What is a catalytic combustor?

A catalytic combustor is a specially engineered ceramic component that fits inside your wood stove. It's made from a temperature-resistant ceramic material extruded into a honeycomb shape, which creates thousands of small channels for smoke to pass through. The surface is coated with noble metals — typically palladium or platinum — that act as a catalyst.

When smoke passes through these channels and contacts the metal coating, a chemical reaction occurs that causes the smoke to ignite at a much lower temperature than it normally would. The result: more heat from every log, less creosote buildup in your chimney, and significantly less air pollution.

What does a catalytic combustor do in a wood burning stove?

In a conventional wood stove, 5–30% of the energy in every log escapes up the chimney as unburned smoke. The catalytic combustor captures that wasted energy by burning the smoke before it leaves the stove. The result is measurably better performance:

• Up to 50% more heat generated from each piece of wood (per independent research)
• 90% less air pollution — the combustor burns the gases and particles in smoke before they exit
• 90% less creosote — burning the smoke dramatically reduces chimney buildup and fire risk
• Less wood consumed to maintain the same level of warmth

How does a catalytic combustor work?

Normally, wood smoke will only ignite and burn at around 1,000°F (540°C) — too hot for practical everyday stove operation. Your catalytic combustor changes this by using a noble-metal coating to lower the ignition temperature of smoke to around 500°F (260°C), which is easily reached inside a wood stove firebox.

Here's the sequence in a typical burn cycle:

1. Build a fire and keep it burning at medium to high intensity with the bypass damper open.
2. After 20–30 minutes, the combustor reaches 500°F — this is called "light-off."
3. Close the bypass damper so exhaust gases are directed through the combustor.
4. The gases ignite within the honeycomb channels. The combustor's exhaust temperature rises to 1,000°F–1,600°F, indicating active combustion.
5. The by-products of catalytic combustion are water vapor (H₂O) and carbon dioxide (CO₂) — far cleaner than untreated smoke.

Important: A catalyst is not consumed in the reaction. By definition, it facilitates the chemical process without being used up — which is why a well-maintained combustor can last for many seasons.

What is a catalytic combustor made of?

The core is a high-temperature ceramic substrate formed into a honeycomb pattern. This shape maximizes surface area so the catalytic coating has as much contact as possible with passing smoke. The ceramic is first coated with alumina oxide for a high-surface-area base, then a noble metal — typically palladium and/or platinum — is applied as the active catalyst. The finished unit is cut to the sizes specified by individual stove manufacturers.

Why was the catalytic combustor developed for wood stoves?

In the 1970s, oil prices rose 272%, electric rates jumped 215%, and natural gas increased 483% in cost. Millions of homeowners turned to wood burning as an affordable heating alternative, and hundreds of new stove companies emerged. While these stoves were efficient in terms of heat output, they came with serious downsides:

• Up to 30% of energy was lost as unburned smoke in some stoves
• Wood smoke produced harmful air pollution
• Airtight stoves burning at low heat produced large amounts of creosote

In the early 1980s, the catalytic combustor emerged as the solution. The U.S. EPA and state DEQ agencies later established standards for stove manufacturers, ushering in a new era of cleaner, more efficient wood burning. Applied Ceramics has been a leading manufacturer of catalytic combustors since this period and remains the world's largest manufacturer of catalytic combustors for wood burning stoves today.

Temperature Quick Reference Guide

Keeping your combustor in the right temperature range is the single most important operating factor. Use this guide:

How do I achieve light-off and operate my stove properly?

Starting from a cold stove:

1. Build a medium-to-high fire with the bypass damper open.
2. Maintain this firing rate for 20–30 minutes. Even if temperatures reach 600°F within a few minutes, do not turn down the fire yet — the combustor and firebox need time to fully stabilize.
3. Once the combustor has reached 500°F for 20–30 minutes, close the bypass damper.
4. The exhaust temperature on the combustor's downstream side should rise immediately, confirming the catalyst is active.

Refueling a hot stove:

• Firebox above 500°F: Open the bypass, load fuel, and close the bypass immediately — as long as the wood is dry.
• Firebox below 500°F: Open the bypass, load fuel, and fire the stove for about 10 minutes before closing the bypass to ensure the catalyst reaches 600°F again.

How do I monitor the combustor's temperature?

Two temperature sensors give you the most complete picture:

• Upstream sensor (inlet side): Tells you when the combustor is approaching light-off temperature and when it's ready to receive gases.
• Downstream sensor (exhaust side): Mounted about ¼" off the combustor surface, centered. This is the most important reading — it tells you whether catalytic combustion is actively occurring.

