Cat overfire - What to do

[Ashful]

This is a thread about what to do when you see your catalytic probe thermometer going high, not the stovetop temp. We all know this happens when your firebox is producing very large amounts of un-burned wood gas, often shortly after turning down the primary air control early in the burn cycle.

Conventional wisdom says to open up the primary air control, to burn more of that wood gas in the firebox, in theory sending less of it as un-burned fuel to the catalytic combuster. However, I believe this theory ignores the reality that when this cat over-temp situation actually happens, you are typically sitting on a very large load of un-burned fuel.

My one or two experiences with this very early in my burning career put me in the situation that raising the air control to burn more of the wood gas in the fire box lead to what felt like a run-away situation. In other words, cat goes hot, raise air control a bit to cool cat, and firebox takes off more. Then try to lower, and cat tries to go even hotter, faster!

What is your experience with this? What have you done that has worked? What have you done that has failed? Do you believe you have the solution?

 

[kennyp2339]

My plan is this, now other stoves my be different due to cat placement in relation to the fire box, On the BK princess I would open the bypass, open the door and let the cooler room air bring things to a manageable level, then turn my air control down and shut the door. Hopefully this will cool things down enough to re engage the cat and have a regular burn.

 

[Poindexter]

Kinda feeling my way on this one. I dunno if a BK can even be made to do that now that I think of it. You would almost have to engage the cat with the loading door open I think, barring mechanical failure type leaks of course.

With my BK if I saw that I would flip the bypass lever first to pretty well empty the firebox of unburned gas in a second or two while bypassing the cat.

I haven't actually got to there with the one and only cat stove I have ever owned in my life. In shoulder season if I engaged the cat too quick or turned the air down to far too fast the stove would just go out.

What I ended up settling on, because it worked consistently, was to run the stove at full throttle in bypass until the cat probe marker needle was up into the active zone to the first hash mark. Once there I could pretty well engage the cat, run it for an hour on high and then turn the Tstat where ever I wanted.

If I skipped either step (heating to hash, adjust Tstat before running for an hour on high) I could pretty much count on the stove going out. But I am on only 13 feet of flue and I don't have this problem in cooler weather, just in the shoulders.

Curious to see what works for others. There was some BK discussion earlier in the season about how fast you can turn the Tstat after engaging the cat, I wasn't familiar enough with my own stove at the time to find a common theme in the commentary on that one.

I guess if you have a box full of dry stuff one a full bed of coals and the whole box is completely engulfed when you close the door and engage the cat - well, I have done that and I get a bunch of smoke out of my stack without the cat taking off much anymore than usual; I wonder if MC of the wood contributes to less steam enough to make a difference.

 

[tarzan]

I have never and hopefuly will never have a cat overtemp situation like I think you are talking about, ie catastrophic? ILeo don't know much about the stove you are running but with mine, I think I could simply bypass the cat.

 

[fox9988]

open the bypass

bypass the cat.

^This^

 

[Ashful]

This may be possible, for me. I cannot run indefinitely on a large load in bypass mode, as the firebox temp will go too high after some time, but I could definitely run for a period (10 minutes?) in bypass.

This stove is probably a little different from the BK's, in that I'm looking straight thru the front doors and directly up the flue, with the cat bypassed. It's literally a straight shot. Not the greatest for slowing the burn, but I think the idea was getting the load going and out of bypass as fast as possible, when the stove was designed.

 

[NinjaTech]

I think it would be fairly difficult to actually damage a cat from a single (or even several) over fire incidents. Turbojoe may show up here, he tests cats for a living and the temps and times they run those things are are pretty insane. I have intentionally tried to overfire mine before just to see how likely it would be in normal running to ever have this happen. I had to try my hardest to actually get to a temp that started to seem unsafe, it was not an easy task.

 

[tarzan]

This may be possible, for me. I cannot run indefinitely on a large load in bypass mode, as the firebox temp will go too high after some time, but I could definitely run for a period (10 minutes?) in bypass.

This stove is probably a little different from the BK's, in that I'm looking straight thru the front doors and directly up the flue, with the cat bypassed. It's literally a straight shot. Not the greatest for slowing the burn, but I think the idea was getting the load going and out of bypass as fast as possible, when the stove was designed.

This may be possible, for me. I cannot run indefinitely on a large load in bypass mode, as the firebox temp will go too high after some time, but I could definitely run for a period (10 minutes?) in bypass.

This stove is probably a little different from the BK's, in that I'm looking straight thru the front doors and directly up the flue, with the cat bypassed. It's literally a straight shot. Not the greatest for slowing the burn, but I think the idea was getting the load going and out of bypass as fast as possible, when the stove was designed.

