Is it possible for too much air to cause a fire to burn poorly

[vdavidoff]

This stupid sounding question brought to you by the forces that are driving me insane as I try to figure out some issues with my stove (which I will for the moment keep to myself).

The fire in question would be either deadfall aspen or well seasoned (12-15%) pine in a Kuma Sequoia insert at 9,500' (if altitude matters - I get the impression this is a contentious issue).

Thanks for any thoughts on this question, and thanks for this forum. I have spent the past few days reading dozens of not 100+ posts and have learned a lot. Unfortunately I am still struggling with an issue, but it takes so many words to explain with full context/history that for the moment I'm trying to keep it simple.

Andy

 

[begreen]

Too much air normally will cause a fire to burn inefficiently, putting most of the heat up the flue. However, at 9500 ft it could get interesting. Altitude definitely has a direct correlation to draft. If the chimney is too short for the altitude and too much air cools things down in the firebox due to weak draft, can that cause issues? Would it cause a cat stall? Interesting question for us lowlanders.

Can you describe the flue system on the stove from stove top to chimney cap including height, diameter, etc.?

 

[vdavidoff]

I have reached out to my installer to get more details, but I can tell you the cap is a 6" vacustack. There is one bend in the pipe right as it exits the stove to go up a masonry chimney, and I don't know what's in place there, but I recall the installer saying it was designed so that even though it's not round and is flatter than the pipe where it bends, the volume it can pass is supposed to be sufficient with regard to the rest of the piping (i.e. it's not a problematic restriction). The distance between where the pipe comes out of the masonry chimney and the top of the cap is ~40". The run from the stove to the top of the cap is about 20'.

The short version of why I'm asking this question is that I routinely (but not always) find myself in the situation where after start up from kindling or hot coals, no matter how well the fire is burning and how correct all the temps look, I get a ton of thick white/slightly yellow smoke out of the chimney (it's definitely not steam), even with the cat engaged. This can continue for hours.

Lowering the intake from full open to 1/3 or 1/4 helps dramatically. I often still get more smoke out of the chimney than I'd think I should, but everything else seems OK (i.e. I don't think I'm starving the fire and making it smolder) . If I then open the intake again, more smoke. So it's like something about more intake air makes that worse. No matter what, my glass always goes black though, so long as I'm burning my primary fuel (pine) and not the deadfall aspen, but I don't think there's anything wrong with the wood (other than pine not being the greatest choice).

This problem actually started even worse, in that I'd shut the door after a new start up and would immediately get smoke puffing back into the house. Transitioning to a start up method that builds heat more slowly seems to have solved that part of the problem - basically, leave the door cracked for considerably less time. I am guessing something about the stove not going from raging hot with tons of intake (door open) to door closed, even with the intake full open, is the factor. That I'm not suddenly starving a raging fire. But I'm guessing here.

I have been and still am playing with a bunch of stuff here in terms of how I load the box, how I start up from kindling, etc, and I may be onto something, but it's hard to tell. I get a few encouraging burns in with some new method then it all goes wrong again and I have to try something new.

Where I'm at now is playing with ideas related to filling the firebox with less fuel in conjunction with all the other tweaking to start up, intake settings, and such. I am wondering if it's the case that when this happens there's just more fuel in the box than it can handle given the amount of intake it can manage, which even if the fire looks OK, is generating a ton of smoke off-gassing from all the fuel, that's just not going to light.

Andy

 

[moresnow]

Burning at that altitude is definitely going to be different than at lower elevation. You are in a rather unique group at that elevation. You are dealing with less oxygen density. As mentioned above a full description of your stove/connector pipe and chimney pipe would be of interest to some here. Pics would also help. Best of luck and feel free to describe your actual issues.

 

[stoveliker]

Not only less oxygen, more importantly less total air pressure which negatively affects the engine of the stove: draft.

 

[Dave_in_ABQ]

The rule I found online for elevation is a 4% loss in draft for every 1000 ft in elevation. 10k ft elevation corresponds to a 40% loss. The Kuma Sequoia manual specs a min chimney height of 12 ft. If the draft at 12 ft is 60% of the min needed, then a height of 20 ft corresponds to the 10k altitude minimum.

I've noted white smoke could be associated with moist wood.

 

[stoveliker]

The rule I found online for elevation is a 4% loss in draft for every 1000 ft in elevation. 10k ft elevation corresponds to a 40% loss. The Kuma Sequoia manual specs a min chimney height of 12 ft. If the draft at 12 ft is 60% of the min needed, then a height of 20 ft corresponds to the 10k altitude minimum.

