What Is A Fireplace Insert’s Secondary Burn? (EPA Emissions)
A fireplace insert’s secondary burn is the burning of smoke gases above the wood or inside a catalyst, after the first flames release them. It uses heat and added air, or a catalyst, to reduce smoke and emissions. It does not make a fire smokeless, and its performance depends on dry fuel, correct operation, and the insert’s design.
Secondary combustion can sound like a complicated feature, but the basic idea is simple: a fire can burn some of the gases that would otherwise leave the firebox as smoke. The details vary. Some inserts use heated air and a baffle; others use a catalyst. That adaptability matters when you read a manual, because the right startup steps and control settings depend on which design you have.
In my equipment teaching, I’ve found that owners often mistake a brief smoky startup for a failed system, or expect a secondary burn to eliminate every trace of chimney smoke. Neither conclusion is reliable on its own. Here’s how to understand the process and use it safely.
What secondary burn means in a fireplace insert
Secondary burn is a second stage of combustion. The first stage burns wood and releases heat, smoke, and gases. In the second stage, the insert provides conditions that let some of those gases burn in the firebox or pass through a catalyst, reducing the amount of unburned material that leaves the appliance.
Wood smoke contains gases and tiny particles released as the wood heats. If the firebox is hot enough and air reaches those gases, they can ignite. This is different from simply burning the visible wood. The insert’s design helps keep heat and air in the right place for that second stage.
Secondary combustion is useful, but it is not a guarantee of zero smoke or zero emissions. Smoke can appear during startup, reloads, or poor-burning conditions. A steady plume after the insert has warmed up may point to wet wood, restricted airflow, weak draft, or another operating issue.
How noncatalytic and catalytic inserts burn smoke
Noncatalytic and catalytic inserts both aim to burn smoke gases, but they do it differently. A noncatalytic model uses heat, added air, and a baffle. A catalytic model sends exhaust through a catalyst that supports gas oxidation at lower temperatures. The manual identifies the design and its operating steps.
| Insert type | How it supports secondary combustion | What the owner should know |
|---|---|---|
| Noncatalytic | Preheated air enters above the fire through ports or tubes. A baffle helps keep hot gases in the firebox. | Keep the baffle and air passages in good condition. Follow the manual for startup and air-control use. |
| Catalytic | Exhaust passes through a catalyst, where smoke gases can oxidize. | Engage or bypass the catalyst only as directed. Inspect and maintain it according to the manual. |
A baffle is a heat-resistant plate near the top of the firebox. It helps guide hot gases and can lengthen their path before they exit. Preheated air is air warmed by the insert before it reaches the fire. These parts work as a system; a damaged baffle or blocked air passage can affect performance.
A catalyst does not mean the insert can burn any fuel or operate at any setting. Both designs need enough heat and suitable airflow. There is no single air-control position or operating temperature that applies to every model.
What EPA emissions ratings do and do not tell you
An EPA emissions rating is a test result for a covered appliance under a specified certification method. Under the 2020 New Source Performance Standards, covered new residential wood heaters must meet a limit of 2.0 grams per hour using the crib-wood test method or 2.5 grams per hour using the cordwood test method, as applicable. Those limits are not a promise of the rate in every home.
The units, grams per hour (g/h), describe particulate emissions measured during a test. The test method matters, so the two limits should not be treated as interchangeable measurements of the same test. Check the appliance documentation or certification information to see which method applies.
| Information | What it means | What it does not mean |
|---|---|---|
| EPA certification limit | The applicable maximum emissions rate for the covered heater under its certification method | A guarantee that every fire will emit at that rate |
| Secondary-burn feature | A design intended to burn some smoke gases | A smokeless fire under all conditions |
| Low visible smoke after warm-up | A possible sign of cleaner combustion | Proof by itself that the appliance meets a specific g/h rate |
| EPA-certified insert | The model was tested for certification | Proof that the chimney, installation, fuel, and operation are correct |
Installation and use affect real-world performance. Certification does not verify that a particular chimney has proper draft, that the insert was installed to its listing, or that an owner is burning dry wood. For your setup, use the rating plate and manual, and have the chimney system inspected.