If you can only install one sensor, make it the downstream (exhaust) sensor. Thermocouplings and stove thermometers are available from your dealer for this purpose.

How will I know the combustor is working?

The most reliable method is temperature monitoring. When the bypass damper is closed after proper light-off, the exhaust temperature will rise noticeably. This is your confirmation the catalyst is active.

Another indicator is chimney smoke: with the bypass closed and the combustor working properly, there should be little to no dark smoke visible from the chimney.

What about glowing? The combustor may glow during the first 20–30% of the burn cycle, when it's burning the most smoke at the highest temperature. However, the combustor does not need to glow to be working — during low-burn cycles it rarely glows, even while operating effectively. Do not rely on visual glow checks as your primary indicator.

If the combustor stops working effectively, you will notice an increase in fuel consumption and creosote buildup in your system.

Is it okay to "hot fire" my catalytic stove?

How do I maintain the combustor's operating condition?

The catalytic combustor largely maintains itself — the high temperatures it operates at burn off most contaminants during normal use. A few practices will significantly extend its life:

• Burn only dry, seasoned wood (see Section 7 for wood preparation guidance)
• Always achieve proper light-off before closing the bypass damper
• Monitor temperatures and avoid prolonged operation above 1,600°F
• Do not burn garbage, treated wood, painted wood, plastic, rubber, cardboard, or any foreign materials
• Inspect and clean the combustor at the start of each burning season

How and when should I clean the catalytic combustor?

Midwest Hearth recommends inspecting and cleaning your combustor before each burning season. Here's how:

• Wait until the stove is completely cool before handling the combustor.
• Shine a bright flashlight onto the surface from above and look through from below. The amount of light passing through indicates how clean it is — heavy blockage suggests fly-ash or creosote buildup.
• For fly-ash buildup, use a soft-bristled brush or paintbrush to gently dust the surface. A vacuum cleaner may also be used carefully. Individual blocked cells can be cleared with a pipe cleaner or cotton swab.
• For soot or creosote masking, open the bypass and build a hot fire. Once hot, close the bypass halfway and burn for 30–60 minutes to burn off the accumulation.

What does a healthy combustor look like?

A properly functioning combustor should have a light gray, powdery appearance. The cells should be open and free from significant buildup of fly-ash or creosote. Small cracks do not necessarily reduce efficiency, as long as the unit remains in place and no large pieces are missing.

What are the DO's and DON'Ts for my stove?

Installation

• Insulate your masonry chimney, especially on outside walls. Insulation promotes proper draft and prevents icing that can block chimney flow.
• Use the manufacturer's recommended flue diameter. An improperly sized flue impairs draft and may allow smoke to leak inside.
• Ensure all seals connecting the stove to the flue are airtight. Tight seals prevent smoke leaks and contribute to good draft.
• Determine correct draft level with a certified installer or chimney sweep. Too much draft causes uncontrolled burn and overheating; too little causes back-puffing.

Operating

• Burn only dry, well-seasoned wood stored at least 12 months. Wet wood yields less heat and can cause thermal shock to the combustor.
• Operate in bypass mode initially; wait for proper light-off before engaging the catalyst. Closing the bypass too soon causes fly-ash masking and can damage the combustor.
• Build and maintain a hot fire quickly after initial wood loading. A hot fire helps light-off occur faster; once lit, the catalyst stays active at lower inlet temperatures.
• Burn moderately sized loads and minimize door openings. Frequent door openings lower temperature and increase pollution.
• Check gaskets, the bypass damper, and cast iron seams annually; replace worn materials. Worn gaskets allow smoke to bypass the combustor, increasing pollution.
• Check and clean the flue twice a month, professionally cleaned at least once a year. Clean flues prevent chimney fires; tight joints prevent leaks.
• Burn trash, treated wood, particle board, plywood, plastic, rubber, or petroleum products. These produce harmful fumes, can overheat your stove, and will poison the catalyst.
• Operate in bypass mode after the catalyst has reached light-off temperature. Once lit, the combustor should be working — producing more heat and reducing pollution.
• Over-fire your stove. Avoid sustained temperatures above 1,800°F. High heat can permanently damage or destroy the combustor.
• Open the ash pan while the catalytic combustor is engaged. This leads to dangerous overheating.
• Remove or tamper with preset operating or temperature controls. This is against federal law, voids your warranty, and creates a safety hazard.
• Operate the stove without the catalytic combustor installed. The stove's efficiency depends on it; operating without one increases pollution and is against federal law.

How long should my combustor last?

When used and operated correctly, independent studies have shown catalytic combustors can remain effective and provide clean burning for up to 8 seasons. Some have operated efficiently for as long as 10 years. Actual lifespan depends on the stove operator, maintenance practices, fuel quality, and stove design.

