Please help me understand new/hyperactive cats

[Guess52]

Recently installed a blaze king princess insert. Overall we're very happy.

Burning ash that was standing dead for years and reads 14-20% moisture at room temp, fresh split, along grain. I have been reloading with the cat still active and running at highest thermostat setting until I have full flames and outside char on the logs. Typically this means I'm turning the air down when the cat probe reads at the very top of the active range.

After our initial small paint curing fires I was a somewhat alarmed to see the cat temp probe far exceed the active range (6 o'clock position). Calling BK, they told me not to be alarmed and just burn as needed for my heating needs, the cats start overactive and then calm down over time. That said, the manual also alludes to damaging the cat by overheating it...

With some colder nights I've been loading more fully and not turning the thermostat down as completely. This morning about 40 minutes after reload the cat thermostat had completed over a full 360 and made it back into the labeled inactive zone (8 o'clock) but was glowing cherry red and obviously cooking. Fan was on high, thermostat was at 25%.

1) when should I be concerned about cat temps causing damage?

2) I'm not really understanding the "hyper active" description on a chemistry / physics basis. My naive assumption would be that the cat temp is defined by the flux of unburnt wood gases passed through it and the rate at which it can transfer heat out to the stove/room. Unless it were to dramatically lose efficiency (and presumably increase emissions) what's going to keep it from overheating similarly after break-in if I'm putting the same amount of fuel through it?

Thanks so much.

Draft is 24' of pretty straight shot insulated chimney liner if that's relevant.

 

[Wisco Shepherd]

Hyperactive catalyst is normal when new. She'll settle down.

 

[Guess52]

I know that it'll settle down. I'm hoping to understand what that means. Is is the case that it's no longer burning all the gases during peak off-gassing?

Likewise, the BK thermostat doesn't show temps but the manual states "
Extreme temperatures (above 1800°F, or
1000°C.) at combustor surface can cause the catalysts to peel. Over firing and flame impingement on the combustor are primary
causes."

If the BK probe anything like this condar cat stove probe (Pic attached), it suggests I'm well, well over 1800F by the time the needle gets to 6 o'clock let alone does a 360. I know there is a warranty but I'd still prefer not to need it.

 

[Wisco Shepherd]

I know that it'll settle down. I'm hoping to understand what that means. Is is the case that it's no longer burning all the gases during peak off-gassing?

Likewise, the BK thermostat doesn't show temps but the manual states "
Extreme temperatures (above 1800°F, or
1000°C.) at combustor surface can cause the catalysts to peel. Over firing and flame impingement on the combustor are primary
causes."

If the BK probe anything like this condar cat stove probe (Pic attached), it suggests I'm well, well over 1800F by the time the needle gets to 6 o'clock let alone does a 360. I know there is a warranty but I'd still prefer not to need it.

Definitely normal concerns, it's possible it's not burning all gasses but likely is doing a good job. When do you close your damper? Sometimes waiting for the catalyst gauge to be in the active is too long. Most around here engage their bypass at 500-600 internal flue temp or when the fire is burning robustly. I have a hybrid stove and once I see my secondary burn system kick on I know I'm good to engage the Cat.

 

[Guess52]

I'm typically reloading while the cat is still active. I close the bypass to engage the cat immediately after I close the stove door. I turn the thermostat/air down once I have char on all the wood. Timing depends on volume of ember bed and load.

 

[gthomas785]

OP is asking an interesting physics/chemistry question and I'll try my best to answer in a way that makes some sense.

It is pretty intuitive that a brand new catalyst will have the most "active sites" of catalyst material where unburned organic material can bind and be oxidized at the highest possible rate in a confined volume which leads to a high temperature. After the catalyst is used for a while, some of the catalyst material becomes masked by contaminants such as ash. Also, the surface becomes annealed by the high temperature and loses surface area. Both of these will reduce the available active sites for the chemistry to occur.

Now, as to whether you lose efficiency after this initial degradation occurs, the short answer is no. BUT it's complicated. The high temperature of the catalyst during initial stages of the burn when you have the most organics offgassing (even after some catalyst degradation) is usually sufficient to promote secondary combustion of flue gas downstream of the catalytic element without directly requiring each molecule to interact with the catalyst. So although the catalyst efficiency itself is reduced, you will still get fairly complete secondary combustion and the heat will be distributed over a larger volume, leading to a lower measured cat temperature. At lower temperatures where the catalyst is the only driver of secondary combustion, there will also be a lower volume of organics passing through the catalyst so it will be able to keep up over a wide range of burn conditions.

