Mini-Puffbacks: Consult Please

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Post Wed. Dec. 22, 2010 9:03 pm

I have a Hitzer 503 insert which heats my entire house quite well--contemporary, open floor plan helps alot. Recently,I noticed something that intrigued and puzzled me. I noticed periodic mini-puffbacks occurring in a hot stove which was not dealing with new coal that could be emitting any volatile gases. I was fortunate to be home to see this happening again tonight and made a short video---

The operating details for tonight were stove at 450 degrees and burning nut coal. Stove had grates shaken and coal added about two hours prior to this video. It had not been touched in the interim. MPD closed as much as stove allows; ashpan vent holes just slightly bigger than perfect circles. Nice stable fire with plenty of orange, no blue ladies. 33 outside; 70 inside.

As can be seen in the video, the mini-puffbacks occur regularly at a certain interval. The nature of the puff appears to indicate to me that volatile gases might be accumulating under at least part of the burning coal given that they puff appears to involve all areas of the firebox. This does not make sense given temps in the fire and the unlikelihood of any gas hanging around long enough to accumulate before igniting in a puff. More believable to me is that the coal in the hopper is generating some volatiles, which periodically ignite when they are forced down into the burning coal. I have observed this phenomenon only on two occasions--everything usually burns more routinely and without puffs. Hopper access door on top of stove is properly seated, i.e. no coal in the mating areas, good gasket, etc.

Just after the video was taken, I opened the MPD full bore and puffing continued at same interval, with perhaps somewhat greater puff intensity. I then kept MPD open, opened the ashpan door--the puffs stopped probably due to better airflow. I then inserted a poker into the burning coal in the center just under the hopper. I stuck it down toward the grates in an effort to create a better air pathway from underneath. Unavoidably, some coal in the hopper fell down onto the fire. I then closed ashpan door (keeping vents in the door opened to the same extent as always through these manipulations) and closed the MPD. No more puffing. Was this due to the better airway from underneath and/or the repositioning of the coal in the hopper as some fell into the fire?

Incidentally, I noticed some time ago and only on 1 occasion, blue lady type flames--very gentle ones--coming out of the top front sides of the hopper around where the hopper sits on the rails. This was alittle intriguing!

Any and all ideas welcome! Thanks for your consult!
Last edited by Beeman on Sat. Apr. 01, 2017 3:21 am, edited 2 times in total.
Reason: Embedded YouTube video

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Post Wed. Dec. 22, 2010 9:20 pm

My first thought, after seeing that "chute" in the center of the firebox, is that it's heating up the fresh coal inside enough for it to gas off, causing the mini puffs. Every time a little air seeps in, you get ignition. No other explanation would make sense. In order for there to be a puff, there has to be gas ( and air ) -- and there cannot be gas in the presence of an open flame.

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Post Wed. Dec. 22, 2010 9:25 pm

Check the hopper load door for some crumbs of coal or a loose gasket...
Easy for some coal to get stuck on the edge and admit some fresh air for combustion...
The 'rents keep a small paint brush, natural bristle, to clean the area after loading coal...

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Post Wed. Dec. 22, 2010 10:24 pm

I'd say fresh coal feeding from the hopper at regular intervals is gassing off fresh volitiles, allowing for puffbacks

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Post Wed. Dec. 22, 2010 10:29 pm

ive seen my hitzer 50-93 do the same thing. I was wondering what it was doing. lol..

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Post Wed. Dec. 22, 2010 11:52 pm

My bet would be that there is (was) a cavity inside the hoppered coal that collected/accumulated the volatile gases from the hopper coal as it was heated, that would eventually grow large enough to leak into the firebox and be ignited, setting off a mini-explosion. The cavity could have been created by a chance arrangement, inside the hopper, of large-ish pieces of coal, a piece of wood, etc. that were arranged in such a way to prevent the coal above it from flowing down by gravity. This would explain why you only observe this rarely, as well as several other characteristics of what you describe:

The regular/periodic nature of the mini-puffbacks would be expected with volatiles being released at a constant rate (stove temp being more or less constant over short time frames) into a space of fixed/constant volume.

I assume the sound that accompanies each puff is from air flow thru your ashpan vent holes. The sound seems to have two distinct elements--an "inhale" and "exhale" phase, or if you prefer expansion and contraction phases suggesting, to me, these are explosions rather than just transient changes in airflow, in one direction, thru the coalbed (which was my first thought)

When you poked down thru the center of the coalbed to the grates, enough coal fell from the bottom of the hopper to destroy the floor of the cavity so, after you did this, you observed no more puffbacks, since the gas being released from the hopper coal was no longer kept separate from the fire.

As you know, I have the hopperless 983 similar in other ways to your stove, and I've never observed anything like this. I believe the 50-93 has a hopper, possibly explaining why RLB112 has seen it in his Hitzer but I never have. I suspect the presence of a hopper is the critical issue here.

If/when it happens again, removing the hopper lid and poking at the coal in the hopper to be sure none of it is hanging up (and creating space for a gas pocket) should resolve the "problem".

My $0.02.
Last edited by Ashcat on Wed. Dec. 22, 2010 11:54 pm, edited 1 time in total.

