Burn pot fire dance

Over the years, we've seen this topic discussed many times and in many different threads, however it's one of the elusive things that never seems to end with a real solution. Figured we could toss it around here and see if we can come up with something. Actually someone did report a solution, but I don't think it was really a solution - more on that later.

Most of you with a P-series Harman stove will have seen this behavior before. It is not clear to me if other makes or models of stoves exhibit a similar behavior or not. Suspect some do.

Description: Instead of a continuous / steady flame in the burn pot - the fire will do a rhythmic or harmonic "dance". I've also seen it described as a flutter or whoof - The effect is very similar to a strobe light. It can happen briefly and then go away, or it can happen for an extended period of time. In extreme cases, when the fire is large (tall) where it's touching or almost touching the heat exchanger - it almost looks like there are two different flames in the pot - a lower portion, like a body, and a separate top portion like a head (yes, it's every bit as bizarre as it sounds).

For me, it used to happen very, very infrequently in past seasons. I would see it every once in a great while, like a couple times per season maybe. This season, starting back in mid to late January, both of the stoves are doing it now, and they are doing it very frequently - every time they run, and the P68 does it more often than not these days.

When the fire does this behavior, it does not appear to affect heat output, nor does it appear to affect the cleanliness of the stove. In other words, I don't notice that the stove is any cleaner or dirtier when cleaning it after this is happening (a little or a lot) and I don't think it's creating a "dirty burn" situation.

I keep the stoves very clean in general, and just did a major cleaning of both stoves last weekend, hitting all the usual suspect areas. The entire firebox, the burn pot, burn pot holes, including the two immediately beneath the flame guide, the flame guide itself, the igniter chamber, both fans, and the fines box. All gaskets, including the hopper gasket are good. The stoves are operationally fine, no issues whatsoever. Just this weird flame pattern, and the cleanings do not appear to have had any impact on it.

Both stoves were recently OAKed (beginning of February). They were doing this before the OAKs were installed, and they continue to do it after the OAKs were installed. I can tell you they do this behavior much worse and more frequently with the OAK attached. I don't want to turn this into an OAK thread, though, so let's just settle now on the fact that the was a pre-existing behavior, and it happens whether the OAK is attached or not.

The only thing I have found that actually makes the odd flame behavior subside is to slightly open the hopper lid, the door, or fines box cover. If you do any of those things (and I mean slightly, not wide open) then the flame returns to what I would call a "normal" flame pattern. Just mentioning it as an observation. Obviously not a real solution, because doing any of those things makes the stove lose vacuum.

What else can I add? In January, we re-insulated the rim joists in the basement. Previously, the house builder had stuffed loose pieces of R11 batts in each one. They were all removed and replaced with 2" styrofoam insulation (then sealed) and then another layer of R15 rock wool all around. That has made a very noticeable difference in heat retention. The first floor of the house does feel warmer and easier to heat now. I wonder if that insulation work somehow changed the air flow pattern / envelope of the house sufficiently enough to cause this? Doesn't seem likely, but the timing of this is too close to ignore. I say it doesn't seem likely because I can open a window or door near the stove and it has no impact on this, so if the stoves were "starved for air / oxygen" let's say, then you'd think opening a window or door would have a pretty immediate impact, but it doesn't seem to do anything. Maybe the air flow simply isn't behaving the way I think it does or should.

The solution I mentioned earlier where someone claimed to have fixed this was they installed a new flame guide. I strongly suspect in the end that really didn't fix the issue, and the person just never came back later with an update to say so. It seems very unlikely, at least to me, that a flame guide would be involved in this, what with it being a stationary part. It seems more likely to be caused generically by an air flow behavior. An unexpected restriction or an obstruction somewhere. For me to be seeing it on both stoves makes me think the root cause is probably external to the stoves themselves, but not really sure. The P68 flue was cleaned New Years day. The P43 flue only gets cleaned at the beginning of the season - it vents into a 25' 6" flue chamber and only burns about a ton per season.

Anyway, does anyone else see this behavior with their stoves? Frequently, infrequently, never? Has anyone spent any time trying to determine a real cause and potential solution? If there is no solution, that's fine - like I said the operation of the stove does not appear the be adversely affected by this. At this time, it's really more of a curiosity and small visual annoyance than anything else.

 

Not sure TT. Harmans have a damper flap or something in the intake dont they? Maybe for some reason it starts to swing, for lack of a better term.

 

IHP you are right about that, there is a flapper on the air inlet.

 

I have seen this in my stove as well, but not as frequently as you are describing and it doesn't last all that long. I haven't investigated this. IHP's conclusion could be the cause, but as TT said, it doesn't seem to effect the heat output so I don't really worry about it. I replaced my flame guide last year and have seen this phenomenon again this year, so I don't think that's the cause. A mystery.

 

I came to the same conclusion as IHP!
Disconnect the OAK and have a visual on the flapper. Connecting a Anemometer might help see what's going on.

 

I just spent a couple of minutes watching my burn and thinking about this. Every time I have seen this there has been some ash buildup in front of the fire. If I scrape the pot I don't see it, so at this point I'm thinking it has something to do with the amount of ash in the pot. Next time I see the stove doing this I will scrape the pot and see if it continues afterward.

 

It is a standard for all drop feeds and is part of why ejection of ash occurs in deep burn pots. Under feed or pusher stoves do not depend upon the dance to eject ash. The dance is a fairly slow turning of the pellets as they burn or a low bounce as the pellets drop into the burn pot. It is caused in most part by the air flow through the bottom of the pellet pile. a properly adjusted drop feed stove doesn't cause ash to build up in the burn pot and it looks like a volcano cone as far as the ash level in the stove is concerned. Does that provide the answer you are seeking. See Pete's 60+ bag burn in his CPM and pictures of his burns in his Astoria.

 

Astoria:

 

Yes, but Mike Holton at Englander was having kittens watching me do it.

 

I don't recall my P61 ever doing that pulse without an OAK, but since installing it kind of annoying. Noticed same observations about cleaning ash and cracking hopper lid. I'm thinking it's a resonance in the Oak that makes it worse. Mine is 3" flex duct pipe and probably 8 feet long, not ideal I'm sure. One of these days I'll try to tune it out with a resonance damper attached to OAK.

 

And other issues as well he hates to see one of his stoves being dirty that long.

 

Don't get me going on burn pot build up. I can go on for hours on how poor most stoves are.