I've noted white smoke could be associated with moist wood.

My manual says to add 0.5 ft for every 1000 ft of elevation. Given that the physics of draft is not different for different stoves, that might apply as well to your stove.

 

[begreen]

How are you loading the wood? Pine is going to outgas quickly. What has been described sounds like the wood is outgassing faster than the cat can eat it. This could be because the cat is getting plugged with fly ash, the cat is dying due to...?, or just too much wood gas is being generated because the pine is super combustible. Can you get some dry alpine spruce? That can be nice and dense.

Have you given Jason at Kuma a call?

 

[vdavidoff]

Up until the last week or two I was loading one row N/S, then another E/W, fairly loosely packed. Now I'm loading everything N/S and packed fairly tight, which was just part of experimentation. Pine is readily available around here, and that's why I have it. I am not sure what else is available in quantity for a whole winter, but I can surely ask my firewood guy for next season.

I emailed Kuma and Jason responded to my email to let me know he got it and will get back to me.

What you're saying about outgassing and pine seems to align with my latest thoughts, so that's encouraging.

I'm starting an overnight burn right now (from kindling), and plan to load the box only about half way, all N/S, packed well, and am expecting that's going to burn well (i.e. not too much smoke) with the intake set low. If it does, that's great, then the next question will be how long of a burn do I get. Still playing with it...

Andy

 

[ABMax24]

I think its a cat issue. I think it's time to be replaced.

If the cat was functioning properly and the fire was oxygen starved while outgassing lots you should be getting soot, likely a lot of it, exiting the chimney as a dark smoke.

Your plumes of white/yellow smoke further my belief in this. That is wood gas, and wouldn't be there if the cat was functioning properly.

Given your smoke rollout issue I don't believe you have too much draft, yes too much draft can cause issues, but your stove should be running too hot at the same time.

How old is the stove, and how old is the catalyst?

 

[vdavidoff]

The stove and catalyst are 3 years old. It's entirely possible I damaged the cat by overheating it or something as I was learning the stove, but to my eye, it looks OK, and the people who serviced it last spring (same people who installed it) said it looked OK to them, too. I sent pictures of the cat to Jason at Kuma. I'm also attaching those here since I've got 'em handy. Sorry one of them is a little blurry. The few damaged spots are, I think, from me accidentally hitting the cat with the end of my bellows one time last year when I was using it to blow out some ash.

Will a cat still glow even if it's not working properly? Because mine does glow nicely still.

Andy

 

[ABMax24]

If its glowing its at least working some. I'm assuming the entire cat glows?

How about the bypass gaskets are those in good shape? Is it possible smoke is allowed to get around the cat and up the flue?

If the cat and bypass are working correctly then I'm with Begreen, it seems like the wood is off gassing very fast and not enough oxygen is being supplied to consume the wood gas.

12-15% is very dry, and may be too dry to burn cleanly. I know I've heard mention on here than 14% is the driest that should be run through the BK stoves. Try loading some wetter pieces in and see if the problem persists. I have some wood in my pile that is in that range too, my non-cat stove is run with the primary air almost fully closed to burn that cleanly and I get very large flames off the secondary tubes because it off gasses so much. If I open up the primary air the stove gets very hot and I get soot out the stack as the secondaries can't provide enough air to consume all the wood gas.

 

[vdavidoff]

I think all the gaskets are OK. They look a little rough but they are in a box full of fire after all. It's possible I get to the point where I replace the cat and seals just because it's something else to do, but for the moment I don't think it's necessary and would like to avoid it if I don't really need to do it. I've attached a picture of the cat glowing.

I have been measuring my wood moisture on the outside of the splits that have been sitting. I'm going to split one of those in half later today and take a measurement from the core of the split and see how much that differs.

So last night I only loaded 5 relatively large splits (probably max 2/3 of the total firebox volume) and for the most part it was a huge improvement. Intake was at about 1/4 the whole time, though I did play with it some. After closing to 1/4 (which happened pretty quickly) there were some flames and bursts of gas igniting, but this slowly died down for an hour or so until it just turned into glowing at the bottom of the stack, but the cat was ripping and temps were great. Given how the cat is meant to work, it didn't bother me that there weren't any visible flames. There was still a small amount of smoke out of the chimney, but it was so minor that it didn't bother me.