Definately different. I can control the burn as well with the thermostat wether bypass is open or shut.

 

[HarvestMan]

Do you believe you have the solution?

Not the solution you are likely looking for, but it is the one I use.

I under load my stove. Max load for my stove is 43 pounds; I will usually have loads between 18 and 24 pounds. Max cat temp I have seen is 1200 degrees; that said, I have a 25 year old stove with rear bottom draw into a back mounted cat chamber, not as likely to get as hot as the newer top mounted cats.

 

[Ashful]

I think it would be fairly difficult to actually damage a cat from a single (or even several) over fire incidents. Turbojoe may show up here, he tests cats for a living and the temps and times they run those things are are pretty insane. I have intentionally tried to overfire mine before just to see how likely it would be in normal running to ever have this happen. I had to try my hardest to actually get to a temp that started to seem unsafe, it was not an easy task.

You have been out of the loop! I just completely melted two cats, one hitting something over 2000F, the other failed at only 1800F. Both cases were a failure of the steel can around the ceramic cat, not the ceramic itself.

Not the solution you are likely looking for, but it is the one I use.

I under load my stove. Max load for my stove is 43 pounds; I will usually have loads between 18 and 24 pounds. Max cat temp I have seen is 1200 degrees; that said, I have a 25 year old stove with rear bottom draw into a back mounted cat chamber, not as likely to get as hot as the newer top mounted cats.

Yeah... if I load my stove only 70% full, then the cat usually stays at 1000F - 1200F. It only hits 1700+ when I'm on a very full load of high-BTU wood. Trouble is, full loads of high-BTU woods are what it takes to half heat this place. Those who know the house know why I say, "half heat".

 

[NinjaTech]

How did you melt two cats?! Got picture? If the can actually melted that would be 2500F +

 

[Ashful]

Yes, plenty of photos, but let's not derail this thread. Take it over to: https://www.hearth.com/talk/threads/yikes-1035f-on-outside-of-stove.140222/

All I'll say here is that the deformation you see described in that thread happened at 2050F in the one stove, and 1850F in the other. I've never seen this deformation happen with Jotul OEM, Corning Long-Life, or Condar cat's. It is a failure apparently unique to the Applied Ceramics cat cans.

Again, let's keep the failing cat conversation over at the other thread.

 

[jeff_t]

I think burning with some flame could help to keep from getting to that point, but I don't think it's going to bring the temp down in any reasonable amount of time. I think the only way to do that is bypassing.

This may be possible, for me. I cannot run indefinitely on a large load in bypass mode, as the firebox temp will go too high after some time, but I could definitely run for a period (10 minutes?) in bypass.

This stove is probably a little different from the BK's, in that I'm looking straight thru the front doors and directly up the flue, with the cat bypassed. It's literally a straight shot. Not the greatest for slowing the burn, but I think the idea was getting the load going and out of bypass as fast as possible, when the stove was designed.

Can your primary air be cut back far enough to keep firebox temps down?

This is down thru the flue collar with the bypass open. Definitely a straight shot up the pipe. That's some creosote on the floor of the firebox from sweeping today.

 

[Ashful]

Hi Jeff,

Yes, I can always control my firebox temp, probably almost as good as a BK. I can hold under 400F on a VERY large load of dry wood.

Giving this some more thought, I've been turning down the stove to the lowest setting as soon as I get cat temps > 1000F. I think that sometimes, depending on the load, sometimes it will start off-gassing quickly after an hour or so. This is what's driving my cat temp way up. Taking a cue from the BK manual, I see they suggest running the stove on a medium-high setting for 20 - 30 minutes after a reload, although their given reasoning is to clean firebox and glass deposits. In any case, it may be possible that running my stove more open for longer after a reload might burn off more of the volatiles that are driving up my cat temp.

I'm experimenting with exactly that now. It seems to have peaked at 1520F on the cat probe, and is now holding 1480F, on a medium setting. Stove top has been holding 480F, and single-wall stove pipe is 200F. Looks very good. Only trouble is, I'm not sure how this will fit into my morning reload schedule, which is quite tight.

Thanks!

edit: I just checked in on it again, holding 1480F on cat probe, 500F stovetop, and pipe is up to 300F. Still running air half open, 70 minutes into burn. Going to start turning down soon.

 

[Charles1981]

This probably doesn't help much but on my VC 2n1 Encore I have yet to install a Cat Probe. I should. I really should, but I just follow the guidelines in the owner manual, and go by stove top temperature. I have a second stove probe on the cast iron flu collar as heat leaves the stove. I can tell from that if the catalyst fired off but it is not very scientific.

If the stove top is 500-600 and the flue collar probe is 300 the cat didn't fire off or the load is all coals now.

