Why catalytic combustors fail

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  • Cat's are consumable, in that the metals plated onto the ceramic substrate are slowly consumed, but the substrate itself should not fail under proper use.

    Wood stove catalysts are typically warrantied for 5 years (pro-rated), so you should get some money back on your cat that failed after 3 seasons.

    Stove temperature and cat temperature have absolutely nothing to do with one another, and in fact operate in complementary form during the early stage of the burn cycle. Closing down the air supply will reduce stove temps, but will almost always raise catalyst temperature, during this early part of the burn cycle. Do you have a temperature probe on your cat? If not, get one.

    Here are the basic failure modes of a cat:

    - Temperatures over 1800F will cause the catalyst plating to delaminate from the ceramic. Not an instant-death scenario, but definitely shortens its useful lifetime. This is most often caused by resinous woods (walnut is the one that gives me the most trouble), or loading too many small splits, which off-gas too quickly in the early part of the burn cycle. When you see your cat probe climbing to 1800F, open up your air control, which will allow more of the gasses to be burned in the firebox, reducing the load on your cat. If you're stuck between overheating your stove and overheating your cat, then it's time to open the bypass Damper.
    - Direct flame impingement can cause the same type of catalyst plating peeling that you see in a classic cat over-temperature scenario.
    - Plugging with ash is caused by burning garbage, cardboard, improper firestarters, etc.
    - Plugging with creosote is caused by engaging the cat too early, especially burning wet or pitchy woods, such that it's in the smoke path without being in the active (500F+) region, as measured on the cat temperature probe.
    - Poisoning is caused by burning many different chemicals, including painted wood, pressure-treated wood, garbage, plastics, lots of galvanized nails, etc.
    - Cracking and all mechanical failures are caused by rapid temperature change. Hot to cold (throwing a big load of cold wood in and re-engaging before the steam is sufficiently baked out), or cold to hot (engaging a cold cat during a vigorous burn, causing direct flame impingement on a cold cat.

    If I saw a hole thru the middle of my cat, I'd assume thermal shock resulting in cracking and failure, nothing else. I don't think you can get a cat hot enough in a wood stove to actually burn thru ceramic.

    original post in this thread:
    http://www.hearth.com/talk/threads/destroyed-my-cat-howd-it-happen.115170/#post-1539834