Technical Information

Non-catalytic vs. Catalytic

Below: Cross section of a non-catalytic stove, showing combustion air/exhaust flow patterns, large baffle and high level combustion air supply.

Below: Cross section of a catalytic stove, showing combustion air/exhaust flow patterns, the catalytic element, and the bypass damper.

The descriptions of combustion options below are courtesy of The Wood Heat Organization. which is a Canadian nonprofit that promotes the use of wood for heating.

The internal design of wood stoves has changed entirely since the EPA issued standards of performance for new wood stoves in 1988. EPA's mandatory smoke emission limit for wood stoves is 7.5 grams of smoke per hour (g/h) for non-catalytic stoves, and 4.1 g/h for catalytic stoves.

Today, all wood stoves and fireplace inserts, and some factory-built fireplaces, sold in the United States must meet this limit. Stove manufacturers have improved their combustion technologies over the years, and now some newer stoves have certified emissions in the 1 to 4 g/h range. The EPA certified emission rate is a reliable number that can be compared from one model to the next, but a one or two gram per hour difference in smoke emissions does not mean much in day-to-day use.

The two general approaches to meeting the EPA smoke emission limits are non-catalytic and catalytic combustion. Both approaches have proved effective, but there are performance differences.

Non-catalytic Stoves

Non-catalytic stoves do not use a catalyst, but have three internal characteristics that create a good environment for complete combustion. These are firebox insulation, a large baffle to produce a longer, hotter gas flow path, and pre-heated combustion air introduced through small holes above the fuel in the firebox.

Non-cats cannot match the even heat output of catalytic stoves, but their owners love watching the beautiful fire they create. The baffle and some other internal parts of a non-catalytic stove will need replacement from time to time as they deteriorate with the high heat of efficient combustion.

Although most of the stoves on the market are non-cats, some of the most popular high-end stoves use catalytic combustion. Because they are slightly more complicated to operate, and the best of them do produce exceptional performance, catalytic stoves are suited to people who like technology and are prepared to maintain the stove properly so it continues to operate at peak performance. Both options have their pros and cons.

Catalytic Stoves

In catalytic combustion the smoky exhaust is passed through a coated ceramic honeycomb inside the stove where the smoke gases and particles ignite and burn. Catalytic stoves are capable of producing a long, even heat output.

All catalytic stoves have a lever-operated catalyst bypass damper, which is opened for starting and reloading. The catalytic honeycomb degrades over time and must be replaced, but its durability is largely in the hands of the stove user. The catalyst can last more than six seasons if the stove is used properly; but if the stove is over-fired, garbage is burned and regular cleaning and maintenance are not done, the catalyst may break down in as little as 2 years. (EPA note: Garbage should never be burned in a wood stove or fireplace.)

For more information, visit The Wood Heat Organization.

Relevant EPA Publications

For a recent list of EPA documents, visit the EPA Library and search using the keywords wood stove and wood stove.

Additional Technical Articles

OMNI-Test Laboratories Publications
OMNI-Test Laboratories, Inc. is a hearth product testing laboratory.

Compilation of Residential Wood Combustion Surveys and Related Studies (PDF, 20 pp, 33 KB)
The authors of this publication are OMNI-Test Laboratories employees. Dr. James E. Houck is a Research Scientist with OMNI Consulting Services and has over 20 years experience with air quality, energy and biomass combustion issues. David R. Broderick is an Environmental Engineer with OMNI Environmental Services with over 10 years experience with air emission and efficiency measurements.