Adventures running a cat stove with cordwood <12% MC

[Xtrl9]

Can I just take a moment to say that I really appreciate the way you are documenting the times and temps like you are setting off on an exploratory mission to mars with your hybrid alien technology?

Half joke but really an interesting read. I'll be watching out for part2 "Poindexter's burn theory: Observer effect and mismeasured metrics"

 

[Woody Stover]

At T+60 I am still not inclined to turn it down. What I am afeerd of is if I turn it down to far too fast the rich mixture I clearly got in there might ignite spontaneously instead of sequentially. House is up to 82dF, the fan kit has been on high for 30 minutes, indicated stack temp is still "300"
FWIW the cat is glowing a brighter blaze orange than any safety gear I ever saw, has been for a while. That 300 indicated on the stack probe tells me I have unburnt fuel getting through the cat. I would really really like to see the cat probe temp peak and be headed back down before I fool with the Tsat knob. I wonder what would happen if I turned the Tsat to medium -before- I close the bypass door.

I am pretty sure it is under 11%MC you have to start worrying/watching for a air fuel mix so rich that it might cook off all at once instead of smoothly.

@Highbeam was right again, most recently for me as "much fuel is puked up the flue during startup." I think if I can turn the Tstat down even 30 minutes sooner I would have had a legitimate 12 hour burn.

Saying you are "afeerd of a rich mixture igniting spontaneously," are you worried about a violent back-puff if you cut the air too fast? You haven't mentioned that happening, from what I've read so far. "Puking heat up up the flue?" Hell yeah, if you aren't cutting the air soon after the cat is active. My Keystone has a primary air lever that goes from 1-4. I might have the air at 1.75 or 2 when ramping the stove up to temp, but I'm already cutting the air to around 1 as I approach my desired light-off temp (which gives me a head start on cutting the rate of off-gassing from the wood.) And once I light off the cat, I'm cutting the air back to about .3 to .5. for the duration (maybe a little higher if it's cold and windy out.) Temps in the house (1000 sq.ft. with iffy insulation and some air leaks) will be 68-72 throughout the burn cycle. In my case, I'm getting 8-10 hour cycles with a firebox half the size of yours. But if you are needing room temps of 85-90 to keep your family happy, and have to leave the thermostat set to medium or higher to achieve that, then yes, you are going to blow through a load relatively quickly.

I hope I'm right with my understanding that the stove is engineered to prevent cat overheating. I know @BKVP wrote that but I too am dealing with low to sub teen % moisture wood. No big deal to replace the cat but what about the rest of the stove?

Seems like the thermostat will protect the stove from getting too hot, no? Also, how are you getting sub-teen wood in that wet climate??

Poindexter! Haven't you any larger diameter logs to burn? With all that surface area, your load will burn much quicker.

If he's got the air open on a load of lower-output wood that's not split big, he sure will toast a load in a hurry. In the pic he posted of the loaded stove, there are some good-sized splits in there. But one reason I like my cat stove is that, even using split sizes of 'medium' or 'medium-small,' I can cut the air low enough so that the load still burns slowly.

 

[Ashful]

11%, 12% is fine. I have burnt a lot of it, probably more than 2 cords.

Your two cords of spruce would be 2 weeks of my heating season, in the dead of winter. Not "a lot".

 

[Woody Stover]

Your two cords of spruce would be 2 weeks of my heating season, in the dead of winter. Not "a lot".

That's a years' worth for me, but not a lot compared to what I have stacked.

 

[Highbeam]

We can get dry wood here pretty easily. I am three years ahead now and store out in the open with full sun and wind. All relatively low btu hard and soft woods.

 

[Poindexter]

Yes, I am bumping this thread.

To recap, when I was burning stove fulls of wood at or under 7%MC my burn times sucked, i was running at the ragged edge of possibly overheating my cat, and I happened to have a fly ash accumulation temporarily deactivate my combustor.


@BKVP and one of the guys at my local BK dealer spent a bunch of time working with me off forum, thank you both for that. I recognized up front I was outside the operating parameters of the stove and intentionally stopped updating this thread since I was more or less alone with my problems.

