BK steel cats vs ceramic and their new efficiency ratings

[Moomeow91]

I looked and haven’t seen anything that specifically deals with BKs newish product lines. I don’t have a specific question per se but I would like people’s opinion on BK sending their stoves with steel cats vs the ceramic. I do recall a post from BKVP where he seemed to think ceramic cats are better. Perhaps the technology changed and they are able to get more efficiency out of them? Also, there are a couple options on BKs website that claim 83% efficiency (sirraco 30 I think?) so by that token wouldn’t their King 40 get 83% if you swapped in a steel cat?

Mods sorry if this is a little all over the place, these are just thoughts I haven’t found answers to in the forums.

Pic for inspiration!

 

[stoveliker]

I think efficiency ratings differing by a few percent are in practice not noticeable. What's more important is how long they last.

Bkvp will say that legally you should use (or they can only sell) the cat it came with (steel or ceramic) which is the one it was tested with.

Steel cats can light off faster I think, because less mass to heat up
Channels are smaller so more easy to clog, especially if you have high draft (ash flying around). And, in my current experience, are burning spruce (I've never had ash on the face of my cat but I can see some now that I burn spruce).

BK has or will have a cat they claim is better, but costs significantly more. I have not seen data suggesting it's worth the extra dollars. (Nor am I sure it is legal, as the stove hasn't been tested with that new cat...)

I like my steel cat. It's quick to light off. So far that's all I have to say - no complaints.

 

[Moomeow91]

I think in about 8 years or when my cat goes out I’ll try a steel. We use big leaf maple, Doug fir, and white oak, and we don’t have much ash build up if any on the cat when I check it.

 

[stoveliker]

Same for me with oak, maples, pine, sassafras, beech, dogwood, black locust.

This d*mn spruce though... Last time I will have that...

Point is that ceramic cats are less prone to ash build up because of their larger channels. So not having issues now doesn't mean you won't have issues with metal cat. (What your flue height?) Some folks go to ceramic because their metal cat got clogged.

 

[Highbeam]

The only good reason that I can think of that manufacturers are using metal cats is that they don't typically physically break like a ceramic cat can. This would reduce warranty claims from a "broken" ceramic cat during manufacturing, shipping, or other beginner mistakes. Though recently we've been seeing a lot of physically deformed metal cats too but that hopefully is just a manufacturing error.

Metal cats are not more efficient as a rule. Certainly not the reason for the most recent increase in efficiency ratings.

I've used both and prefer ceramic. The much larger cells allow better flow with less likelihood of clogging. Both cat materials work great when new. The ceramics last longer in my experience.

 

[stoveliker]

I think they light off faster; less heat input needed to heat up the substrate.

I would think the better flow equates to less scattering of gas molecules off of the channel walls. And given that the efficacy of a cat depends on the number of contacts with gas molecules (because each contact has a probability of the reaction happening that is lower than 1), and given the same channel length, the metal cat would give s somewhat more complete combustion?

 

[Tron]

I would think the better flow equates to less scattering of gas molecules off of the channel walls.

Not really. It all depends on gas flow vs. cat surface.
Although I don't know if the surface area is different between metal and ceramic cats. Probably not.

 

[stoveliker]

With the same draft, the lower impedance necessarily is due to less scattering off of surfaces as that is what's causing the impedance...

And surface area seems hugely different. I think by a factor of two at least

 

[Tron]

And surface area seems hugely different. I think by a factor of two at least

In favor of the steel cat or the ceramic cat?

 

[stoveliker]

The surface area of the steel cat is far larger.
And with surface I mean active area.

 

[Tron]

I'll observe it. My current (ceramic) cat is on it's last legs, so I'll replace that with the (metal) cat I got for the next season.

 

[stoveliker]

compare your ceramic cat with the pics here

Post in thread 'Catalyst Failure again?!'

Jan 15, 2025

So, I took out my cat today (for the first time) because I ran the sooteater through the system.
It has 6600 measured hours of activity plus some (200?) from the first season when I got the stove.

This is how it looks. Front and back. Interestingly the gasket fell apart but less so than I expected.
A little bit of fly ash in the bottom 1/2" on the back, as you can see. Vacuumed that out and it's as good as new.
New gasket, slide it in, done.

• stoveliker

• 

-edit: I thought you had not seen the metal cat. But you already have one.
Performance comparison is nice to see indeed.
(Though I think without a labbook and proper data it's hard to compare, given that one exchanges an old and tired cat for a new one, and our brain (mine at least) will be used to the old and tired cat, so any new one will appear to be much better.)

