Cat overfire - What to do

[Ashful]

Lucky for you, Highbeam. This is more for all the non-BK / traditional cat stove owners out there.

I've been putting more thought into the situation, and it seems to me there are two ways to run a cat stove:

1. I have been getting the load charred and baked out enough to where exhaust temps hit 550F. Engage cat, get cat lit off, and turn down the stove for a nice controlled burn. 99% of the time this will prevent the load from ever getting going well enough to produce problematic rates of wood-gas production, but perhaps 1% of the time a shift in the load or other factors will cause it to out-gas so fast that your cat temp can skyrocket. Doing 300 - 400 loads per year in a stove means at least a few cat over-fires every year.

2. Burn out the load until there are simply not enough volatiles left to cause a cat overfire, when you turn it down. This has the disadvantages of shortening burn time, running the stove up hotter than you might otherwise want or need, and requiring you to stay in the house longer after a reload, but leaves almost no possibility of cat overfire after turning down the stove.

I've been doing version 1, but I'm going to spend the next several days experimenting with version 2.

 

[rideau]

Very interesting, and thanks for the correction.

I've never seen my cat take off, in11 years burning, using # 1 above, so I have no advice. Will be watching, and be interested to see if anyone chimes in who has had the problem and found a solution to prevent it, while still always loading and burning in manner #1

 

[Highbeam]

Lucky for you, Highbeam. This is more for all the non-BK / traditional cat stove owners out there.

Well since only you seem to have that particular stove and since other BK posts were made about opening the bypass to dump heat I thought it was important to mention the risk.

 

[mellow]

I get my flue probe up to 550-600 then close the bypass to engage the cat which is usually around 800 by this time, then I immediately cut the air in half, then in 5 minutes I close the air sliders all the way and crack one of them ever so slightly open.

I also try to put all my hot coals to the back of the stove so they are directly under the bypass and then stack my wood E/W on top of that, that way I get a back to front burn. I also try to get an air trench on the ashes down the middle from back to front.

 

[tarzan]

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.

Thanks for the heads up but as BK owners I think we were only theorizing what we would do if the cat went nuclear! Remember though, his wood fire temps never got very high during the cat killing.

 

[Ashful]

Well since only you seem to have that particular stove and since other BK posts were made about opening the bypass to dump heat I thought it was important to mention the risk.

Don't mistake my envy for hostility.

I've never seen my cat take off, in11 years burning, using # 1 above, so I have no advice.

Actually, that itself is good info. In using #1 above, there is one thing I've been doing, upon which I did not elaborate. If the cat hits only 600 - 800 on initial engagement, I will often let the stove run a while at 1/4 air, waiting for the cat to climb above 1000F, before lowering the air. This is mostly only an issue on large loads of oak, which are slower to really bake out and get cat temps up, versus other woods. In any case, it is worth mentioning, as the sole difference to what you seem to be doing.

I get my flue probe up to 550-600 then close the bypass to engage the cat which is usually around 800 by this time, then I immediately cut the air in half, then in 5 minutes I close the air sliders all the way and crack one of them ever so slightly open.

Pretty much what I've been doing. You've never, ever, ever (even once in 200 loads) seen a cat take off above 1800F?

Remember though, his wood fire temps never got very high during the cat killing.

This is true. Stovetop temp was holding a perfect 450F when the cat hit 2000+.

 

[tarzan]

Don't mistake my envy for hostility.

Come on, you can do it! Just need a new user name.

Maybe, Joful King Me?

 

[mellow]

Ohh yea, I have had my cat take off, I just open my damper which was already mentioned to cool the cat off.

 

[Ashful]

So, I checked the cat probe meter this morning, and found the cat peaked at 1812F at some point during the night, despite having been cruising at 1000 - 1200F for at least an hour or two before I went to bed. I did not reload the stove this morning, so I can open and inspect. I suspect it will look fine, as this is a Corning Long-Life (Jotul's OEM) cat.

10:15p load stove on very small coal bed with 7 medium splits of oak and ash
10:30p close bypass damper
10:35p reduce to half throttle (cat at 505F)
10:40p reduce to quarter throttle (cat at 510F)
11:15p shut'er down tight (cat at 1000F)
11:30p - 12:00a cat peaks at 1200F
12:30a cat back down to 1050F and cruising happy
6:30a cat at 550F, but data logger shows it hit 1812F sometime during the night

I wonder how many people here, claiming their cat never goes that hot, actually have data logging on their cat to capture unseen events in the middle of the night...