Eventually, as the catalyst surface continues to degrade there will be more heat input required to sustain secondary combustion, and you'll see some incomplete secondary action at lower temps. That's when you'll see the dreaded smoke from the chimney and know it's time to clean or replace the catalyst.

 

[Todd]

I don’t think those cat probes are the most accurate instruments. An Auber thermocouple probe would be much more accurate imo.

 

[stoveliker]

I do think that the cat settling down is indicative of the cat doing less. (The cat temp is primarily originating in how much it combusts.)

So I do think that these initial burns will be the most clean exhaust that you can get.

Yet, there is nothing one can do about this.I think these cats are made such that they can't sustain their initial combustion efficiency/heat production (they degrade, aka "settle down") - but are able to maintain a sufficient combustion efficiency over the long term.

(I do think that the production of the cat, its wash coating surface area and distribution of active metals, is unlikely to be cheaper if they would be produced at a lower "quality point", i.e. at the lower, more steady state efficiency they reach when they have degraded a bit.)

To me this asks the question whether the EPA testing is done with this initial high efficiency, or (also) later in the cat life. (I vaguely remember that some later datapoints are also involved, but I am not sure.)

 

[Todd]

I do think that the cat settling down is indicative of the cat doing less. (The cat temp is primarily originating in how much it combusts.)

So I do think that these initial burns will be the most clean exhaust that you can get.

Yet, there is nothing one can do about this.I think these cats are made such that they can't sustain their initial combustion efficiency/heat production (they degrade, aka "settle down") - but are able to maintain a sufficient combustion efficiency over the long term.

(I do think that the production of the cat, its wash coating surface area and distribution of active metals, is unlikely to be cheaper if they would be produced at a lower "quality point", i.e. at the lower, more steady state efficiency they reach when they have degraded a bit.)

To me this asks the question whether the EPA testing is done with this initial high efficiency, or (also) later in the cat life. (I vaguely remember that some later datapoints are also involved, but I am not sure.)

I think the EPA tests require a stove be broke in for 50 hours before the test. What that entails I do t know.

 

[stoveliker]

50 hrs might not settle it down, at least completely so (in my case it took about a cord).

 

[Highbeam]

I too recall that the testing is done with an "aged" catalyst.

As to the OP question, I think there is also a feedback loop. The extra hot cat produces an extra strong draft that sucks more air through the fixed minimum air orifice even when the thermostat is closed and feeds the primary fire extra air. When that cat is ripping hot, the stove is ripping hot too and I think the minimum output is a bit higher than it will be later in the cat's life.

As the cat settles down this whole feedback loop settles down so you can get longer, lower, burns while not missing much smoke.

 

[stoveliker]

I too recall that the testing is done with an "aged" catalyst.

As to the OP question, I think there is also a feedback loop. The extra hot cat produces an extra strong draft that sucks more air through the fixed minimum air orifice even when the thermostat is closed and feeds the primary fire extra air. When that cat is ripping hot, the stove is ripping hot too and I think the minimum output is a bit higher than it will be later in the cat's life.

As the cat settles down this whole feedback loop settles down so you can get longer, lower, burns while not missing much smoke.

Reading your post, I thought you were going to reach the opposite conclusion: an extra hot cat produces extra strong draft, which sucks more air into the firebox, which increases primary combustion and thus decreases cat-fuel.

Also, an observation from my stove: when the cat is so ripping hot, the stove does not (and often is not) need to be hot. Indeed, it's often the opposite; a mediocre output pile of glowing chunks and a bright yellow light from the cat.

 

[Highbeam]

Reading your post, I thought you were going to reach the opposite conclusion: an extra hot cat produces extra strong draft, which sucks more air into the firebox, which increases primary combustion and thus decreases cat-fuel.

Also, an observation from my stove: when the cat is so ripping hot, the stove does not (and often is not) need to be hot. Indeed, it's often the opposite; a mediocre output pile of glowing chunks and a bright yellow light from the cat.

In my experience the primary fire stays smoldering. No active flame when the fresh cat is overactive so the primary "fire" is not eating smoke. Since I almost always run a cat only fire, the cat is the source of heat and a ripping hot cat means higher output.

It's all academic, we don't have any control really.

 

[stoveliker]

ah. That's a difference; I regularly do have some flame (running the stove from the basement for a 1700+825 sqft home, I often run at the border of flame and no flame - seeing the Tstat open and close by the appearance of flames).

Academic is good


I do think an even better explanation/illustration of this hyperactive cat issue (that pops up in *every* new BK user post here) might be useful in the manual. Even if only to avoid many questions sent to dealers or BK themselves.
THe many questions here suggest that many folks don't get it sufficiently from the manual. It'd be good to avoid initial misunderstanding, or worse frustration or fear. @BKVP