If the stove top is 500-600 and the flue collar probe is >450 after closing the bypass and engaging the cat and it stays there or creeps up to 500-600 I know the cat is firing off.

If I have every had over fire conditions in the cat combustion chamber I will never know probably. I have left the bypass open a few times before unfortunately and gotten stove top temps >800 and that isn't fun.

But so far 3 season in and things are going well. I can also tell the cat is active because it can often be easily visualized glowing through the refractory slits.

I think I am afraid of what I might find if I had a cat probe lol.

 

[Ashful]

Charles,

It's been about two years since I looked at the Encore 2-in-1, so my memory is rusty, but doesn't that stove have a non-catalytic reburn system upstream of the catalytic? If so, it's not likely you will suffer catalyst overfires, as the non-cat system can help to burn off a lot of the volatiles before they ever reach the cat. The only situation which might lead to a cat overfire in such a stove might be killing the air real fast, where you get the firebox temps down low while the wood is really outgassing fast.

 

[Ashful]

Well, it does appear maybe my memory on running the Firelight more open might be correct. Running it half open, for an hour, I worked myself into a corner where it was going so well in the firebox that I couldn't shut down the air without going cat > 1700F. Stovetop thermo was safely holding 550F, so I suppose more time would've gotten me over that hump, but that's not the situation I want to be in when trying to get out the door to work in the AM.

Maybe it's just not possible to bake-out a full load this big (Jotul Firelight is a large stove) in under an hour? BK gives you the advantage of the thermostatic damper, to sort of take care of this automatically, but it does seem likely a traditional (manual) stove might just take more time than I'm giving it to get to the point where you can shut'er down tight.

 

[Charles1981]

I think they shoved the catalyst IN the non-catalytic reburn system chamber....So I would think that the cat in the VC is exposed to even higher temperature than normal because it takes a higher temp for the ole never burn system to work...

But I am not 100% positive... but the Cat is just shoved in a big ole chamber...and to convert it to non-cat burning you just remove the catalyst. I figured it was all happening in the same chamber because thats the only one I know about.

 

[Ashful]

Interesting.

Well, I'm cruising at 1400F now, so it's time for bed. Here's what has been learned tonight:

1. Getting a big load of wood underway, and shutting down too fast, is recipe for some high cat temps. Of course, we all knew this already.
2. It might take a REAL long time to burn a very large load of wood down to where you can shut your stove down tight, without causing excessive cat temps. I had previously thought 10 minutes in bypass + 20 minutes incrementally turning it down to nil, was plenty of time. It appears that perhaps an hour or more is required, and some manufacturers do allude to this sort of timing in their manuals.
3. Going into bypass and shutting the air for 5 minutes will slow you down right quick, even on this old stove. Jotul states it should be done for 5 - 10 minutes, and that the stove should not be left unattended during this time.

Remaining question: Could I have given the stove more time, and gotten "past the hump," without going to bypass. I think, "yes," but I'll have to experiment with that another night.

 

[Poindexter]

Couple items.

On my short stack I leave the morning load on high, 3/3 and go to work, the wife shuts it down later. BKVP has posted at least twice most of the outgassing takes place in the first hour of burn time, so turning the TSat down after most of the outgassing has already happened isn't a big deal.

This works for me and my observations, but I am on a short 13-14' stack. I am out on a limb here, but I would think turning the Tstat down on a big load at 20-30 minutes with a tall stack and low ambients (lots of draft) might be a problem. Or not, it might be the taller stack and greater draft get the load off gassed faster.

I would hazard a guess that there is a ambient temperatrue range of danger zone and not far below that is a temperature range where you would leave the stove set on full throttle anyway.

I just don't see my BK overfiring at all without a leaky loading door gasket or similar air leak outside the design parameters.

 

[rideau]

I don't have a catalyst probe, have a flue temp probe.

Heated with a Fireview for 7 years, ceramic cat, I think Applied Ceramics. I never had a problem with the cat deteriorating.

Woodstock recommends burning the stove with only very small flame coming off the wood for maximum efficiency. I usually do this. When it is very cold, I burn with some secondary flame in the box, and stovetop temps peaking at about 500, gradually dropping to 400 range where they stay for a long time before dropping into the 300's. This has never resulted in any issue with the cat.