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Post Wed. Dec. 22, 2010 11:53 pm

Back in my wood burning days, I'd occasionally see this with my stove. Invariably it would occur in the middle of the night during temperature inversions. I read how the sequences set up the pattern of puff backs and I'll give a whirl at putting them down in this thread. It's pretty much what everyone above points out as the cause - heating coal gas in the hopper and air getting into the waiting coal.
  • Very weak draft slows the pull of fresh air through the fuel. As the column of new air moves up through the combusting fuel, the oxygen is consumed.
  • The zone above the fuel becomes rich in fuel gasses but not enough oxygen is present to completely burn it. The oxygen having been used up in the bed of fuel.
  • Air from a secondary inlet would slowly feed into the rich fuel gas zone above the fuel bed until the flash level of fuel gas/oxygen/temperature was just right for it to light off. It's a mini explosion of the gasses. POOF - puff back!
  • The resulting pressure from the puff back increases pressure in the stove and chimney overcoming the week daft. The chimney momentarily begins to draft and pulls air into the fuel bed where the oxygen is again consumed. The draft weakens again and the entire process repeats at a given cycle.
In my wood stove, it would sound like a small steam engine. The house would slowly fill up with smoke that was pushed out of the main and secondary air vents. I'd be woken up by the screaming smoke alarms! Run down stairs to the stove and hear puff-puff-puff-puff-puff... at about one second intervals. For my wood stove, a first gen Vermont Casting Defiant, I'd have to close down the secondary air inlet.

Like CapeCoaler posted, it looks like the above sequence is happening in your hopper. Do you know what your draft measures when this happens?

Edit: Ashcat's was faster than my connection!

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Post Thu. Dec. 23, 2010 2:49 am

In a Steam Locomotive this phenomenon is called, "Drumming." It is a common occurrence when a young man learns the art of Firing. It happens when you have a strong draft, insufficient air from below because of blockage (ash or heavy coal bed) and high gas production at the same time. The draft is pulling out more combustion air faster than it can come in and unite with the coal gas and ignite, but the temperature in the firebox is hot enough to ignite the gasses. As new air gets pulled into the firebox the proportion of air, heat and gas are correct you get ignition in small pulses. The solution is to change one the variables. My suggestion is to give the fire a tiny bit more secondary air and the problem should stop. You also might have a small air leak somewhere in the lid at the top of your stove. This might be allowing air to come down through the coal in the magazine, picking up the heated gas as it goes along, and when this mixture hits the heat it causes the stove to burp like that.
But first look for a leak somewhere, if you can't find one, then try changes in the draft settings.

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Post Thu. Dec. 23, 2010 9:53 am

Thats very interesting. I'd guess it's volatile gases being generated from the coal in the magazine. When I was talking with a guy about building a magazine for my stove, he told me the original had four 3/4" holes in the top to allow the gases to come out. I'm wondering if what you are seeing is because your stove doesn't have that designed into it. Perhaps the gases build up enough to push out and cause the puffs you are seeing...


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Post Thu. Dec. 23, 2010 1:20 pm

It is going to be air related. Usually a puffing like that indicates a very minor lack of air. As the gases burn they use up the oxygen and drop the air pressure in the stove. As the need for air increases and the pressure differential gets to a certain point, it will suck in air like a gasp. It's satisfied and the extra air it gulped in does the little mini puffs you see as the gases ignite. Your stove is telling you that it needs attention. Like in the previous posts, make any changes very small and note what you do along with the results. Let us know what it turns out to be.

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Post Thu. Dec. 23, 2010 5:41 pm

Since the hopper is sealed from above the draft pattern will create a negative pressure inside the hopper. Gasses inside the hopper will be pulled down through the unburnt coal directly under the hopper where they will disseminate into the burning part of the bed. When they get enough air they puff. Because the hopper top is so well sealed, but not perfectly, it takes awhile for the gasses to slowly work down accounting for the time intervals. If this is correct, then cracking open the hopper lid should immediately cause it to stop. Again, if this works the fix might be to drill a few holes in the hopper. Not sure just where is best. Your example is with a pretty lively fire, and if the video was shot with an open door there was plenty of over fire air, probably will not happen with a slower burn.

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Post Thu. Dec. 23, 2010 5:48 pm

Is there a baffle near the breach?

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Post Thu. Dec. 23, 2010 6:26 pm

Thank you one and all for some great feedback and thoughtful comments and ideas to consider. This morning, after shaking grates and refilling the hopper with coal, I checked carefully around the area on the top of the stove where the hopper door sits, as well as turned the hopper door over to check the gasket. I found some very small coal "crumbs" sitting on the mating surface where the hopper door rests--hard to imagine these would have caused enough air to come into the hopper from the top.

I did, however, find an interesting situation on the gasket under the hopper door. I found a very small fragment of coal--estimated to be no larger than an 1/8" fragment, stuck to the gasket itself. IMHO, this might have been large enough, especially when added to the thickness of the gasket, to raise the hopper lid ever so slightly, even with a heavy cast iron pot filled with water atop it. Given that I use a jury-rigged, galvanized register fitting as a funnel when filling the stove and I am quite careful not to spill anything, I think the hopper lid gasket picked up this coal fragment when I set it down on the hearth next to the stove while filling the hopper.

I will watch the stove carefully tonight to see if I see a similar puffing as last night. I also plan to keep the area where I set the hopper lid much more clean of small coal pieces. Finally, I might try to replicate the puffing by putting a tiny coal fragment under the gasket as I had found the other one this morning. I will report back on what I find. Thanks again for great help!!

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Post Thu. Dec. 23, 2010 10:00 pm

Like I said before just use the natural bristle brush to give the hopper area a quick clean...
Those little pieces of coal are very sneeky...
Two short pieces of round 1/4" steel will keep the lid off the ground...
And picking up some strays... :lol:

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Post Fri. Dec. 24, 2010 2:16 pm

My 30-95 does the same thing but sometimes in a bigger way, :o enough to slam the baro damper door shut. I had also concluded that it was gas comming out of the hopper. However I didn't know why it didn't do it consistantly after shakedown. My draft is nearly always good so wsherrick's theory of ash buildup on the grates reducing the flow of intake air is probably correct in my situation. Next time I get this series of puffs, I will rake the grates from the underside and see if that makes a difference.

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