Care and Maintenance of
Woodstove Catalytic Combustors

Tips to extend life of your Combustor

  • Avoid Thermal Shock: Thermal shock occurs when your ceramic combustor is heated or cooled too quickly. Thermal shock can lead to cracking or crumbling of the combustor’s ceramic honeycomb. A very active combustor can easily reach temperatures over 1800°F. When a fresh load of fuel is put into the stove, relatively cool gasses and steam are released from the wood as it ignites. If a very hot combustor is re-engaged at this phase, the sudden drop in temperature may damage ceramic. To avoid damage, always allow fresh wood to get burning well before re-engaging. The introduction of moisture-laden wood to a burning stove can damage a combustor by thermal shock.

  • Monitor catalytic temperatures:. A catalytic meter will tell you when to engage your combustor, when to turn down your stove, and when your combustor needs to be cleaned or replaced. It signals when to avoid damaging catalyst at temperatures exceeding 1500° Fahrenheit (1050° Celsius).

  • Don't overfire the stove: It is not necessary to reach ultra-high temperatures to burn cleanly in a catalytic stove. Once 1000°F catalytic temperature has been reached, adjust the stove draft for study heat output. The combustor does not have to glow to be working.

  • Avoid direct flame contact with the ceramic material: Under high fire or high draft conditions, flames can be pulled up into the combustor, which can lessen the catalytic reaction, shorten combustor life, and possibly damage the stove. Immediately reduce draft if flame contact is observed!

  • Leave damper in bypass position until wood is burning nicely: Even when adding wood to a burning stove and the combustor is already hot, the stove bypass should be open. This lets steam from fresh wood escape. Meanwhile, your combustor will cool somewhat from its peak temperature.

  • Burn Seasoned, dry wood: Fuel with high moisture content burns poorly, creates little heat, and makes it difficult for your combustor to recover heat from your wood – which is the primary benefit of your catalytic combustor!

  • Never burn trash in your catalytic stove: Use only untreated natural wood to avoid chemicals that may damage the catalyst on your combustor.

  • Use only natural kindling and small amounts of uncolored paper to ignite your fire: Chemical fire starters, wax, etc. can poison the combustor, rendering it inactive.

  • Inspect and clean your combustor: Whenever you clean your stove and chimney, your combustor should also be cleaned. Or more frequently if needed.) Evidence of creosote, fly ash, and plugging suggest a need to adjust your burning practices before a problem arises.

Inspecting the condition of combustor

Inspect your combustor before each heating season, and during the season if your stove's performance seems to change. Ash (a fluffy light grey powder) and soot (a darker granular material) accumulate on the combustor surfaces in normal use. Ashe accumulates both as a result of the smoke being burned within the combustor and by being carried out of the firebox by the chimney draft. Soot is often the result of previous creosote deposits having been burned off or foreign materials having been burned in the firebox.

Signs of clogging include reduced heat output and sluggish performance. If inspection reveals combustor is becoming clogged, brush away soot or ash with a soft brush or vacuum with a shop vacuum cleaner. If soot or ash remains in the cells, gently dislodge with cotton swab or pipe cleaner. Avoid using high-pressure air or a hard object to push the material out, as these may remove catalytic coating.

Creosote is a tarry brown substance that accumulates on the combustor if it is engaged before smoke is hot enough to activate the catalyst. Once in place, creosote “masks” the catalytic material on the surface of the combustor and does not allow the combustor to become active. At this point, the combustor can quickly accumulate additional creosote.

The only way to remove creosote is to burn it off the combustor. To do this, you will have to burn your stove at higher than normal temperatures before engaging the combustor, and leave the combustor engaged until it has all burned away.

This is a potentially damaging procedure to both the stove and combustor. Avoid the necessity of this tricky procedure by monitoring catalyst temperature for properly engaging bypass, which means your combustor won't accumulate creosote in the first place.

If the combustor is coated with creosote, it is possible the pipe and chimney are also coated with creosote. The very hot fire needed to clean the creosote off your combustor could also ignite this accumulation. Check the chimney and pipe for build-up. Clean them if needed before attempting to burn off the combustor creosote with a hot fire.

