• A stainless liner matched to the insert's outlet size, running from the collar up to the chimney top
• A connector or adapter that joins the liner securely to the insert's collar
• The old damper removed or fixed fully open so the liner can pass through the throat
• A block-off plate at the damper area on many installs, so room heat does not drift up around the liner
• A top plate that seals the chimney around the liner, finished with a cap
• Insulation around the liner when the liner's listing or the clearances call for it

Sizing the liner to the insert

The starting point is the insert's flue collar. The installation manual states the outlet size and whether the liner may be the same diameter, and that manual is what the job has to follow. A liner smaller than the outlet chokes the exhaust; one much larger repeats the problem of the old flue on a smaller scale.

Chimney height and shape matter too. A short chimney gives less draft, and offsets, which are common in rowhouse and brownstone stacks where the flue jogs around a floor or a neighboring flue, add resistance. Where a round liner will not pass a tight spot, an oval liner of equivalent area is sometimes used, provided the insert and liner instructions allow it. Measuring the flue along its full length, not only at the top, is what tells the technician which liner will fit.

When the liner needs insulation

Liners are tested and listed as systems, and for wood-burning appliances insulation is often part of that listing. Its first job is safety: in many older houses the brick chimney sits tight against floor joists and wall framing with no air gap at all, and the insulation is what lets the liner meet the clearance it was tested for.

Its second job is performance. An insulated liner stays hotter, so the stove reaches a steady draft sooner after lighting and less creosote forms on the way up. That matters most on chimneys built on an outside wall, which stay cold through a northeast winter. Insulation is either a blanket wrapped around the liner before it goes down or a poured mix that fills the space afterward, and the choice depends on how much room the old flue leaves.

What gets checked before the liner goes in

A liner protects the house only if the chimney around it is sound. Before quoting, a technician looks at the flue tiles from top and bottom, the smoke chamber, the crown and the brick above the roof. A crown that is cracked will keep letting water into the space around the new liner, and loose brick at the top has to be repaired before a top plate can be sealed to it.

Inside, the hearth and the floor in front of the insert get a look as well, since the insert's manual sets requirements for floor protection and clearances to the mantel and trim. In multi-flue chimneys, which are common in attached houses, the crew also confirms which flue belongs to the fireplace and that the neighboring flues are not cracked into it.

Older insert installs that cause trouble

Many inserts were installed by sliding the unit into the fireplace and letting it exhaust into the smoke chamber, sometimes with a short pipe that stops just above the damper. These setups tend to deposit creosote in the smoke chamber and lower flue, right where it is hardest to reach, and cleaning usually means pulling the insert out of the fireplace first.

Signs that an existing insert is not venting well include smoke rolling out when the door opens, a sharp creosote smell in the room, glass that turns black after one or two fires, and a fire that struggles unless the air control is wide open. Any of those is a reason to stop and have the venting looked at before the next cold spell, rather than burning through it.

Freestanding stoves and the chimney sleeve

A stove that sits in the room and vents into a masonry chimney connects with stovepipe, either through a thimble in the chimney wall or up through the old fireplace opening. If the flue is much larger than the stove's outlet, or the clay tiles are cracked or missing, it usually needs a liner too. Homeowners often call this a chimney sleeve. The idea is the same as with an insert: one continuous flue of the right size, from the appliance to the top.

A thimble connection deserves its own look in older houses. The opening in the brick has to be sound, sealed around the pipe and kept clear of wood framing and plaster lath by the distances the stove and pipe instructions require. Old thimbles left from coal stoves or heaters, sometimes hidden behind a wall or a metal cover plate, are often too close to combustibles or open into a flue that also serves something else, so reusing one is decided only after it has been inspected.

What sets the cost and how the job goes

The price comes from the chimney's height, the liner's diameter and grade, insulation, the number of offsets, how hard the roof is to work on and any repairs needed first. A tall stack on a steep slate roof takes more setup than a short one reached from a flat rowhouse roof.

On the day, the crew protects the room, removes or opens the damper, lowers the liner from the top, connects it to the insert and seals the top with a plate and cap. The insert is set in place and the first fire is watched to confirm the draft. Our estimate spells out each of those items in writing, with upfront pricing.