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.