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Understanding puffing......in simple terms

Post in 'The Boiler Room - Wood Boilers and Furnaces' started by heaterman, Jan 21, 2013.

  1. heaterman

    heaterman Minister of Fire

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    I was browsing here and caught a couple threads where people were asking about puffing with various kinds of wood. A little discussion about the cause of this might be helpful so I'll share what I know about it.

    Puffing.....the woof woof woof woof you hear and the tell tale smoke coming out around the door is caused by an incorrect fuel to air mixture/ratio. If you would take a flue gas sample during the time the puffing is occuring you would find that CO (carbon monoxide) is "off the chart". It will be extremely high while the excess air/O2 levels will be very low.
    Simply put, this means that you have an oxygen starved situation in your combustion chamber. There is not enough air to properly burn the fuel available.
    What is actually happening inside the firebox when any wood burner starts to puff is that the fire is partially going out due to lack of oxygen and reigniting when more fresh air comes into the firebox. It's basically a mini explosion every time you get a woof or two out of the unit.Understand that when you get into a puffing situation your efficiency is going to go downhill rapidly.
    I have seen about every kind and/or type of wood burner puff but good loading practice and consistent wood will do a lot to alleviate the problem.

    The basic fact of nearly all wood burners.is that once you reach maximum air input you can do nothing other than decrease the fuel load available. Even the Lambda controlled units will puff if there is more fuel than there is air to combust everything. The exception would be a burner that is set up to control primary and secondary air independently of each other and is able to vary those settings in real time throughout the entire burn. (This is what Tom Caldwell has done with his highly modified Garn)

    So...........what's a guy to do? Since most of us are not able to sit around and monitor the entire burn we have to be careful about loading density along with being judicious with the small stuff and extremely dry stuff we load.
    If a person has a lot of smallish wood like 2-3" diameter sticks or fine splits, or one is burning scrap dimension lumber such as 2x4 or 4x4 stuff, he is just going to have to load less of that in at one time, and make sure to mix it with some bigger stuff.
    Puffing can also occur if you have exceptionally dry wood. And by that I mean a moisture content of 15% or less. I have to say that in my travels I have rarely found anyone using wood in that range because it just about takes 3 years worth of seasoning in at least a semi controlled environment. Wood size and type is a far more common cause than being too dry.

    Be careful about increasing air to eliminate puffing at the start of your burn because unless you are there to dial it back once the peak part of the fire is past, you;ll be running way too much O2 and losing a lot of heat up the stack. It's best just to learn what size and type of wood your burner can handle and try to not go past that point.
    muncybob likes this.

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  2. muncybob

    muncybob Minister of Fire

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    Sorta, kinda what I thought was going on. Too dry of wood is not a problem for us....just too humid around here most of the year. I did learn not to load up on small stuff though and it seems to help.
  3. Nofossil

    Nofossil Moderator Emeritus

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    Great explanation - lines up exactly with what I observe.

    Any wisdom on typical changes in primary/secondary ratio over the course of a burn? Seems like the content of the woodgas being generated varies over time - more complex stuff early on, mostly CO later.
  4. heaterman

    heaterman Minister of Fire

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    Typically once the flame is established, which of course varies with the fuel load and MC, one would want to see more secondary air input to handle the high wood gas output at the first part of the burn. Later on in the process you would want more primary air to get the fuel fully involved and to "eat up" the charcoal while maintaining a satisfactory flue temp.
  5. maple1

    maple1 Minister of Fire

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    With all the playing with the (limited) air adjustments I have available to me, in my first 3 1/2 months of running, I had deduced from watching how the fire reacts to them that I need more primary air in the intitial stages, and less secondary air. Sometimes I leave my primary door open a crack AND my bottom draft door wide open (which also lets more air into the primary chamber) to get even more air in there on startup, depending exactly what I have in there for wood (i.e. especially dryer & finer). If I do that, then after a while I'll close the primary door, then close the bottom door half way through the first burn. I have my secondary draft window door thing closed almost all the way - I was opening it when I closed the draft door, but I now just leave it the way it is all the time since it still burns pretty good later in the burn so isn't really worth fiddling with. I did have a few occasions early on where the gassifying flame would go out not long after firing up. Since I have closed my secondary draft window way down, that hasn't happened. Mine's a bit unique, having no fan, and sometimes it takes a bit to get it spooled up - but that's what I found.
  6. TCaldwell