Insert setup, fuel checks, and safety preparation
Before lighting a fire, confirm that the insert and chimney are in serviceable condition and that you understand the model’s controls. Use the manual for required clearances, chimney configuration, and any draft limits. These details are appliance- and installation-specific; EPA certification alone does not confirm that an installation meets them.
Check these items before use:
- Read the manual’s instructions for startup, air controls, catalyst bypass, reloads, and ash removal.
- Check the door seals, baffle or catalyst, and visible air passages as directed by the manufacturer.
- Use seasoned wood at 20% moisture or less. Measure with a moisture meter on a freshly split face, not the outer surface.
- Do not burn wet or green wood, treated or painted wood, trash, or anything prohibited by the manual.
- Keep combustibles outside the listed clearances. Maintain working smoke and carbon monoxide alarms.
Wet wood can suppress firebox temperature and increase smoke and creosote. That can make secondary combustion harder to establish, even if the insert itself is in good condition. If you cannot confirm the wood is dry, do not assume that a strong-looking flame means it is ready for clean burning.
Starting, establishing secondary combustion, and reloading
To start the secondary burn, first build a stable fire with dry kindling and wood, following the insert’s startup instructions. Keep the air control open as directed while the fire catches and the flue warms. Reduce air only after the fire is established. A catalytic model may have a bypass control; engage the catalyst only when the manual says conditions are right.
A practical sequence is:
- Arrange a small kindling fire and dry fuel as the manual recommends.
- Start the fire with the specified air-control position.
- Allow it to become stable and warm the flue before reducing the air.
- If the insert has a catalyst bypass, use the manual’s directions for engaging it.
- Add wood without overfilling, then reduce air gradually as instructed.
During a secondary burn, you may see flames above the wood or activity at the catalyst, depending on the design. The appearance varies by model and burn stage. Do not use one visual cue as the only test of correct operation.
When reloading, follow the manual’s guidance on timing and air settings. Avoid closing the air control too early or smoldering a load to stretch the burn. Restricting air before the fire is established can defeat secondary combustion and increase smoke and creosote.
Reading smoke and diagnosing a weak secondary burn
A short period of smoke during startup or after adding wood does not, by itself, prove there is a problem. The more useful question is whether smoke continues after the insert has warmed up and the fire is established. Persistent chimney smoke can indicate wet fuel, poor draft, restricted air, or incorrect operation; inspection may be needed to identify the cause.
| Observation | Possible explanation | Sensible next step |
|---|---|---|
| Smoke during startup | The fire and flue may still be warming | Follow the startup steps; use dry kindling and fuel |
| Persistent smoke after warm-up | Wet wood, poor draft, or incorrect airflow are possible | Check fuel moisture and manual settings; arrange inspection if it continues |
| Little or no visible secondary flame | The insert may not be at the right stage, or conditions may not support combustion | Check the model’s instructions; do not assume all designs show the same flame pattern |
| Repeated cool, smoky fires | Creosote may build up in the connector or chimney | Stop treating it as normal; have the system inspected and cleaned as advised |
These signs are clues, not a diagnosis. Draft depends on the appliance and chimney setup, so there is no universal draft setting to recommend here. If the insert does not respond as the manual describes, have a qualified professional assess the appliance and chimney instead of forcing a control setting.
Cleaning, component inspection, and catalyst care
Cleaning and inspection help preserve the conditions secondary combustion needs. Let the insert cool fully before handling ash or checking components. Follow the manual for cleaning intervals and methods, especially for a catalyst, baffle, and air passages. Avoid scraping or cleaning parts in a way the manufacturer does not allow.