However, at about 3 hours in, some flames kicked back up (I think the bottom row had burnt down enough that there was now a lot more surface area showing) and the smoke increased a bit. I played with the intake all over the place and couldn't seem to really make a difference on the smoke, even fully closed, which I don't think I have even ever tried before and was surprised to see how well the fire kept burning, though I do think it was too aggressive

For the most part further open (which induced more flames) caused even more smoke, which is what I have been trying to figure out as described previously, but in this case it wasn't as dramatic a difference as it has been in the past with the firebox more full of fuel. Because I couldn't seem to find a position that made a dramatic improvement in terms of smoke from the chimney, I left the intake it at 1/4 thinking that even if there was perhaps a little less smoke in some other position (it was kinda hard to tell), I'd probably rip through the fuel way faster than I wanted for an overnight burn.

About that time the glass finally started going black. That I got 3 hours of it before that happened was impressive. Basically at 3 hours I started getting more smoke that I couldn't seem to burn off, and so that blackened the glass as well as was visible out of the chimney.

By the time I woke up the glass was totally opaque. It's the kind of soot I can get off pretty easily (it's not like, hardened on or anything), so I guess that's something. There are a few chunks of char left that are larger than is probably ideal, but there's gonna be a trade off here with the intake that low I would imagine.

It looks like I got about 8 hours of the burn before the blower shut off, which for only having 5 splits in there is pretty good. That's about on par with what I was getting when I was filling the box more fully and fighting with the puffing and smoking way more. I would bet I could load it up with 1 or 2 more splits without it being a problem and get another couple of hours out if it, but we'll see how it goes.

I started the kindling at about 9:30pm.

Images:
* IMG_6644.jpeg - Shortly after the load caught decently and I shut the door (~10pm)
* IMG_6652.jpeg - Glowing cat and mostly glowing fuel. This was still flaming up burning off gasses in the box quite regularly. (~10:30pm)
* fire2.mov - How the fire was burning around 10:40pm. I think the intake was already at about 1/4 at this time.
* IMG_6653.jpeg - Inside of the glass this morning
* IMG_6654.jpeg - Remaining coals and ash this morning (~8:40am)
* Screen Shot 2021-12-03 at 8.41.33 AM.png - Catalyst and blower temps for the burn. The blower temps are measured by a thermocouple that's mounted right in front of the blower exhaust. The cat temp uses a similar thermocouple that's in the location meant for a cat probe. The jump in the blower temp at ~5:15am is when the blower shut off. The large spike in the blower temp near the beginning of the burn is from before I turned the blower on.
* Screen Shot 2021-12-03 at 8.45.40 AM.png - Temps from my main room and upstairs above it (which is open air - the main room goes up multiple stories to the roof) which I'm just showing to help indicate what the fire was actually doing to the home in terms of heating over the life of the fire.

It has been a really mild winter so far and it only got down into the mid 30s last night, probably also worth mentioning just in terms of how that impacts draft and such.

Andy

 

[Highbeam]

Nobody can visually rate a cat as good or bad unless it is deemed bad due to physical clogging or damage. When they wear out due to normal use they can look perfect. They only last about 12000 hours of burn time so many of us full time burners need to replace them every two years. How many hours do you think this cat has?

Even a mostly dead cat can glow if you really send a ton of hot smoke at it using high burn rates, they lose the bottom end first. Visible smoke during the burn is a common sign of a dead cat.

I burn dry Doug fir in my cat stove and get a lot of heavy white smoke during warm up but once up to temp and cruising it goes away and burns mostly clear. Lots of heavy white smoke during warm up. It’s embarrassing and I try to light fires after dark. The same dry softwoods burn with much much less smoke in my noncat stove. Point is that a cat stove will smoke early on as it comes up to temp but should stop pretty early on in the cycle.

Verify dry wood and verify cat is good. Your stove and install sound fine.

 

[kborndale]

If your wood is measuring 12-15% on the outside than your problem is probably wet wood. It's gonna be over 25% when you measure it correctly. Bring a piece inside and let it get to room temperature then split it and check the freshly split inside.