I use no kindling, load my splits onto a small coal bed, coals raked forward and into the two corners, center left open for easy entrance of the accessory air at the bottom front of the stove (small hole there). I have flames before the door is closed. Have a very strong draft, so with flue damper open the fire is very active and the chimney roaring within a minute or so. CLose the flue damper to only a bit open (maybe 75-80% closed), and the air by at least 1/4, depends on weather conditions. As soon as the flue probe temp is 500, I close the flue damper completely, the air to at least 1/2, and, after a minute during which the flames drop enough for me to actually observe the secondary flames rolling out of the secondary air holes (rather than the entire box being a mass of flames), I engage the cat (at this point I feel I have enough flames to not get flames getting through the tortuous path to my cat: in your case, I would close the air almost all the way in two or three steps, to get down to very mild flames, then engage the cat, since you have a direct path to the cat). As long as your flue temp is 500 when you engage the cat, it should light off just fine. With the air very low, I don't know why your cat would get too hot. The wood should off gas fairly slowly and give your cat time to burn the wood smoke, if the air being fed to the box is low.
At the lowest air setting, my stovetop usually creeps to 400 and stays there. I'm sure my cat isn't unreasonably hot.

If you need to burn with a hotter stove to heat your home, you may need to get a different stove. Pretty hard to burn hot with a lot of flame and not damage a cat that is in a short direct path from the firebox.

 

[tarzan]

Well, it does appear maybe my memory on running the Firelight more open might be correct. Running it half open, for an hour, I worked myself into a corner where it was going so well in the firebox that I couldn't shut down the air without going cat > 1700F. Stovetop thermo was safely holding 550F, so I suppose more time would've gotten me over that hump, but that's not the situation I want to be in when trying to get out the door to work in the AM.

Maybe it's just not possible to bake-out a full load this big (Jotul Firelight is a large stove) in under an hour? BK gives you the advantage of the thermostatic damper, to sort of take care of this automatically, but it does seem likely a traditional (manual) stove might just take more time than I'm giving it to get to the point where you can shut'er down tight.

I think you may have answered your question with this post. I don't suppose a smaller load in the AM could be an option?

Rideau said "if you need to burn with a hotter stove to heat your home, you may need to get a different stove"

Not sure what you could recommend? There are shopping malls smaller than his home

 

[rideau]

He pointed out that he has a very short direct path to his cat. He may be better off with a stove that has a longer path to the cat, which would allow him to burn with more flame without concern about flame impingement on the cat/excessive cat temps.

That's all I meant.

 

[Ashful]

Tarzan really summarized everything above pretty well, so answering his post...

I think you may have answered your question with this post. I don't suppose a smaller load in the AM could be an option?

Anything is possible. Our longest burn cycle is on that AM load, since we're out of the house at least 11 hours, during that cycle. So, I'm usually loading for a 12+ hour burn, in the AM. If I load smaller, it will still go toward reducing my oil bill, but I fear temps will really drop in the late afternoon, right before the automatic T-stat starts calling for heat. In other words, on a smaller load I'd maybe be keeping that half of the first floor warm 3/4 of the day, only to lose it when it would be most beneficial.

Rideau said "if you need to burn with a hotter stove to heat your home, you may need to get a different stove"

LOL... there aren't many bigger heaters than a Firelight 12. The word "monster" is used by most F12 owners, in describing its capacity.

Not sure what you could recommend? There are shopping malls smaller than his home

That is a slight exaggeration... but only slight.

He pointed out that he has a very short direct path to his cat. He may be better off with a stove that has a longer path to the cat, which would allow him to burn with more flame without concern about flame impingement on the cat/excessive cat temps.

You mis-read me there, rideau. What I meant is that in bypass mode, I have a very straight shot up the chimney, which is why I don't like running long periods in bypass on a very full load. However, this is a down-draft catalytic stove, which is about as indirect a path as you can imagine, with the cat engaged. Exhaust gas gets drawn thru ports in the rear of the firebox (mid-height), pulled down into the lower chamber of the cat assembly, flows vertically up thru the cat combuster, then out thru side ports into the smoke chamber, and then up the flue. It is a very good system, in most regards, but I think Jotul should have used a much larger catalytic combuster (more surface area) for such a large firebox. That is the only potential flaw I have found in three years of running multiple Firelight 12's.

 

[Highbeam]

On the BK, we don't have to worry about overfiring the cat. The cat meter provided and the owner's manual do not indicate operation based on a cat temp, only to engage the bypass when the cat goes active. There aren't even numbers on our cat meters to reinforce this. Just set the intake to a setting within the specified standard range and go eat a hamburger.

What I would warn readers about is opening the bypass to cool a stove. That's a great way to ruin your stove. The bypass door is not designed to pass that much heat and you can warp and destroy the bypass gasket retainer assembly. Replacement requires cutting and welding for those that blow too much heat up the bypass.

Also, don't go opening the loading door. That will send a bunch of cold air into the hot cat and thermal shock is the biggest cause of cat failures on the BK line. Just turn down the stat and enjoy the glow.