Catalytic Combustor Maintenance - Woodstove Combustors by Condar

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.

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 tell when you need a new 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 the 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 catalytic combustor is beginning to fail. 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 does not come up to temperature as quickly, or that it does not reach temperatures as high as in the past, replace the combustor. Remember to also check the thermometer probe for deterioration and replace if needed.

Creosote Accumulation

A properly operating catalytic stove will make very little creosote, so noticeable accumulation is a sign of decreased combustor performance.

Sluggish Performance

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 "light-off temperature" of 500 degrees for ceramic combustors or 400 degrees 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 amounts of 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 will be 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.

How to Replace 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.

To Clean Your Combustor

Inspect and clean the combustor at least annually. In addition to keeping the unit clear of ash and debris, a major cleaning every other year (or every year in heavy-use situations) will help prevent deactivation of the combustor due to accumulation of deposits on the surface of the catalytic material. DO NOT attempt to remove the combustor from its stainless steel band (if it is so equipped) for this procedure. Always be sure the combustor has cooled completely before handling. Handle with care to avoid breakage.

Using a clean, unused paint brush and a vacuum cleaner, remove any fly ash and loose soot from the combustor. If some cells are totally blocked, use a cotton swab or pipe cleaner. Be careful not to damage any of the cell walls by scraping or scratching them with a hard object.

Pour distilled water and vinegar in a pot to form a 50/50 mixture of the solution. Heat the solution until it is boiling.

Take the cord or coated wire and form a handle for dipping the combustor into the boiling cleaning solution.

cleaning combustor

Slowly lower combustor into the boiling solution and slightly reduce the heat. There is no need to keep the solution boiling once the combustor is in place. Do not allow combustor to bump the bottom of the pot. Allow vinegar solution to flow easily through the combustor cells. Let it soak in the hot cleaning solution for at least 30 minutes.

After soaking combustor in the vinegar solution, dry with a soft towel. Dump the vinegar solution, rinse the pot and boil half of the remainder of the distilled water. Again, place combustor in the boiling water to rinse. With the water just below boiling, let it simmer for 15 minutes. Rinse combustor using this procedure at least twice. Use fresh distilled water each time.

Shake excess water from combustor cells and dry it thoroughly. Place combustor back into your stove according to the instructions in your owners’ manual.

Allow combustor to dry 24 hours before lighting a fire. Steam generated from a damp combustor could damage the catalyst material. If the stove must be put back into operation immediately, place combustor in an oven and heat at 300°F for at least 1 hour. Allow combustor to cool completely before reinstalling it.

Tips to extend the 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 1800º 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. Burn seasoned, dry wood.Wood with a high moisture content burns poorly, creates little heat, and makes it difficult to maintain a catalytic reaction. The introduction of moisture-laden wood to a burning stove can damage a combustor by thermal shock.

• Monitor catalytic temperatures. A proper catalytic monitor will tell you when to engage your combustor, when to turn down your stove, and when your combustor needs to be cleaned or replaced. A catalytic thermometer is the most important single factor in proper operation and longevity of your combustor.

• Do not overfire the stove. It is not necessary to reach high temperatures to burn cleanly in a catalytic stove. Once catalytic temperatures have been reached, adjust the stove for desired heat output. The combustor does not have to glow to be working.

• Avoid direct flame contact with the ceramic material. Under high fire and/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 unit. 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 bypass should be open. This lets steam from fresh wood escape. Meanwhile, your combustor will cool somewhat from its peak temperature.

• Never burn trash in your catalytic stove. Use only seasoned dry wood to avoid chemicals that may damage the catalyst on your combustor. 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!

• Burn only natural wood and small amounts of uncolored paper to ignite your fire. Chemical fire starters, garbage, treated wood, painted wood, colored paper, rubber, and plastics 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.

Protect your Combustor with a Probe Thermometer

The proper way to operate a catalytic combustor is to monitor its temperature with a catalytic probe thermometer. When properly placed in the stove, it tells you when the unit is operating normally, and (most importantly) when you are reaching dangerous temperatures.

With a temperature scale of 2000°F, the catalytic thermometer inserts through a quarter inch hole (usually provided by the manufacturer). Monitoring from this point, you know exactly what your combustor is doing. This lets you burn more efficiently, save fuel, and extend the life of your combustor. When adding wood, you know whether to bypass the combustor to get your stove up to temperature, or whether you can re-engage it right away.

For some stoves and inserts, a thermometer placed in the catalytic access hole may not be easily visible. In this case, a digital Condar 9-86 Catalytic monitor with a remote display can be used to monitor temperatures. A thermometer also signals when your combustor is performing properly. When temperatures don’t rise as far or as fast, you know it’s time to inspect the combustor. When a unit cannot sustain catalytic temperatures even after cleaning, it’s time to replace the combustor. Due to the harsh environment they operate in, catalytic probe thermometers should be replaced along with the combustor (or earlier if needed).

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