Cracking and crumbling of ceramic combustors result from thermal shock, which occurs when the combustor is heated or cooled too quickly (see tips for proper operation). A cracked or slightly crumbling combustor will continue to work well as long as there are no large pieces of the ceramic missing.

If your combustor has a metal band around the ceramic, this is designed to hold the ceramic in place even when cracked. Handle a cracked combustor with extra care to avoid losing pieces. If large parts of the ceramic are missing or if the combustor cannot easily be put back into the stove due to cracks, replace the combustor.

A catalytic combustor is made of ceramic or steel "honeycomb" which has a coating that contains the catalytic metals palladium and/or platinum. On a new combustor, this coating will be brown, but after the first fire it will turn gray. This brown coating is functional inside the cell walls when wood smoke passes though the combustor.

Occasionally, this coating may come off the honeycomb, revealing the white ceramic or bright steel beneath. This can be caused by cleaning the unit too aggressively, such as with a stiff brush or with high pressure air, or by abrasion caused by ash particles in the case of a very strong chimney draft. Areas where the coating is peeling may appear "fuzzy" or may have patches of white. In some cases, turning the combustor over to expose the opposite side will gain an additional season of use, but you should replace a combustor which is severely peeled or mostly "bright."

How to know when you need to replace the combustor

A visual inspection of your combustor will reveal any obvious problems, but it doesn't tell the entire story. Sometimes what appears to be a flawless combustor will be failing to do its job properly. In addition to a visual inspection,your stove's performance will tell you a lot about how the combustor is functioning and whether it needs to be replaced.

  • Decreased Heat Output: If your stove seems to be making less heat than it has in the past, this indicates your catalytic combustor is beginning to wear out. Since many factors can affect heat output, such as the quality of your wood and draft conditions, the only sure way to tell if your combustor is not coming up to temperature is with a catalytic thermometer (find out more about Condar's Catalytic Probe Thermometers and Digital Catalytic Monitor.) If it indicates that the combustor no longer comes up to temperature as quickly, or it does not reach temperatures as high as in the past, replace the combustor.

  • Creosote Accumulation in your stovepipe: A properly operating catalytic stove will allow very little creosote in your chimney flue. Stovepipe creosote is a sign of decreased combustor performance.

  • Sluggish Draw: If your stove seems to draw well when the combustor is not engaged, but draws poorly when the combustor is engaged, this is a sign of combustor failure. Remember, however, your catalyst won't work until it has reached a "light-off temperature" of 500° for ceramic combustors or 400° for STEELCAT combustors.

  • Smoke from Your Chimney: Your combustor was designed to dramatically reduce the amount of smoke that comes out of your chimney. If you see significant smoke while the combustor is engaged, this is a sign that the combustor is not doing its job. A properly operating combustor produces water vapor as a byproduct of combustion, so be sure that what you see coming from your chimney is smoke rather than water vapor. Water vapor is white and will dissipate quickly in the atmosphere. Smoke is darker, thicker and tends to persist in the atmosphere.

    Please Note: Water vapor may condense on the walls of your chimney. This may be especially noticeable on exterior masonry chimneys that tend to be cooler. The water that forms may have enough condensed smoke in it to smell strongly of creosote, but you should not see any significant accumulation of creosote on chimney walls.

  • Replacing the Combustor: It is important to replace your combustor with the same size and thickness as the original one. Note whether the original combustor was banded in metal. The metal band helps to hold the combustor together.

    In some cases, the combustor may fit into a holder supplied by the stove manufacturer, which will typically be made out of heavy-duty steel or cast iron. Do not discard this holder, as your new combustor will need to be fitted into it prior to installation.

    Some stoves also require a gasket to insure that the combustor fits back into the stove properly and that smoke cannot go around it when it is engaged. Special catalytic gaskets are required to withstand the intense heat around the combustor. These gaskets, such as Condar's CatGard™ may fit loosely when first installed, but swell to many times their size when heated. It is sometimes useful to tape the gasket onto the combustor with masking tape to keep it in place. You may leave the ordinary masking tape in place, as it will burn off when the combustor is used.

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