    TCaldwell Minister of Fire

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    Truth be known, even the modified garn can get into trouble. I have a choice, to totally get away from puffing the o2 setpoint has to be must be high, then causes the excess air and flue temp also to be too high. Or reduce the o2 setpoint, that increases secondary burn temp, reduces excess air and flue temp. For a little occasional puffing I choose the latter. Consistent control of batch fired cordwood is just plain difficult!, but fun to try.
    Taylor Sutherland likes this.
  7. heaterman

    heaterman Minister of Fire

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    Primary air = air directly on the fuel/wood causing flames

    Secondary air = air used to combust wood gas and smoke given off by the above.

    The old trick with the Garns in adjusting air patterns as the load burns works pretty well. That is; placing a small block of wood (2x4 size) just inside the primary air inlet on the bottom at the start of a burn. This forces more air up and over the fire instead of right on the wood. As the fire burns, the small block in front of the primary is consumed and that let's more of the airflow directly on the wood to finish things off. It isn't perfect but it works pretty dang well from my primitive point of view.

    Martin tried to do this with the very early Garns by installing a damper (user operated) that would change airflow from the bottom to the top of the firebox. Alas...it was a good idea on paper and worked well when used but customers being interested in things other than maximum efficiency just loaded it and walked away. Most of the old ones I have seen have the damper stuck at about the halfway point.

    Tom, you are so right. Burning cordwood clean and at maximum efficiency through the entire burn is an incredible challenge because there are so many dynamically changing factors through each load. Throw in the fact that every load is different and you have something that is nearly impossible to do. In other words, even in something like a Garn, you will get it puffing and you will get smoke under certain circumstances.
  8. goosegunner

    goosegunner Minister of Fire

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    Heaterman,

    Thanks for posting the explanation of puffing. I run very dry wood with my Econoburn, most at 15%. When my boiler is hot I do not get puffing. Maybe you could help with an explanation for my method below?

    I have found if I load 20lbs of wood and burn for 1/2 hour to bring the boiler up to temp, then load the boiler with remaining wood on the coals puffing is very,very rare.

    My initial start takes about five minutes to load, light, get flue temp to 230-250 and close the damper.

    gg
  9. heaterman

    heaterman Minister of Fire

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    What exactly do you mean when you say "hot"? Are you referring to water temperature?
  10. goosegunner

    goosegunner Minister of Fire

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    Yes the water temp will reach 165-170 with a 25-30 degree delta T increase from return. During this initial 1/2 hour burn the refractory also heats up so that is hot too.

    vs

    Loading full at cold start it seems to struggle more with puffing. I don't know if my wood is too dry and small causing too much fuel/ smoke in the primary chamber with lack of heat at the refractory and nozzle to maintain gasification? But if I do a small fire for 20-30 minutes and then load it just cruises along with rarely an incident of puffing.

    Same holds true if I load more wood after large load is burnt down to coals,no issues with puffing.


    gg
  11. maple1

    maple1 Minister of Fire

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    I know mine is sluggish on the gassifying until the refractory gets hot. I've also taken to getting a smallish kindling fire going first - I let that take off & get established & get the draft going good while I round up what will make up the rest of the first load (usually some re-split hardwood) & get it ready to go into the boiler. Also split more kindling at this time if needed for the next fire.
  12. heaterman

    heaterman Minister of Fire

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    I think the key issue there would be draft. When the flue is "cold" you aren't going to clear combustion gases out of the firebox as well as when the flue is hot and drawing good.
  13. BoilerMan

    BoilerMan Minister of Fire

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    I have equated puffing to a hot engine with the choke still on, way overly rich mixture, visible smoke, stuttering, high hydrocarbons (or CO in our case). The only way I can get my boiler to puff seems to be if I open the lower door in the middle of a really hot burn to see what's going on in there, then when I cloes it all the accumulated wood gas in drawn quickly into the lower chamber and combusted in an oxygen starved enviroment, and POOF lotsa smoke from air inlets and smelly boiler room, time for fan in window.