 

[mellow]

I have a high temp alarm on my exhaust set to go off at 600, which correlates to 1900+ on my cat. I have had it go off, usually due to my flipping door gasket/door latch not sealing right.

 

[brant2000]

I have no idea how hot mine have ever gotten, but through periodically monitoring throughout relatively large loads, I've never seen temperatures get much above 600 C (1100 F). Probably doesn't really apply to most of you, as I have a somewhat unique unit (AFAIK, I'm the only one on hearth using this model).

Just FYI, I've spent the past couple years tweaking my air controls and just wanted to let you all know what I've found to work the best for me. My unit has a single lever that adjusts both primary and secondary air simultaneously. I have installed an electric damper actuator on that lever and use a cat sensing temperature controller to operate the actuator. The actuator has the ability to do proportional modulation (set itself anywhere between 0-100% open), but it tends to cycle open and closed instead, since the temperatures are very difficult to control. I set that controller to 380 C (715 F), so upon light off the temperatures will be below that and the damper fully open. After light off, it will go to fully closed, perhaps hover/cycle a bit, but closed until the back end of the burn cycle. Then a few hours later it will open up again to cut down on the amount of coaling that builds up.

 

[jatoxico]

Joful not a cat owner but do you think if you separated out some monster splits from your woodpile and used one whopper per load it could help? Use your version #1 technique with a few "normal" splits so you get the burn going and have a full load to achieve the burn time you want but with at least one extra large split you might minimize effects of load shifting etc that sometimes lead to bursts of outgassing. You must have a big ugly or two to play with.

 

[tarzan]

You have two Firelight 12's, right? Is the other one running and being monitored?

Just asking as I wonder what role, if any, last nights weather may have played in this.

Hate to keep going back to the BK! But it's all I have for comparison. The draft on it seems to regulate itself well to outside temps though.

 

[Ashful]

Thanks, guys. Last night was windy as hell here... out windows were literally rattling. However, I do not suspect it was a factor, as the cat was cruising at 1000F more than two hours into the burn, when the wind was probably at its strongest. The 1812F temp was recorded after I was asleep, more than 2.25 hours after loading.

I'm gathering that if I can reduce secondary air, I can control the cat temp. Without BK's thermostatically controlled damper on secondary air, I guess the designer has to find the compromise between easy light off and safe secondary burn rate, two contradictory conditions. I could come up with something to partially block off some of the secondary air, but without it being automatic, I'm not sure how useful it would be.

 

[mellow]

That is the one thing I wish I could do on my Appalachian, open the primary air towards the end of the burn to keep the cat active, I shut my air just about all the way down before bed, I come down to a firebox still full of chunks of charred wood and a cat barely still active until I open up the air a bit and it will go for another couple hours with the cat climbing back up to 800-1000.

I have no secondary air on mine, unless you want to count the gasket leaks

 

[Woody Stover]

In using #1 above, there is one thing I've been doing, upon which I did not elaborate. If the cat hits only 600 - 800 on initial engagement, I will often let the stove run a while at 1/4 air, waiting for the cat to climb above 1000F, before lowering the air. This is mostly only an issue on large loads of oak, which are slower to really bake out and get cat temps up, versus other woods.

Is the cat at 600-800 a problem? Is there smoke out of the stack, or are you risking a cat crash if you cut the air before you get to 1000? I don't like burning up any more of the load ramping up than I have to, so the #2 approach doesn't appeal to me if there's a better option.

not a cat owner but do you think if you separated out some monster splits from your woodpile and used one whopper per load it could help? Use your version #1 technique with a few "normal" splits so you get the burn going and have a full load to achieve the burn time you want but with at least one extra large split you might minimize effects of load shifting etc that sometimes lead to bursts of outgassing.

Yep, a lot of the gassing rates and timing is going to depend on the load. What kind of wood and how big are the splits? I think the higher-BTU stuff generally gasses a bit slower and at more even rate than, say, soft Maple, Cherry or Ash. You should also be able to control how much wood is involved early, depending on how the coals are distributed and how you get the load started. You can use different woods' gassing rates to your advantage; I'll put some soft Maple in front on the coals to get the stove temp up quicker without the rest of the load gassing too much, too early. I've seen the same thing as you with a few big White Oak splits I've used lately. They've been stacked 3 summers but apparently aren't totally dry; They will crash the Dutchwest cat if I have more than one in there, or load it where it will need to gas early to sustain the cat.

