I Turned My Hybrid Stove into a Rocket Stove and Nearly Burned My House Down

Scared the bejeezus out of me and made me seriously question life decisions. Hopefully my story helps someone else.

Background:
* The stove is a four year old Hearthstone Heritage hybrid stove. About 2 cubic foot firebox.
* The catalysts were original and needed to be replaced leading up to the event, though at the time I thought they still had some life. Additionally, they were partially clogged. I'd forgotten to brush them clean when I'd had the stove open for ash cleaning about a week prior to the incident. I didn't want to shut the stove down just for that, so was limping them through until the next ash-cleanout.
* I have minimal experience burning cat stoves. This was my third season.
* It was a January evening, outside temp was ~20*, and the stove has an outside air kit.
* I have 36' of STRONG draft, vented out a masonry chimney.

Story:
I was preparing the stove for the overnight burn. I was proud of this particular load of fuel: the splits were perfect to really pack in a big load. It was about 50% black birch, 25% red oak, and 25% ash. All jammed in.

For the past week, I'd been struggling to get the stove/cats to light-off as the cats were a bit clogged and also old.

On this night, I was in the final stages of getting the air turned down to the appropriate point that I could leave the stove for the night. I'd been upstairs getting ready for bed and came down to a relatively unusual condition: catalyst probe indicating "too hot" and a stove temperature that was pretty low, maybe 325* Additionally, there was no visible flame. In my stove, that's always meant I turned the air down too fast and the fire was smoldering. Usually I'm right there to catch it, but on this night I wasn't and the probe was allowed to get to the overtemp zone.

Big mistake: I thought the procedure in my manual for an overtemp'd catalyst was to close the air control and open the bypass (like- to protect the catalyst from too hot conditions). The manual does not say to open the bypass, but that's what I did. Then I made the second big mistake: I went upstairs to brush my teeth, leaving the stove unattended.

Unbeknownst to me while I was upstairs brushing my fangs, when I opened the bypass, I took a smoldering large load of very high BTU wood and gave it unadulterated, unrestricted 20* air through 36' of super strong draft.

When I came back down, maybe four minutes later, the stove had become a blowtorch. Literally. Flames were exiting the rear of the stove and were long/strong enough to continue up through the tee and into the chimney liner. Like a rocket stove. It obviously sounded like a tornado. The stove pipe and liner were red hot, and I still believe I could actually see the flames through the pipe and liner, though I'm now not sure how that's possible. Flames began to compromise the connection between the tee and liner, and were starting to exit the venting system.

I immediately went for the fire extinguisher. Opened the side load door and shot a single burst of agent into the firebox, then closed the door. Combustion stopped immediately, but only momentarily as the blaze then reignited. I opened the door a second time and shot a second burst. This time it worked and the fire was extinguished.

Smoke and extinguishing agent were all over the house. Alarms blazing. My wife, whom I'd dragged out of bed, was confused and frightened. But the house survived, thankfully. So did we.

I've now fully cleaned the stove, replaced all compromised components, and upgraded from the factory catalyst probe to a quality Auber Instruments device. Level 2 inspection determined no damage. I'd considered doing away with a hybrid stove altogether and going back to open fireplace enjoyment, but in the end, decided I needed to get back on the wagon. A couple burns into this season, and everything is fantastic.

Learning Points
* My stove does not like to burn with degraded cats. At all. Just replace them when the need comes, regardless of cost.
* If a catalyst is partially plugged, make the small sacrifice to cool the stove and clean them off.
* DO NOT OPEN THE BYPASS WITH A SMOLDERING FIRE. (!!!)
* DO NOT LEAVE THE STOVE UNATTENDED IF IT'S NOT FUNCTIONING NORMALLY. (!!!)
* Is it really so critical I fill every ounce of space with 60lbs of wood?
* Thank gosh I had a fire extinguisher ready.

I'm still a little jumpy around this thing, but time and normal burns should fix that. Burning wood should be fun, and on this particular night, it was anything but.

 

glad to read everything turned out ok and your back to burning as usual.

 

I know the stove , my guess is you provided too much fuel for the cat

glad your safe

 

I think I recall well experienced cat stove users saying that if the stove overfires for some reason, just open the bypass (to try and keep the hot cat out of the direct shot of cold air) and then open loading door, wide open, not just cracked. That lets a load of cool air into the firebox, which cools the fire and calms things down...seems like the wrong thing to do, but it then it does make sense once you think about it. Backwoods Savage, Highbeam
Glad nothing more major happened!

 

I've seen that "translucent metal" phenomenon before, on exhaust headers on engines being tested on stands, and on exhaust pipes on stoves and oil burners of different types. We used to use an old oil drum wood stove outdoors on construction sites for heating chilled hands and such, and it wasn't rare for that stove pipe, and sometimes almost the whole stove, to be glowing red with mottled colors if we fed it plenty of scraps.

I think what we're seeing is the flow of rapid temperature changes on the surface responding to the inside "drafts" of exhaust, when the metal is hot enough that just a few degrees results in different colors changing rapidly on the surface, and it is also probably influenced by the outside air drafts caused by all that heat too, changing the surface colors when the tempperatures are right at those boundaries. It really does look for all the world as if you can almost see through the metal, but whatever causes it you know the metal is HOT when that happens!

Glad that all turned out okay for you!

 

Yah, that makes a lot more sense than the metal turning translucent or some such.

 

Somehow that was in my head at that moment, but I went back and specifically checked the manual and it simply says to turn the air down. ChatGPT, which I trust for almost nothing but consult often, said I should have left the bypass closed and cracked the door. Dunno, maybe that'd work too, though I hear a lot about thermal shock. But it'd at least been better than simply opening the bypass and disappearing upstairs.

