For just a moment forget about thermal shock. Instead consider the #1 failure for combustors can be excessive temps. The wash coat, which binds the precious metals to the substrate and adds tremendous surface area (think English muffin in appearance), cannot tolerate higher than 1600F. The ability of the environmental factors to increase draft will most often result in increased burn rates and correspondingly elevated cats temps. If a stove has a cat and does not have a mechanism to prevent excessive temps of 1600F, it doesn't matter what the substrate is made of. After repeatedly hitting 1600F, the wash coat flattens out and is likely to peel away from the substrate. In ceramic based substrates, this can lead to the combustor turning back into a powder. In metal substrates, the appearance may be deceiving because it won't revert to a fluid but instead just become ineffective and then begin plugging. Under Federal law, manufacturers can only sell the same combustor that was used in certification. I worked with EPA to get a suitable replacement program in place for cat stove mfg's. It requires we test a second cat for the low and high burn rates and not exceed .5 gr/hr of the original cat test and it is deemed a suitable replacement. We need this in the event a supplier cannot deliver for multiple reasons. There are no such requirements on after market combustor sellers. Outside of the opinions of what really is a very tiny user population that hang out in wood stove forums, our global position is both substrates have their advantages. Certainly metal with thinner wall tolerances do reach 550F in half the time of ceramic. Ceramics with more mass definitely remain above 550F longer on the exact same application. In excessive draft situations, it is possible for the higher cell per square inch metal substrate to accumulate more fly ash. Globally, 99% of in the field users do not experience any significant difference.
We need more industry insiders like BKVP who can objectively inform consumers without any hard sell or an authoritative attitude. Much thanks Chris.
DNH I hope you don't mind if I tag along. Stalling the CAT? I have seen this term used before on here and made up my mind that I was going to ask the next time I saw it. Stalling out the CAT. What exactly is it? How do I avoid it?
It's possible to have a combustor begin to moving into an active range, but something done by the user result in a pause and then decrease in the cat temp. A good example is loading very large load of frozen wood into the firebox. The introduction of such a large mass of cold, moist fuel can stall and reverse what was an acceleration of cat temps. In our units, a rapid change to the thermostat from high to low before combustion has stabilized can stall the cat. If you wait until more burnable gases are released before changing thermostat settings, you can avoid the stall. Also, don't over think this. Cat stalls happen to new users, those that want to load and run etc. If you find yourself always running behind, get in the practice of bringing the next load into your home, preferably near the stove 24 hours in advance. This permits the load to acclimate to the room conditions and temperatures.