One thing I did do was open the side curtains on my solar kilns back in October. This prevented the kilns from generating solar gain around my wood stacks and sent my stacks seeking an EMC ~9% instead of the 5%EMC they were seeking with the solar gain from closed curtains.

So I got a couple PMs from a nameless lurker here. It really sounded to me like his wood was too wet. I got two pics in my PM box today. One of them is a MM stuck in a freshly split face of red oak, seasoned 7 years, reading 7.0%MC. The other is a freshly split face of pine reading 11.3%MC.

Rather than PM him back, I am putting this out here where he, and others like him, can maybe find this without cluttering up my PM box.

1. I was blowing a BUNCH of unburnt fuel up the stack when starting from a cold stove getting the cat hot enough to engage. Like half my burn time was used up before I could get the cat engaged. I was literally getting 4-5 hour burns before hot reload when starting with a cold stove. My exhaust plume was epic opaque black on start up. Not just puking fuel, pumping unburnt fuel out the stack, and pumping it hard. Not like an 18 wheeler downshifting, not like a diesel locomotive working up a grade, more like a coal fired battleship at flank speed moving against a strong current.

2. Once hot enough to engage my plume would clean right up, but the cat indicator was soaring up the dial far enough and fast enough to concern me. I am only one data point, but from monitoring the thermometer in the back bedroom of my house furthest from the stove i think the convection deck fans pull a lot of heat off the stove. I doubt they alter the reading on the probe by cooling the coil very much, compared to how much heat is actually being taken off the stove.

What is the number one complaint about the fan decks on BKs? Is it stalled cats, overheated cats, or shortened burn times?

3. I did pick up a legacy cat probe thermometer. It is by Vermont Technologies Inc, from right around the time (early 1980s) when energy efficiency was becoming a thing up here as the pipeline construction ended, the economy crashed and people started caring about energy prices here. We got a ton of lava lamps and orange shag carpet at estate sales too if you are looking for any late 1970s - early 1980s vintage stuff. What I don't have is the install and operation manual for the darn thing. What it has is two conductors in a cloth jacket as the temp pickup. If I can figure out how to install and run it it should be more or less immune to the air flow from the fan deck.

4. What I did in shoulder season, finally, was to just do hot reloads with a half load of wood. It burned quickly, but didn't off gas enough VOC fast enough to push the indicator needle up past the end of the active zone. What I see with wood under 11% MC is every surface of every stick becomes fully engulfed in maybe 15-30 minutes. At 12-16% MC my loads burn across the stove from the coals to the opposite wall, front to back, left to right, whatever. 11% and under, every stick just goes up in smoke all at once. By backing off to a half load I limited the amount of surface area on fire all at once and thereby limited the amount of VOC going into the cat. That kept the needle down in the printed active range, and heck it was shoulder season. I got a insulation envelope, and it works good.

5. By the time I got into 24/7 burning my wood was more moist and the stove has been running pretty well. Now is a good time for to head down to the garage and see where my stacks are. I have 2-3 loads left in my ready rack in the garage, brought it in last Thursday I think, about 5 days ago. The wood I have been burning the last couple months lights pretty darn easy.

5a. Easy enough for visitors to the house to comment on how easily my wife could light the cold stove Christmas Eve. We had a "do" Xmas Eve afternoon and let the house cool off to +75dF from our usual 80-85dF. My wife relit the the stove with "not nearly enough kindling" and "you must be kidding" on a full load of splits. Took right off I am told, I had escaped to go do "man stuff" at the scheduled end of the "thing." I'll go see how dry my current stacks are, BRB, maybe 20 minutes....

 

[Poindexter]

OK. The wood that came in from -20dF outdoors about five days ago into my garage took about 48 hours to thaw. My garage is +55dF and 15%RH, so the wood has been seeking ( https://www.hearth.com/talk/threads/solar-cord-wood-kiln-operation.152699/page-2#post-2105773 ) something like 4% MC for about 72 hours and is at 11%MC measured right now.