The combustor carries a general 6-year life expectancy under standard usage. Midwest Hearth offers a 5-year prorated warranty on its combustors.

When is it time to replace my combustor?

Consider replacing your combustor if you notice any of the following:

• Large pieces of the honeycomb substrate are missing
• The combustor is crumbling or severely cracked with pieces falling out
• Cells are heavily plugged and cannot be cleaned
• Fuel consumption has increased noticeably
• Creosote is building up faster than usual
• The combustor fails to reach or maintain light-off under normal conditions

Small hairline cracks without missing material are generally not a reason to replace — a cracked combustor can still function well as long as it stays in place.

Why do combustors fail or need replacement?

• Thermal shock: Caused by refueling with wet or snowy wood, sending moist smoke to a combustor running above 1,000°F. The sudden temperature contrast causes cracking.
• Flame impingement: Direct contact with flames (not just smoke) changes the catalyst's chemical makeup and breaks down the ceramic. Can be caused by high draft, too many logs, or air controls left wide open.
• Poisoning: Burning garbage, painted wood, rubber, plastic, or other non-wood materials coats the catalyst with substances that block catalytic activity.
• Mishandling: Dropping the combustor, using abrasive tools, applying high-pressure air, or using solvents to clean it.
• Chemical saturation: Over time, byproducts like potassium fill the catalyst's active sites. This process takes years and can be slowed with regular maintenance.
• Normal fatigue: After many seasons of high-temperature operation, the substrate gradually deteriorates — this is normal end-of-life wear.

Why is flame impingement so harmful?

A catalyst burns the chemical byproducts in smoke. When direct flames contact the combustor, they change the chemical structure of the noble metal coating and begin breaking down the ceramic substrate itself. Modern wood stoves are designed to prevent this, but it can still happen from a strong fast draft, an over-fueled firebox, or doors left ajar.

What causes my stove to "back puff"?

Back puffing occurs when the combustor temperature exceeds 1,400°F and acts like a glow plug. Occasionally, the gas-to-air mixture inside the firebox is just right for spontaneous ignition, causing the stove to vent puffs of smoke back into the room. If this happens, reduce the fire and monitor your temperatures more closely.

Why does the combustor struggle to work properly?

The most common causes are:

• Light-off not achieved: The bypass was closed before the combustor reached 500°F for 20–30 minutes. Result: cells plug with fly-ash and creosote.
• Wet wood used: Moist smoke immediately shuts the combustor down and causes thermal shock.
• Masking: Burning non-wood materials coats the catalyst surface, blocking catalytic activity.
• Plugging: Soot, creosote, or fly-ash accumulation in the cells — often from low-temperature operation or burning paper/cardboard.

Troubleshooting Guide

What kind of wood should I burn?

The catalytic combustor is designed to burn seasoned, dried wood only. For serious wood burners using a catalytic stove, hardwoods are the best choice. Hardwoods like oak and birch provide a longer-lasting fire with a shorter flame, produce less creosote, and pair well with catalytic combustion.

Softwoods (fir, pine, spruce) are cheaper and easier to ignite, but they burn faster and produce more creosote — which is harder on your combustor over time.

How do I properly season firewood?

Seasoned wood is defined as logs that have been sawn, split, and stacked loosely under a roof for at least 12 months, reaching a moisture content of 18–21%.

• Split logs: Splitting exposes the wet interior and dramatically speeds drying. Unsplit rounds take much longer to season.
• Stack loosely: Air must circulate freely through the pile. Do not pack tightly.
• Cover from above: Protect from rain and snow but leave the sides open for airflow.
• Store away from the house: Wood piles can attract insects.
• Never refuel with wet wood: Even snow or rain on the surface of otherwise seasoned wood can cause thermal shock when it contacts a hot combustor.

Firewood Species Comparison

* Oak heat value data was missing from source documentation but is widely recognized as a top-tier hardwood fuel.

How is firewood measured and sold?

The standard unit is a cord — a neatly stacked pile measuring 8 ft. long × 4 ft. high × 4 ft. wide (128 cubic feet total). Because most people don't burn 4-foot logs, wood is often sold by the "face cord" or "rick" — 8 ft. long × 4 ft. high, with a variable depth of 12", 16", or 24". A 16" face cord is approximately ⅓ of a full cord.

A "truckload" is not a standard measurement and depends entirely on the dealer's truck. A standard pickup holds only about ⅓–½ of a full cord. If you believe you received short measure or the wrong species, contact your state Bureau of Weights and Measures.