Crackles when air shut down

[Rob711]

using stove since I've joined, tonight I packed it with seasoned oak and maple, packed like Jenga, same as previous Eve's. I've been able to keep enough coal alive to run this intrepid 1 24/7 if I wanted too.
Ran it full until red coals below the last horizontal bar, then packed it tight with oak logs and cut offs I had to fit tiny fire box.
For fun, I shut air down via thermostat once it was all a fire. Minute later, snap crackle, loud! Ran outside, nothing! Not even smoke. Cracklings getting scary at this point, so I open air to half, via the mirror I placed behind my intrepid just to see what thermostat did. Quiet now , minus the occasional pop from wood. It's quiet now, skittle 600 as per my hf ir thermometer, running happily.
Was snap crackle sound liner shrinking from temps going up and down?
Scary, was about to get everyone out of house, solution seemed to be run stove hotter?! Liner brand new and although it's 1983 vintage it's dirtier now then when I purchased it. I believe I have a VC at its peak!

 

[peakbagger]

What you were doing is backpuffing. Definitely not the way to run a stove. When you have a hot stove and add wood, the wood rapidly gasifies into a cloud of potentially combustible gases. They cant burn unless there is enough oxygen around. The stack draft is pulling these gases up through the stove and out into the chimney. At some point the very hot gases find enough oxygen to combust and when it does things go boom.

VCs are known for this issue as they use a lightweight air flap controlled by thermostatic coil. It is quite responsive, as if the stove cools down a bit it adds primary air to the fire. They are relatively airtight and usually the backpuff initiates in the chimney or stove pipe and runs back into the stove but it also can start in the secondary burn chamber which is fed by a separate air port. This makes the inside of the stove pressurize and the primary air flap pops open which give a big puff of air into the fire box which also can causes a boom. If you are near the stove and catch it right you may be able to hear a rapid whomp whomp in fast succession which is the primary backpuff and the secondary larger backpuff from the primary air damper.

Switch over to feeding smaller amounts of fuel more often to match the stove output to the heating demand and the problem will go away. One possibly very bad result of a backpuff is the stove pipe blows off the chimney connection and then the stove proceeds to vent into the house and very rapidly lights the house on fire. This is one very good reason to make sure the stovepipe is screwed at all joints and also at the connection to the chimney and the stove. On the Defiant's there is a cooking plate insert in the top. It will lift up out of the top of the stove, most of the time it ends up back in the hole but it may not and again the stove now vents into the room instead of the chimney. Some people leave a heavy cast iron pot on this plate and coincidentally VC sold a cast iron steamer that conveniently was sized to cover this plate.

 

[Ashful]

My old Jotuls, downdraft cat stoves similar to some of the VC’s, also exhibited backpuffing problems. I’ve never seen one anywhere near as violent as peakbagger discusses, but it’s been known to happen.

However, are we sure that’s what’s happening, here? Backpuffing makes a “whoof!” sound, there’s no way I’d describe it as a “crackle”.

I know of two people who have had a backpuff violent enough to blow the latched front doors open, one of whom lost their house in the event, the other was able to quickly put things out. Those are rare extreme cases, but it can happen.

 

[Rob711]

Wait, wait, wait, so this 500 degree cast iron box in my living room could be dangerous?! Thanks for the important info peakbagger, I've been running it on hotter side to minimize any build up in new liner. Air door, and I believe there's only one, is open full cold and down to almost nothing when it's hot, even then I tap thermostat back just to get it slightly open.
So far, I haven't been a part of a chimney fire and would like to remain in this club. There was no air sounds, almost like a heavy rain on metal, definite crackles, scary. It was odd when it cleared up with more air, and hotter fire, counter intuitive but worked. I won't do that again!
Rob

 

[jatoxico]

Good call @Ashful sussing that out. I get some crackles from rapidly expanding or contracting metal but nothing too concerning or that I didn't recognize as what it was. Wonder if you could have been slowly burning off some build up on a baffle etc that when you added the air began burning more completely.

 

[Ashful]

Wait, wait, wait, so this 500 degree cast iron box in my living room could be dangerous?! Thanks for the important info peakbagger, I've been running it on hotter side to minimize any build up in new liner. Air door, and I believe there's only one, is open full cold and down to almost nothing when it's hot, even then I tap thermostat back just to get it slightly open.
So far, I haven't been a part of a chimney fire and would like to remain in this club. There was no air sounds, almost like a heavy rain on metal, definite crackles, scary. It was odd when it cleared up with more air, and hotter fire, counter intuitive but worked. I won't do that again!
Rob

I've also experienced that heavy rain on metal sound, after leaving the bypass open way too long on a cat stove. I always suspected it was the creosote in my pipe being baked out and popping loose, but I've never verified that was what was really happening. It only happens when I get the pipe scary hot, while running with the combustor bypassed, which seems to happen about once each year when I get distracted and forget it too long. Thankfully, it has never been followed by the rushing roar of a chimney fire.

Do you have a stove pipe thermometer, and if so, is it a probe type or surface mount? Is your stove pipe single or double wall?

 

[Rob711]

Surface mount magnet thermometer kept on top of skillet. Highest I've seen 600. Stove set in fire place, comes straight out top into 6" stainless about 8" long collar then into 18' of non insulated 6" stainless flex, to cap.
Highest temp on collar 4-500. Liner highest 300. And thanks for replys

 

[fbelec]

i get that same noise when the stove is cranking flame and you close it down to quick the hot air starts burning what ever buildup is on the pipe because it slows down so much. if you did the same thing but cleaned the pipe before hand it doesn't do it. when i was a kid and dumb to the ways of burning once every two weeks i started the noise on a old stove we had in the kitchen and if i were to let it go the pipe would turn cherry red starting from the stove and in a 5 inch space it would travel it's way up the pipe and i would put it out just as it got to the chimney. thats how i would clean the smoke pipe. like i said a dumb kid.