By James Rodriguez, Central Chimney Service.
Putting an insert into an existing masonry fireplace changes what you own. It looks like a fireplace upgrade and it is genuinely one, but from the chimney’s point of view the appliance, the venting, the deposits, and the service work all become different. Homeowners who install an insert and keep the same maintenance assumptions are the ones we most often find with a problem.
Two different machines
An open fireplace burns with unlimited air and vents into a large masonry flue. A wood-burning insert is a sealed steel or cast-iron firebox with a controllable air supply, a gasketed door, and a much smaller, dedicated flue.
That air control is the whole difference. An open fireplace cannot be damped down — it burns fast and hot and sends most of its heat up the chimney. An insert can be run low and slow, which is why it heats a room far better and why it produces a completely different deposit profile in the flue.
Why an insert needs its own liner
An insert must vent through a correctly sized liner running the full height of the chimney and connected directly to the appliance. Venting an insert into the original oversized masonry flue is the single most common defect we find on inserts installed without a chimney professional involved.
The reason is temperature. The insert produces a smaller volume of cooler exhaust than the open fireplace did, and in a flue sized for the open fireplace it cools rapidly, drafts poorly, and condenses heavily — which means creosote accumulating fast, in the hardest form, in a flue that is difficult to clean. A properly sized stainless liner keeps the exhaust hot and moving. Chimney relining is the same process whether it is done for an insert or for a deteriorated flue.
The deposits are different, and harder
Slow, air-restricted burning is exactly the condition that produces stage two and stage three creosote — the tarry and glazed forms — rather than the light flaky deposit an open fireplace tends to leave.
Glazed creosote is the difficult one. It adheres hard, it does not brush off, it frequently requires chemical treatment to make it brittle before mechanical removal, and it ignites readily. An insert run efficiently on well-seasoned wood is not a problem; an insert run damped down overnight on marginal wood builds difficult deposits quickly. The appliance rewards good practice more than an open fireplace does, and punishes bad practice harder.
What sweeping an insert involves
The work is more involved than sweeping an open fireplace. Many inserts have to be partially or fully pulled from the fireplace opening to reach the connection and the base of the liner, and the appliance’s own internals — baffles, secondary air tubes, and the catalytic combustor on units that have one — need attention as part of the same visit.
That last part matters and is routinely skipped. A blocked baffle or a degraded catalyst changes how the appliance burns, which changes what it deposits in the flue. Our chimney sweeping page covers the process, and the services page lists what else we handle. Around Downers Grove chimney sweep calls, inserts installed by a previous owner with no documentation are common enough that identifying the unit is part of the first visit.
Which one you should have
If you want heat, an insert. If you want an occasional fire on a winter evening and the look of an open hearth, the fireplace you have is fine — and it costs nothing to keep.
The honest caveat is that an insert is a real installation, not an appliance you slide in. It needs a correctly sized liner, a proper connection, adequate clearances, and a chimney sound enough to carry all of it, which is what a chimney inspection establishes first. Doing that assessment before buying an insert avoids discovering afterward that the chimney needs work before anything can go in.
Price up front on our pricing page, before any work. Insured and bonded, 29 years, 7 days a week from 7 AM to 7 PM at (815) 305-5510.
Frequently asked questions about inserts and open fireplaces
Does a wood-burning insert need a new chimney liner?
Yes, in nearly every case. The insert requires a correctly sized liner running the full chimney height and connected directly to the appliance. Venting into the original oversized flue causes poor draft, rapid creosote accumulation, and a flue that is hard to clean.
Does an insert need sweeping more often than an open fireplace?
Frequency depends on how it is burned rather than on the appliance type, but inserts run slow and damped down build deposits faster and in harder forms. The annual inspection determines what your particular unit and burning habits actually require.
Why does an insert produce glazed creosote?
Because restricted-air burning creates cooler, smokier combustion, and cooler smoke condenses more heavily on the flue wall. Burning properly seasoned wood in hot, well-supplied fires largely avoids it; overnight smoldering on damp wood is what produces it.
Can I sweep an insert flue myself with a brush from the top?
Generally not effectively. Reaching the connection and the base of the liner usually requires pulling the insert, and top-down brushing tends to pile debris where you cannot get it out. Glazed deposits also do not respond to brushing at all.
Will an insert actually heat my house?
It will heat the room it is in and adjacent space considerably better than an open fireplace, because the sealed firebox radiates heat into the room instead of sending it up the flue. Whole-house heating depends on layout and air circulation, so treat it as strong zone heating rather than a furnace replacement.
Can I use my existing damper with an insert?
No. The liner passes through the damper location, so the damper is typically removed or permanently fixed open as part of the installation. Air control moves to the appliance itself, which is what makes the insert controllable.
What if I bought a house with an insert already installed?
Have it inspected before using it. Undocumented installations are frequently found without a proper liner, with an incorrect connection, or with clearance problems, and none of that is visible from in front of the unit. Identifying the make and model is part of that first visit.
Is a gas insert a simpler option?
It avoids the wood-burning maintenance profile entirely — no creosote, no ash — but it introduces its own venting requirements and needs annual service on the burner, pilot assembly, and vent terminal. Simpler in some ways, not maintenance-free.
Published September 10, 2026 · Last updated August 27, 2026