Check for visible damage, blockage, or deterioration in the firebox components the manual identifies. A catalyst needs model-specific care; do not assume it can be cleaned or replaced using the same method as another brand. If a part looks damaged or the insert’s performance has changed, consult the manual or a qualified service professional.
Remove ash safely only after the appliance is fully cool, using a suitable metal container and following the maker’s instructions. Do not leave hot ash where it could ignite nearby materials. Routine care cannot replace chimney inspection, and a clean firebox does not prove the chimney is free of deposits.
Chimney maintenance and seasonal shutdown
Creosote is a combustible deposit that can form when smoke cools and condenses in the connector or chimney. Repeated cool, smoky burns, often caused by wet wood, air restricted too early, or inadequate draft, can increase buildup. Have the chimney system inspected and cleaned based on its condition and use; do not rely on a fixed interval without considering those factors.
At the end of a fire, let the fuel burn down naturally. Do not use flammable liquids, and do not close controls contrary to the manual. After the insert is fully cool, handle ash and inspect components as instructed. Before the next season, make sure the appliance and chimney are in serviceable condition.
If you suspect a chimney fire, evacuate, call emergency services, and do not use the insert again until the system has been professionally inspected. A suspected chimney fire is not a reason to restart the appliance to check whether it still works.
Illustrative installation scenarios
These examples are common situations, not reports of specific homes. They show why the same secondary-burn explanation can lead to different next steps. The insert manual, rating plate, fuel condition, and chimney inspection remain the sources for decisions about an individual installation.
- A noncatalytic insert with smoke that continues after warm-up: The owner checks that the wood is at or below 20% moisture on a freshly split face, confirms the air-control steps in the manual, and arranges a chimney assessment if smoke persists. They do not assume the baffle or draft is correct based only on EPA certification.
- A catalytic insert after a reload: The owner follows the model’s reload and bypass instructions rather than engaging the catalyst at a guessed temperature. If performance stays poor, they check the manual’s catalyst-care guidance and seek service when needed.
The key point is that secondary combustion is a process, not a single control position. Dry fuel, adequate heat, correct airflow, and a sound chimney all matter.
Frequently asked questions
These short answers clarify what secondary combustion can tell you, what EPA emissions figures mean, and which operating choices should always come from the insert’s own instructions. Use them as a starting point, not a substitute for the manual or a professional inspection.
Does secondary burn mean the fire is smokeless?
No. It can reduce smoke by burning some released gases, but smoke may still appear during startup, reloads, or poor-burning conditions.
What is burning during a secondary burn?
Some gases and particles released as wood heats are burning. The insert supplies heat and air, or uses a catalyst, to support that second stage.
Do all fireplace inserts have a catalyst?
No. Noncatalytic inserts use heated air and a baffle; catalytic inserts route exhaust through a catalyst. Check your model’s manual.
What does an EPA rating in g/h mean?
It is an emissions rate in grams per hour measured under a certification test method. It is not a guaranteed rate for every fire or installation.
What wood moisture level should I use?
Use seasoned wood at 20% moisture or less, measured with a moisture meter on a freshly split face.
When should I reduce the air control?
Only after the fire is established, and according to the manual. There is no universal setting for all inserts.
Can I engage a catalyst as soon as I light the fire?
Not necessarily. Use the model’s startup and bypass instructions. Catalyst timing is specific to the insert.
Is chimney smoke always a sign of a problem?
Not always, especially during startup or reloading. Persistent smoke after warm-up can point to wet fuel, poor draft, or incorrect operation and deserves attention.
Can I burn painted or treated wood?
Do not burn it unless the appliance manual explicitly allows it. The guidance here is to avoid treated or painted wood, trash, and other prohibited materials.
What should I do if I suspect a chimney fire?
Evacuate and call emergency services. Do not use the insert again until the chimney system has been professionally inspected.
(This article was written by one of our staff writers, Evan Bradley. Visit our Meet the Team page.)