 

[vdavidoff]

I split and measured the moisture on 3 pieces that seemed representative of my stock for this winter.
* ~ 8% outside / ~ 12.5% split side (has been inside for a few days)
* ~12% outside / ~ 16.5% split side (stacked in the garage, ~40* F)
* ~12.5% outside / ~17% split side (stacked in the garage, ~40* F)

Of course if that's not actually a representative sampling, then it doesn't mean much, but I think it is, and I guess that suggests I might indeed be close to too wet on some pieces. If it reads higher at warmer temp (not sure?) then I guess it's even worse. It's the wood I've got this year so that's that, but I this is good to know for next year in terms of how to measure the moisture.

That's frustrating if this has been the or a significant part of the problem, since it means I wasn't measuring the moisture correctly and also apparently don't have a good feel for wood that's ready to burn by feel/weight, but I'll take whatever answers I can find. I'll bear this in mind as the season continues and more regularly take measurements to see how things seem to trend.

Regarding cat life, this is ballpark but I probably have about 6000 hours on it with temps over 500*, and probably like 9000 hours with temps over 100 * (i.e. a fire was going even if the cat wasn't active). So I guess maybe I'm getting close but unless I'm way off on those numbers or the cat got damaged, seems like I probably still have some time. It's not outrageously expensive to do though, maybe I should go ahead and do it.

Andy

 

[bholler]

I split and measured the moisture on 3 pieces that seemed representative of my stock for this winter.
* ~ 8% outside / ~ 12.5% split side (has been inside for a few days)
* ~12% outside / ~ 16.5% split side (stacked in the garage, ~40* F)
* ~12.5% outside / ~17% split side (stacked in the garage, ~40* F)

Of course if that's not actually a representative sampling, then it doesn't mean much, but I think it is, and I guess that suggests I might indeed be close to too wet on some pieces. If it reads higher at warmer temp (not sure?) then I guess it's even worse. It's the wood I've got this year so that's that, but I this is good to know for next year in terms of how to measure the moisture.

That's frustrating if this has been the or a significant part of the problem, since it means I wasn't measuring the moisture correctly and also apparently don't have a good feel for wood that's ready to burn by feel/weight, but I'll take whatever answers I can find. I'll bear this in mind as the season continues and more regularly take measurements to see how things seem to trend.

Regarding cat life, this is ballpark but I probably have about 6000 hours on it with temps over 500*, and probably like 9000 hours with temps over 100 * (i.e. a fire was going even if the cat wasn't active). So I guess maybe I'm getting close but unless I'm way off on those numbers or the cat got damaged, seems like I probably still have some time. It's not outrageously expensive to do though, maybe I should go ahead and do it.

Andy

So that's roughly 15000 hours on your cat. Time for a new one

 

[begreen]

Under 20% moisture is ok. Our firewood settles in at 17% even in the shed. It's probably ok. Replace the cat.

 

[vdavidoff]

I went ahead and ordered a cat. I'll keep playing with stuff in the meantime and beyond (certainly I have more to learn) but I'll report back with results after the cat gets swapped.

Thanks for all the input so far. I really appreciate it.
Andy

 

[vdavidoff]

I installed the new cat last Monday (6 days ago). It seems to be working better than the cat I pulled out, but I think the cat I removed was essentially fine, and the new cat hasn't impacted any of my issues, unfortunately. I'm glad I did the swap regardless, as part of troubleshooting.

I think I'm at the end of my rope on this, which sucks, but it seems like there's either something going on that I just totally don't understand, or that I can't control. Maybe both. I don't think there's anything wrong with the stove, and I can't figure out what's wrong with my methods or materials, so I just feel stuck.

What I have found works best both in terms of avoiding smoke puffing into the house and less smoking out of the chimney is as follows. The results are the most consistent I have achieved so far, but the outcome is still only really good maybe 50% of the time, and I don't know why that is.

1. Load the firebox N/S with kindling on the left side of the box
2. Start the fire on the left side of the box with intake full open and door cracked
3. Once the left side of the box is lit enough that there are decent flames and shutting the door doesn't seem to starve them, shut the door and leave the intake full open. This is usually after only a few minutes. This goes against everything I have ever heard about how to warm up a firebox, but it's what's working more often than not.
4. Run the stove like that until the cat reports temps over 500. This seems to take about 40 minutes, but depending on how well the fuel lights off during this time, it can require some fiddling to get the fire going well enough to get the cat hot, but not running so well that I've got too much fuel burning. It's also a little tricky since fire is only burning to one side of the firebox, and the temp sensor for the cat is on the other side. Presumably the cat is being heated somewhat unevenly during this time, which is probably not great.
5. Engage the cat and wait a bit to ensure the temp is going up / staying hot enough that the cat is engaged, then turn the intake down to 1/2 and watch the smoke from the chimney just to get a baseline
6. After a few minutes turn the intake down to about 1/3-1/4, confirm smoke lessens from the chimney and that the temps are still good / fire seems to be burning OK