 

[Tron]

-edit: I thought you had not seen the metal cat. But you already have one.

yes, but I did not compare them side-by-side.
And I agree with your statement. It's probably the same as with, say, motorcycle tires. Once you got the new ones on, you're always amazed how great they are, compared with the old, run-down ones. Whose deterioration you didn't really notice because it was so gradual.

 

[Highbeam]

The new metal cat will work great and I do believe it has more surface area but don't know if that actually matters. In my experience the steel cat did not clog or deform but did wear out much faster than the ceramics.

 

[stoveliker]

I wonder if the new v.3 metal cat is out yet (the one that ashful tested). If so, I'd like to see if that makes up for the time decrease you see.
(But it only makes sense to spend extra $$ if the testing numbers are out there showing it's worth the $$ - but we've had that discussion before...)

I'm hesitant to get a ceramic cat. I do quite often see "cloudy flames" dancing between the flame shield and the cat (disconnected from any other flame in the firebox) .
I don't know what flame impingement means, but flame surely is touching my cat many hours.

I think (but don't have a manometer) that my draft is reasonable, but I'm not sure whether it's too much for the ceramic cat (no ash clogging though in the metal cat, so that's good). Tall flue but long horizontal section. No smoke roll out, easy snuffing out of the fire. Even a fire of two buckets full of finger-thick sticks with much air between them.

 

[Tron]

I'm hesitant to get a ceramic cat. I do quite often see "cloudy flames" dancing between the flame shield and the cat (disconnected from any other flame in the firebox) .
I don't know what flame impingement means, but flame surely is touching my cat many hours.

That's not going to hurt it.
What a cat does is to lower the initial energy needed for a certain chemical reaction. That doesn't mean that said reaction won't happen without a cat, it's just less likely.
So the stove cat converts CO and O2 to CO2. If the mixture is just right before the cat, and the cat is pretty hot already, that reaction can happen outside the cat, which is what you see as those "disconnected" flames between shield and cat.
And that's not enough to impinge anything. Not even the flames coming from the firebox.

Maybe @BKVP knows, but I strongly suspect that the substrate of the ceramic cats is corundum, or aluminum oxide. And that has a melting point of 3700F, nothing you'd ever see in your stove.
(I've managed to melt down corundum crucibles in the HF furnace I built in University, but that's a different story)

 

[Dieselhead]

I know it’s not apples to apples but this year I’ve burnt about 2 cord of spruce and no sign of cat clogging whatsoever. The steelcats have been an issue in the past threads with ash accumulation where the ash just flys right through the ceramics.

 

[stoveliker]

That's not going to hurt it.
What a cat does is to lower the initial energy needed for a certain chemical reaction. That doesn't mean that said reaction won't happen without a cat, it's just less likely.
So the stove cat converts CO and O2 to CO2. If the mixture is just right before the cat, and the cat is pretty hot already, that reaction can happen outside the cat, which is what you see as those "disconnected" flames between shield and cat.
And that's not enough to impinge anything. Not even the flames coming from the firebox.

Maybe @BKVP knows, but I strongly suspect that the substrate of the ceramic cats is corundum, or aluminum oxide. And that has a melting point of 3700F, nothing you'd ever see in your stove.
(I've managed to melt down corundum crucibles in the HF furnace I built in University, but that's a different story)

I know how cats work. Did some basic uhv electron spectroscopy on a system. (I thought it was an aluminum silicate substrate tho with only a coating of Al2O3 on which the Pd (Ru?) was deposited).
The problem is I think that ceramic cats crumble from flame impingement due to the strong temperature fluctuations that entails.

Midwest hearth says metal cats actually have better flow even tho they have more active surface area. I guess they are smoother.

 

[Tron]

The problem is I think that ceramic cats crumble from flame impingement due to the strong temperature fluctuations that entails.

At least Al2O3 has no problems with that. But then again, we don't know what the substrate actually is. Probably company secret.
And I dont' have an X-ray diffractometer around, that'd be an easy solution.

 

[Highbeam]

Midwest hearth says metal cats actually have better flow even tho they have more active surface area. I guess they are smoother.

It would seem that smoother is less good. Turbulence through the catalyst should increase contact. The very thin cell walls are probably a larger contributor to minimizing resistance.

Ashful has put a ceramic cat into the 30 box. He did not seem to have any regrets about it.

I do think that both metal and ceramic will work great when new and not clogged or deformed.