You've never, ever, ever (even once in 200 loads) seen a cat take off above 1800F?

The Dutchwest is pretty solid with the air anywhere under maybe 1/8" open...just tops out at 650/1500. I had some instances of the cat going high on the Buck, but I don't have to be as careful now with how I burn in the load (how much wood's gassing,) now that I've replaced all the gaskets and got better control of the air. I'll still get an occasional load where the marker shows it went up around 1700, a little warmer than I'd like. Keystone, I don't have a good cat probe on it yet but I don't think it gets overheated, judging from the glow. I know what an 1800 cat looks like from the Buck. It sounds like you have pretty good air control, but earlier you said the stove would get hot with the bypass open, even if the air was cut? And apparently you've got a separate air supply dedicated to the cat? Like you said though, sounds like that might be a lot of fiddling around...

I also try to put all my hot coals to the back of the stove so they are directly under the bypass and then stack my wood E/W on top of that, that way I get a back to front burn. I also try to get an air trench on the ashes down the middle from back to front.

Joful, you could try something wild like that... Would that light off the rear cat quicker, without much other wood in the box gassing early in the burn? You'd also think that maybe gassing would be slower since the load is burning "upstream" toward the air supply, flames not going "downstream," involving more wood quicker.

 

[Highbeam]

Thanks, guys. Last night was windy as hell

I'm gathering that if I can reduce secondary air, I can control the cat temp. Without BK's thermostatically controlled damper on secondary air, I guess the designer has to find the compromise between easy light off and safe secondary burn rate, two contradictory conditions. I could come up with something to partially block off some of the secondary air, but without it being automatic, I'm not sure how useful it would be.

The bk has no secondary air. Only the thermostatically regulated primary air.

 

[rdust]

If it's ever happened to me I've never noticed. I'm guessing the older cat stoves just didn't work as well as the newer models like the BK or WS stoves.

My OEM ceramic cat is on it's 4th season and still works pretty good, it has a couple cracks but it's not missing any pieces. I'm noticing it's not as efficient as it was when new but reading the Condar site it says the cat losses efficiency after 12000 hours, I probably had around that on it after season 3. I'll probably pick up a replacement after this season.

 

[Ashful]

I'm guessing the older cat stoves just didn't work as well as the newer models like the BK or WS stoves.

Even the best stove companies are pretty low-tech. So, it took a few decades to really get the formula right, I think.

What I have noticed is that the catalytic combusters seem to be getting larger on more recent stoves, than on stoves of the 1990's. I had theorized that this might be why some of the older cat stoves seem to destroy combusters, whereas the newer stoves seem to have no problem.

 

[Poindexter]

Just for the record, I have never seen my cat probe into the overfired zone. If I ever do, opening the bypass door is what I would INTEND to do.

The baking out thing I think is going to depend partly on the wood you got. With the birch and spruce I have access to, I leave the Tstat on high when I leave for work in the AM, my wife wakes up an hour later to a warm house and chops the Tstat before she leaves for work, bake out done. In the afternoon I run it on high for an hour to warm the house up before the wife gets home from work, and then chop the Tstat. I work 7-3, my wife works 8-4.

I got no idea what to do with oak or hickory or any of those other highly desirable hardwoods.

I do notice automotive combustors have come a long long way. On the 2004 GTO, stock on the dyno 335 wheel hp is pretty average. Replacing just the cats with thru pipes for back to back dyno runs might get the car to 340 at the wheels, so about 375-380ish at the flywheel. Only a 1-2% gain, with attendant maintenance and warranty issues, but there are plenty of legal places to look for 10 and 20 and 40 hp gains. More surface area in the cat is relatively cheap from the factory and a good thing to have in day to day operation.

 

[kennyp2339]

Joful - I think you need to give those 10 cords of wood away to other members here, it appears you have bad wood, its simply just burning way to hot, I'll take some from you so you can start over and keep your cat temps better regulated lol

 

[Ashful]

Hah... if only it were only 10 cords! I'm sitting on close to double that.