Note that starting the fire on the left was a result of me playing with things, but also based on how the intake appears to work on this stove. There are two ports at the front of the stove on the inside of the box. When you slide the intake to full open, both ports are full open. When you slide it to full shut, the port on the left side of the box closes fully while the port on the right side stays maybe 1/3 open. The idea here was that as the fire dies down burning to the right and can benefit from more air without causing excessive smoking, it'll naturally be burning towards the port that's more open. Starting the fire on the right is also problematic because the temp sensor for the blower is on the left.

I had that working like a charm - I was super happy - for maybe 6 or 7 fires in a row. After step 6 there'd be hardly any smoke visible from the chimney, temps were great, and the glass stayed at least somewhat transparent for 2-3 hours before going opaque. But then I started running into issues where I couldn't get the cat warm enough to engage without getting pretty much the entire firebox going (which I know otherwise is what you'd expect), but then I'd be producing way too much smoke both in the firebox and out the chimney, which not only dirties up the glass and chimney, but keeps the temps cooler than I'd like. When this happens opening the intake more only makes it worse, and there's only so much better I can make it by turning the intake down, and it's still bad. This is essentially where the original question of "too much air?" came from. When thing are running really well, the cat will be somewhere between 800-1000 degrees for a few hours. When they are not going well (tons of smoke) it'll be in the range of 650-750 and for less time than when the temps are higher. Also when things are running well, the temp at my blower exhaust will be 220-250, and when they're not, it'll be 160-170 for a while, then raise to maybe a little over 200, then drop again. Everything is just way cooler.

I am not certain about anything but I am thinking that I have two primary issues.

1. Draft - If I get my stove really hot with kindling and a good coal bed before I really load it up, like most people would say is exactly what you should do, I can't close the door after getting the primary fuel going without getting puffing back into the house through the secondary air intake holes (at the top of the front of the stove near the cat). At that point the only way to mitigate is to shut the intake air way down, which just seems generally wrong. The glass goes opaque pretty much instantly and I still get a lot of smoke out of the chimney (same as described above in terms of playing with the intake). Interestingly during the kindling phase, I usually get very little smoke out of the chimney and the glass says clear. It's like with very little fuel in the box, things behave, but with any more, not even a packed box, they go south. Perhaps another interesting observation is that even if I start the fire slowly, if I leave the intake wide open, the fire will get to point, maybe and hour or two in, where it starts puffing into the house. It seems as though it finally gets hot enough that it starts gasping, basically. Point being, left to its own devices with the intake wide open, it will eventually start puffing even if it takes and hour or two, and even if it has been running well that whole time.

I think the issue is that once the fire is going really strong, even with the intake wide open, it can't pull enough air once the door is shut to properly fuel such a strong fire. Whether it's strictly the volume of air or that plus the impact of my altitude, I couldn't say, assuming I'm even right about this theory. If I start my fires so that they slowly build heat (as described above) I totally avoid the issue of puffing smoke back into the house. So basically I know how to mitigate the smoke in the house issue, but it just seems to go against everything you'd think you should do when starting a fire.

I am not sure if there's anything I can do to improve the draft. I don't think I can go higher with the chimney, at least not to any notable degree. I don't know if I could get the stove pipe replaced with something larger, but if I could, I don't know if that'd actually help, or if it'd maybe even make things worse. I have seen attachments for stove pipes that are basically blowers to force draft, but they look very sketchy and since this is an insert and the stove pipe is in a masonry chimney, I don't think there's anywhere I could install it, anyway.

2. Wood quality - As far as I can tell, for what that's worth (maybe less than I realize), my wood is fine. However, even with a relatively small amount of it burning, I often get way more smoke from the chimney than seems to make sense, and it doesn't get noticeably better once the cat is engaged, even with the new cat. Sometimes even if I crack the door and really let the fire rage super hard the amount of smoke pouring out of the chimney still seem extremely excessive. I don't know how to explain that other than to say it's the wood, but like I say, I have no idea what could be wrong with it. I'm not splitting every piece I put in to check moisture content, but as noted in this thread I have checked here and there and it seems fine. The feel of the wood (texture, weight, etc) also feels fine, at least as far as I am able to tell.