    TS
  14. horrocksd

    horrocksd Member

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    I find that my Econoburn 150 tends to puff either when I have partially occluded the nozzle when I loaded it (log covering the slit in the floor of the upper chamber), or when there are strong winds and I'm getting some downdrafts into the flu pipe, blowing smoke back into the burn chamber. In its more serious form this can make for some very scary and loud "bangs", small explosions. This past weekends we had steady 40 MPH winds with gusts over 50 MPH and I had this condition. One loud bang blew the flapper valve of the barometric damper out of the flu pipe and blew open the main damper. Scared the hell out of me. If I hadn't been present I'm not sure what would have happened, but at a minimum I would have had a major overfire condition with the damper open and the strong drawing winds.

    After fooling with the boiler into the wee hours of the morning I decided to shut it down and let my oil boiler take over. I have wired a timer on the fan, so I left the boiler circulating and just shut the fan down. The boiler temp dropped within forty minutes and the puffing stopped as the fire went out. Next morning I went out and removed the remaining wood and found, as I suspected, that the nozzle was blocked. After clearing that and restarting the boiler it ran fine even though there were still 25 mph winds. It's been 5 days and everything seems fine, but I still get the "heebeegeebees" when it gets windy. I'm also a lot more careful when I do a full wood load about not blocking the nozzle. I would think Econburn would put a grate on it about 1-2 inches high. Has anyone done this?
  15. huffdawg

    huffdawg Minister of Fire

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    Horrocksd. Maybe make some sort of safety to hold your damper shut. Or find out how to eliminate the puffing.
  16. bupalos

    bupalos Member

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    This is right on. I have an atmos which is induced draft with a thermo-mechanical control that lowers the air flap as the water gets to full heat. This year when I've taken to burning these little kiln-dried cutoffs from a hardwood flooring company ($40/ton) I was getting bad puffing even though I was loading the stuff as tight as I could manage and changing the ratio to maximum secondary. The fire was taking off too fast, and as the thermomechanical gradually choked off the air, despite my careful packing it just had tons of hot dry surface area without air and it puffed and puffed. The key to making this work right was closing the flap down at the start and disengaging the thermomechanical altogether. Then the fire works it's way through more slowly and quite evenly. The output basically grows moderately all through a 2hr burn, no puffing and incredible output on natural draft from about 30 minutes in. The only problem is towards the end it's a just all a bunch of glowing charcoal probably without enough primary air and too much secondary. And much more than 30lbs and it still will gasp a bit. But overall I get nearly as much heat from this little 30lb 2 hr. burn as from the 4 hour 50lb burns of regular cordwood I was doing last year. And the stuff is almost free in terms of money and labor.
  17. CTFIRE

    CTFIRE Member

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    Not sure my problem is the same, but had an odor in the house last night. I think I loaded too much wood before bed. I noticed it one before puff smoke out the air intake. I couldn't find a problem at 3am, but did have an odor. Watched it for 30 min and no visible smoke and the odor dissipated.
  18. Fred61

    Fred61 Minister of Fire

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    Thought I'd bring back this thread as there is some good information here. I experienced a puffing situation last evening about half way through my daily burn. Sitting there watching the news and I could here something abnormal in the basement. Went down and found the eko having a woofing fit. A huge woof every 10 seconds or less. I have the fan trim panel removed because I had been playing with the air and I could see that wimpy panel that the fan id mounted to billowing out with each woof and blowing some smoke out of a small section where it is attached. I had sealed the joint all the way around with babitrite but this one section evidently broke.

    Anyway as you might imagine, this situation is totally unacceptable when the boiler is in the basement. The fire was half chunks of charcoal with half burning splits on top and burning like hell. I reached under the fire with the poker, lifted it up (It didn't weigh much at this point in the burn) and discovered a large chunk of glowing charcoal almost completely blocking the nozzle. Hooked the far end of it and pulled it toward me freeing up the nozzle. Closed the door, pulled the bypass handle and waited for a couple minutes. NO puffing.