 

I will probably never have a cat stove. If my current one craps out, I'll seek out a used unit. I'm unbothered completely by whatever 'pollution' that the old stoves may produce as I watch the contrails of multiple civilian airliners pass by overhead.

 

Ever notice the black smoke from ships?
Large freightliners can burn up to 7000 gallons PER HOUR!

 

The use the leftovers....essentially sludge. Hats off I suppose to whomever figured out how to utilyze it.

 

Glad you're safe.

 

You experienced a hyper active cat. Google can provide lots of info on that. It’s sort of rare. I’ve seen it once on my hybrid, they weren’t new cats as Google may suggest tho. I also don’t have an internal thermometer so I may not always be aware.
For me it was the cast iron on my PH reaching 700+ temps while the firebox was still black. I consider that nirvana LOL
I’m betting your cats aren’t as worn out as you think.

Your 36’ masonry chimney can be an issue tho. Personally I’d never recommend running a cat stove in a masonry chimney. It will draft like crazy with the bypass open. Once you go into cat mode the chimney temperatures greatly decrease and it can be very difficult to keep the masonry hot enough to keep the stove functioning. Your height plays a role in that too. Hard for anyone to say exactly how that will all play out throughout the entire burn. Just be aware your stove will act a little funny using a masonry chimney instead of 6” liner.

 

I've learned this season using the Auber probe that the Hearthstone probe I'd been using since buying the stove was totally useless and presented horribly inaccurate information. So when this incident happened, and the hearthstone probe indicated "too hot", I can't be sure that the cats were actually too hot... in fact, I'd have to say they weren't even close. BUT, it sounds like the situation you're describing is different in that you're also seeing hot stove top temps. That situation sounds great, since that's the whole purpose.... hot stoves to warm our homes. Am I understanding you correctly? What I saw on that night was a coolish stove doing nothing to heat my house.

Regarding my venting.... there's a liner inside my masonry chimney that the stove uses. Sorry for the confusion.

 

Well,if you think your probe was outright lying to you it’s possible you just stalled the cats.

If your probe was accurate our scenarios would be the same except for I have no probe to read. Ergo I would be reading the outside temps at a later time in the burn where the hyperactive cat would have heated the stove top much higher than normal while the wood was still just off gassing and but not actively burning.
Visually it looks the same. You just have a probe that gives you information in real time. I have to wait quite a it longer to know if the cat is active, hyperactive or stalled.
For the most part I just let mine heat up the top to 400 or more before closing the bypass. Especially with older cats.
Better to burn a little hotter than stall them out.

Do you have metal cats? Woodstock NOW claims about ten years on metal. Never saw that claim when I first got the stove. After five years I bought a new one to experience the difference. Noticed no difference. So I just swap them out about half way through the season and clean the dirty one at my leisure. It’ll be 12 years this next season and those two cats are doing fine. I can’t even visually tell the difference between the one that has an extra five years on it.

 

BTW probably a good idea to keep a chimfex chimney fire extinguisher on hand. Pretty sure its all the fire department uses when they respond to chimney fires that are still contained to the chimney.

 

Dang, 10-12 years?!? I've never seen anything explicitly written about the life of my cats, so I was just going off the 10,000-12,000 hours. Plus, they just didn't seem right.

And that's fantastic advice about chimfex. I've seen them and after this incident, yes, I'll probably get them.

 

I thought that chimney cleaning pixie dust, and ANYTHING other than just plain ole tree parts were forboden from being burned in a cat stove? Especially chemicals like that...poisons the cat supposedly...

 

To the manufacturers, and especially their marketing departments, 10-12 years is the same as 10-12 thousand hours. It's a trick because nobody knows how many hours a year you might be burning and on average, nationwide, people don't burn as much as we on this forum do. I would definitely only expect 10-12 thousand hours but at that point start verifying cat function by looking at your chimney for visible emissions. Sometimes you get a little extra but be ready to replace when the cats fail.

I don't have a hybrid but on my cat stove when I open the bypass there is no additional combustion air added to create a blow torch effect or fan the flames. The exhaust just goes around the catalyst instead of through it. This event could have happened with or without opening the bypass.

Your 36' vertical liner is a major suck. Super strong draft that can pull lots of air though the minimum sized openings that hearthstone provides at low heat. Way more than the designers at Hearthstone anticipated. Any way to add a key damper or two?

I have to wonder if you didn't just have a chimney fire. The suck from the chimney fire could also make your stove get hot.

 

I agree with Highbeam my chimney is 28’ and was told it draws like a freight train. A key damper is really cheap insurance.

I figure I burn at least 4,000 hours year so 3 years is Max out of one my cats.. yes the still work but takes more skill to run that way.

I compare cats to snow tires; yes the tires still hold air and work.. but in a blizzard on bad roads; you want your wife and kids to have a “good” pair

 

Yes, I run my cat stove low and slow for maximum burn times and comfort. Plus our climate here is usually just not that cold. Anyway, running this way the cat is doing most of the work in turning wood into heat as there is usually no flame. Then, when the cat nears end of life it fails rather suddenly and completely since it was the star of the show. If I try and run the stove really hot with the flames carrying a lot of the load then the cats that are too worn out to work on low can sorta kinda polish the smoke out of the exhaust.

Really though, cats are cheap. They save me at least a cord of wood every year, enough to pay for a cat once per year, and so replacing them early and often is financially and operationally better. I'm still a cheapskate though so I actually wait until the failure happens.