I am comfortable saying the wood in my outdoor stacks is currently at about 12%MC. Thus my stove is running great, like the 16-20% MC crowd has never appreciated.

 

[Poindexter]

I shall now invite @Highbeam , @begreen and @Ashful to comment.

This un-named lurker in my PM box does not have wet wood. His red oak is at 7% MC. His pine is at 11.3%MC. He has 23 feet of uninsulated liner in a masonry chimney, but the chimney is inside the house. No block off plate, no insulation between liner and stonework..

At +10dF to +40dF he gets back puffs, and plenty of smoke in the house when he opens the door. This si not, in my experience, from his wood being too dry.

With sustained winds at 40-60mph he can run the BK insert on low, but his burns only last 4-6 hours.

What I have observed with 7%MC spruce is very fast, very hot burns with calm winds on my 15' stack.

I am reliably informed, but have not obsevered, that back puffs can happen easily with too dry wood and too low a Tstat. I think @BKVP has had a few. When the wood is dry enough and the Tstat low enough VOCs build up, dramatic combustion can push smoke out the air intake into the stove room. A severe enough back puff can blow the glass out of the front door, potentially releasing embers onto the carpet.

6 hour burns with 60 mph winds and pine only is quite believable to me, no reference point for oak.

What do y'all think? Insert to liner connection? Block off plate? Insulation?

I do hereby ask the un-named lurker to stuff that firebox full of 11.3% pine, run it wide open down to coals and give me burn time and average wind speed, that data point I can relate to.

 

[begreen]

Just a guess, maybe an oscillation caused by the extremes of wind, cold, draft and dry wood? I'm wondering if the wood outgasses strongly which heats the stove up quickly, which then causes the thermostat to close fully, snuffs the wood and it smolders, then the thermostat opens and ignites the wood gases do to strong draft and a rapid inrush of air?

 

[BKVP]

I shall now invite @Highbeam , @begreen and @Ashful to comment.

This un-named lurker in my PM box does not have wet wood. His red oak is at 7% MC. His pine is at 11.3%MC. He has 23 feet of uninsulated liner in a masonry chimney, but the chimney is inside the house. No block off plate, no insulation between liner and stonework..

At +10dF to +40dF he gets back puffs, and plenty of smoke in the house when he opens the door. This si not, in my experience, from his wood being too dry.

With sustained winds at 40-60mph he can run the BK insert on low, but his burns only last 4-6 hours.

What I have observed with 7%MC spruce is very fast, very hot burns with calm winds on my 15' stack.

I am reliably informed, but have not obsevered, that back puffs can happen easily with too dry wood and too low a Tstat. I think @BKVP has had a few. When the wood is dry enough and the Tstat low enough VOCs build up, dramatic combustion can push smoke out the air intake into the stove room. A severe enough back puff can blow the glass out of the front door, potentially releasing embers onto the carpet.

6 hour burns with 60 mph winds and pine only is quite believable to me, no reference point for oak.

What do y'all think? Insert to liner connection? Block off plate? Insulation?

I do hereby ask the un-named lurker to stuff that firebox full of 11.3% pine, run it wide open down to coals and give me burn time and average wind speed, that data point I can relate to.

Any "back puffing" only occurred when I was burning exclusively manufactured fuels. Never when burning cordwood. Second, NEVER have we heard of any "back puffing" that was anywhere near powerful enough to flex the door frame, thus allowing the glass to break and have anything described in this post happen.

Sitting at my desk, getting dozens of calls a week for nearly 18 years and following up on those calls with owners, I can assure all that too dry a fuel load can indeed cause issues in all wood stoves, regardless of technology. Additionally, visiting more than 150 retailers each year, they too have the same experience.

Alaska officials did a study along these lines. They first burned cordwood and then gathered data on emissions. Then they burned strictly manufactured fuel from an Alaska mfg. Again, they collected the data. Finally, they burned a mix of cordwood and manufactured fuels. The data from all three were reviewed and compared. The cleanest burning load was that which was a blend of manufactured fuels and cordwood.

All of today's wood stoves (all) are engineered to pass a test burning dimensional lumber that measures between 19-23% m.c.