I can imagine how wood that's not seasoned well enough, too pitchy or too wet, would cause the smoke I'm experiencing, and how having less of it burning or hot enough to be giving off gas at any time would make the situation better, but again, I just don't see what's wrong with this wood. It is pine, which I know is generally pitchy, but I just can't make sense of this. When I get a really optimal burn going with very little smoke, the wood that goes into the box doesn't seem noticeably different to me than other times, but maybe it is, as I work through my stacks.

So at this point my strategy is to try my best to run fires the way I described above, and just accept that sometimes it's not going to go well and I'm going to have a ton of smoke. I hate that that's the case, but I am stuck. I did purchase the same type of chimney sweep (the kind that you use with a drill attachment) that my installer uses, and I cleaned the chimney out once basically just to get the experience under my belt, so I know I can do that throughout the season as necessary, it just sucks that I should have to.

Just because I have them, I am attaching some video and pictures of how the firebox can appear to be burning well, but there's still tons of smoke pouring out of the chimney. The filenames explain the images. This is from this morning.

¯\_(ツ)_/¯

(╯°□°)╯︵ ┻━┻

If anyone has any thoughts I continue to be all ears, but I realize this probably sounds crazy, like there's something obviously wrong with my methods or materials that you'd see if you were here, that I am not picking up on. I suspect that the opinion will be "bad wood".

Update: I just reloaded the wood rack in my house in the way I normally do, but I measured the moisture content in literally every piece of wood, including splitting larger pieces to measure from the inside. Most everything is 14-17. A few pieces were 17-18. One was a bit over 19. I left that one, and one or two of the ones closer to 19, out. But overall, this seems to agree with what I checked before, just spot checking, and seems, to me, reasonable.

Thanks again for the all input so far. I know I must sound like I'm crazy.
Andy

 

[FPX Dude]

Looks like it's a struggle with the fuel. After 30+ mins. or so, it should be burning clean and no smoke whatsoever. What happens with just kindling, or 2x4's, pallets etc. We used to burn aspen all the time, but it eventually led to a chimney fire. I don't know if it's sappy or barky or what, but it's a softwood, and yes that's a lot of smoke.

 

[Dave_in_ABQ]

Did you measure the moisture content with the wood still cold? Best if it at room temp.

 

[vdavidoff]

Did you measure the moisture content with the wood still cold? Best if it at room temp.

Half was at room temp (it had been inside a couple of days but I dragged it back outside to split the larger pieces to check the moisture) the other half was from the garage at ~34* F when I split it, measured it, then took it inside. How much of a difference does the temperature make in the measurement - is it fairly deterministic?

Andy

 

[vdavidoff]

Previously in this thread I had said the flue was about 20' full length. After running the chimney sweep down it, I think it's more like 18.5'. Not a huge difference, but perhaps enough, to make a difference. This is still somewhat of an approximation, though.

The chimney sweep went down about 16.5' before it hit resistance that I am just about certain is where the pipe curves to meet the stove. I couldn't see the sweep looking up from inside the stove, however, so I think it stopped right at the start of this curve (and I wasn't willing to force it further). There's a curved bit of pipe and possibly an offset box in there (I am still trying to get confirmation from my installer). I am guessing that's 1 to 1.5 feet in total length. Then there's about a foot of pipe to the top of the cap that was pulled off to get the sweep into the top of the chimney. So that's how I'm coming up with about ~18.5'. Next time I clean it I'll run some fish tape down there so I can get a more accurate measurement, but I don't think I'm way off here.

Andy

 

[ABMax24]

Okay, so you replaced the catalyst, which has had little effect. On to the next step.

If the catalyst is working fine, then the smoke should be consumed there and not be making it through as smoke, that should all be burnt.
The next most likely point is the cat bypass door and it's seal, or the entire catalyst assembly itself. Use a flashlight, ideally one that radiates light in many directions, open the bypass and put the flashlight on the shelf behind the catalyst inside the bypass, close the bypass. Now very carefully look for light coming out, the only place light should come out is through the catalyst itself, if it escapes anywhere else that is a leak and is allowing smoke to bypass the cat and exit the chimney.

The entire catalyst/bypass assembly is held into the stove by 4 bolts, make sure these are tight and make sure this surface seals properly with the top of the stove. I believe Kuma calls this entire piece the baffle assembly.