    That's the first time I've been able to successfully address the situation.
  19. infinitymike

    infinitymike Minister of Fire

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    I had a huge puffing situation last night. It was around 10 pm and I wanted to load it up for the night. The unit was running, as the house was calling for heat.
    I opened the firebox and found a 3-4" thick bed of red hot coals. (I wanted to get a grill and cook a thick steak:cool: ). I tried to expose the 6 nozzles but its impossible since the bottom of the box is curved and they just keep falling back over.
    I began loaded splits in the range of 3-4" and 4-5" and filled it with about 10-12 splits. I closed the door and instantaneously it began puffing.
    It started puffing so bad that it sounded like a locomotive in the old cowboy movies:eek:
    The WG only has a primary damper adjustment which was fully open. I cracked the load door and that helped a bit, but I wasn't going to stand there all night , so I closed the load door and it was puffing again.
    The only way to stop it is to close the primary air damper. But that just lets the fire smolder<>
    I went back out later when the unit shut down because the heat demand was satisfied and opened the primary air damper.
    I went to bed and hoped that it wouldn't happen during the night.
  20. avc8130

    avc8130 Minister of Fire

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    Mike,

    The last (and only time I am aware of) that I had puffing with my WG I had a similar experience.

    Did you ever clean out your boiler? Without a secondary air induction the WG seems to rely on a free flow approach to keep enough oxygen where it needs to be.

    ac
  21. infinitymike

    infinitymike Minister of Fire

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    Cleaned everything out and shes humming right along. Gasifing very quickly, very little smoke at start up and the steam disappears in a few minutes.
  22. EffectaBoilerUser (USA)

    EffectaBoilerUser (USA) Member

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    Heaterman,

    You mention in your post:

    The basic fact of nearly all wood burners is that once you reach maximum air input you can do nothing other than decrease the fuel load available. Even the Lambda controlled units will puff if there is more fuel than there is air to combust everything. The exception would be a burner that is set up to control primary and secondary air independently of each other and is able to vary those settings in real time throughout the entire burn. (This is what Tom Caldwell has done with his highly modified Garn).

    It was my understanding that ALL Lambda controlled boilers had independently controlled/operated primary and secondary dampers that are able to vary the primary/secondary damper openings in real time throughout the entire burn.

    If a boiler is Lambda controlled and does not operate with independently controlled primary/secondary controlled dampers than how do they operate?

    I have operated my Effecta Lambda 35kW for 3 years (which operates exactly as you describe - the Lambda control panel controls the primary and secondary air dampers independently of each other throughout the entire burn cycle).

    When I first start the boiler it defaults to 70% Primary/30% Secondary. Once the smoke temp. exceeds 100C the dampers than go to 100% Primary and 0% Secondary (this helps to more quickly develop a nice bed of coals which is crucial to converting the wood into gas). After the first 30-45 minutes of operation when the boiler is operating at WOT, it is common to see the boiler operating at 40/60, 50/50, 60/40 etc.

    The dampers will continually change slightly (in 10% increments) throughout the entire burn (I suppose as wood is burning and new wood is dropping down from above, thus changing the CO2% at the chimney exhaust).

    In the 3 years of operating the boiler I do not recall every experiencing the puffing as is mentioned.

    This type of control system can also be compared to an oxygen/acetylene torch (too little oxygen and the flame produces excessive amounts of black soot, too little acetylene and there is no flame at all, with the proper amount of both the flame is blue and can cut steel).

    Brian
    nrford and infinitymike like this.
  23. maple1

    maple1 Minister of Fire

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    Can your lambda increase fan speed?

    I see you're at 100% open during one stage of your burn. So that is letting in all it can - so I could see there being a situation where that might not be enough (nothing left on the table to get more air in). Plus, your percent figures always add up to 100% - therefore it appears from that that they are not independant. I.e., can you go 100% primary & 70% secondary?
  24. heaterman

    heaterman Minister of Fire

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    The issue is the fuel load. Once maximum air is entering the firebox, be it primary or secondary, any solid fuel burner will struggle if there is more fuel available and involved in the fire than there is air to properly burn it. Firebox size comes into play here and also the burn pattern (top to bottom/side to side) in that it can serve as a limiting factor on how much fuel can be loaded in the first place. Proper seasoning and split size for a given firebox will also eliminate most of the problem in any boiler, Lambda or not.
    I think that Tom mentioned in one of his posts that he can get even his highly modified Garn in trouble with some loads of wood.
  25. ComnRailPwr

    ComnRailPwr New Member

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    100% agree. Load it plum full of small well seasons splits and it will more than likely do it. There is just too much surface area to ignite. When I throw large wood in mine it doesn't do it, only when there is a bunch of small stuff or a heaping pile of coals. I don't have a garn but any wood burning appliance will do it under certain conditions. Ones without secondary burns are just more prone IMO.

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