I agree that his fuel is not too wet. However, when it is cold out or when the wind blows, the problem does not occur. It was shared with me the problem arises when it is warmer out. It is possible given all the parameters, that this chimney is so long that when the thermostat is dialed back, to produce less heat on warmer days, there is insufficient heat loss to stimulate draft. Again, not all installs are alike.

 

[Ashful]

I unfortunately became very familiar with back-puffing, what causes it, and how to prevent it. Just saw this, and on my way out the door in a moment for dinner and then a meeting, but marking to comment later tonight.

 

[Ashful]

Just got home at 11:30pm (left for work at 7:30am), so not as much time to re-read your posts as I had hoped, @Poindexter. I will have to settle for stating these recollections:

I had enormous trouble with back-puffing on one of my Jotul 12's. This was especially bad in my first year or two, when I was battling several less than ideal factors:

1. Inexperience
2. Poorly seasoned firewood
3. Firewood loads that were often 100% Walnut
4. A very short chimney which consisted of about 5 feet of single 6" wall stove pipe and an additional ten feet of exterior brick chimney with 8" ID clay tile.
5. Poorly designed stoves.

I found that the stove would go into an oscillatory cycle of backpuffing at least one hour, but less than two hours into the burn. Item 1 above was addressed when after experimentation I learned that I could avoid back-puffing on this stove by not closing the air all the way. Some of our other members found similar results with their cat stoves (eg. Woodstock Fireview), and we came to a joint consensus that one sure way to avoid the problem was to always maintain some small flame in the stove.

This solution was tricky, though. If I opened the air control lever a millimeter too far, I would get into a situation where the stove would build enough momentum that I'd get into combustor-melting temperatures. Too far closed, back-puffing. Too far open, ominous scary glowing stove, and then a higher threshold for back-puffing when I'd try to turn it down.

That was item 1 above. I have my own theories on why item 2 was a factor, but since they're un-proven, I'll leave them out for now.

Item 3: Walnut. Some woods cause the combustor to run much hotter than others. Walnut seems to be one of these woods, perhaps being more resinous than some. These woods also seemed to cause more back-puffing. Because I was burning so much walnut at the time, I began to notice I had more back-puffing trouble on all-walnut loads than any other mix of woods. I'm sure someone here can theorize why, I just noted the effect, and then endeavored to not have loads of 100% walnut.

Item 4: short un-lined masonry chimney. Draft was less than ideal. Back-puffing occurs when the firebox is hot enough to ignite volatile woodgas compounds, and airflow through the stove is slow. Remember, the visible flame you see in your stove is not burning wood, but burning wood gas. The theory is that (in the no-flame situation I mention above), these gasses build until you hit some ideal air/fuel ratio, and then the hot firebox leads to spontaneous combustion of this mix. Boom! As mentioned above, maintaining some small flame in the firebox has resolved this issue in every case I have experienced or of which I have read here.

Item 5: I eventually realized the combustor cross section on my old stoves (Jotul 12's) were roughly half that of any modern catalytic stove of similar size. This was a compounding factor, in reducing air flow volume through the firebox for a given chimney pressure.

Fast forward to today, and I daily get my BK's going full-steam for 30 minutes, and then cut the air back to a setting corresponding to a 12 hour or 24 hour burn. In my old stoves, this would have almost surely resulted in a back puff capable of lifting the chimney off the stove, but in the BK's... nothing. They're as simple and reliable as my kitchen faucet.

 

[Xtrl9]

I don't have any deep or meaningful insight here but 40-60 mph winds are literally strong gale force winds, and again not a physicist/meteorologist/aerospace engineer here, but cold air is high pressure, warm air is low pressure. Logic tells me that cold air is going to want to try to come on down the flue.

What size liner is he running, and the liner does extend from the stove through the roof correct? the number of people in my area who told me to wrap some of the pink stuff around a 6' piece of drier hose was kinda scary.

 

[Ashful]

I guess anything is possible, but I ran my stoves in winds approaching 70 mph, and had no such problems.

The trees in my yard? That's another story...