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Backpuffing anew...

Post in 'The Hearth Room - Wood Stoves and Fireplaces' started by Joful, Nov 14, 2012.

  1. Joful

    Joful Minister of Fire

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    Stove has been cruising all evening, a few hours into the latest reload, stovetop holding 350F and cat holding 1300F, both steady for the last 2+ hours. Noticed the smell of smoke in the room a half hour ago, but figured it was my imagination. Then, while sitting here checking emails, I heard and saw a big backpuff, with the classic smoke pushing out through every gasket.

    Criteria is a Jotul 12 cat stove, 60% load probably loaded around 2.5 - 3 hours ago, all Walnut. Been burning with primary air set to minimum for at least the last 1.5 hours, likely longer. Stove had a back-puffing problem last year, but we thought we had solved it with the installation of a 6" insulated liner inside our exterior masonry chimney (was previously running into 8" clay liner).

    Solution was to bump air control open a hair, and now the stove's starting to really take off. Apparently a large part of my load was just starting to ignite at the same time, and likely the stove was not moving the gasses thru quickly enough. Not sure what the cure is to that, though. Thinking maybe a stop or drilling a bypass hole near the primary air damper, that keeps the air control from being shut down quite so far, but I hate doing something like that.

    Makes me nervous to leave the stove burning unattended. I've heard of too many backpuffing stoves blowing the doors open and throwing firebox contents into the room. Had it happen to a close family friend several years back.

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  2. begreen

    begreen Mooderator Staff Member

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    Which chimney/stove is this? I would not expect this to be an issue with the tall flue.

    I wouldn't add a stop to the air control. It's not uncommon that the stove does not run the best with the air control all the way closed. But it's nice to have that extra step of control when it's cold outside and very windy.
  3. Joful

    Joful Minister of Fire

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    Thanks, begreen. This was indeed the shorter flue. I agree with not disabling my ability to shut her down as tight as I can, since there are times I'll surely want or need that.

    The rub here is that with a full load, 20% open can be way too hot, and there's probably 20% play in the air control lever. In other words, loading the stove full in the morning, and setting the air control to a low, safe, consistent position is starting to look like an impossible task.
  4. mellow

    mellow Resident Stove Connoisseur

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    Same issues here, found I have to have the air cracked a teeny tiny bit to allow a bit of flame towards the middle-end of the burn or I will get micro backpuffing.

    Sure would be nice to have a Tstat to do this for me. Haven't figured out how to connect the SmartStove to this insert yet.
  5. blwncrewchief

    blwncrewchief Burning Hunk

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    I really think this is just learning the limits of your stove and flue setup. Even though we have completely different stoves with mine being a secondary burn stove and 15' of chimney, I can produce the same kind of situation. In order to get mine to do that I have to: 1) Fully load the stove with coals at say at least 200-250* or a cold start with kindling. 2) Load dense hardwood. 3) The outside temp is above 25*. What seems to happen is the load lites up, I end up shutting the air down too low too fast to keep the stove temp down. Then awhile (usually 1/2-1 1/2 hours) in to the burn the coals or kindling have burned down and the stove starts to stall because the new load has not been burning hot enough for long enough to burn well enough on it's own with the air turned way down. After stalling for a while it builds up enough heat in the firebox to take off again but the flue temp and draft have dropped during the stall so it starts back puffing as it takes back off. It really took me awhile to put all the pieces together to figure out what the heck was going on as it would only happen to some extent occasionally. The things I noticed along the way were: Once in a while when working outside in more mild weather, like in the fall, I would notice occasional light smoke starting and then clearing up from the chimney during a full load burn in the daytime. This was of course setting up the situation as the house was not very cold in the morning causing me to not run the stove hotter at first to warm the house up and having a decent amount of coals left. It also was during the day meaning the temps were rising outside causing a lower draft. I also had it happen at night on mild nights starting up with a full load and kindling. This set up the same kind of situation where the kindling would start up the stove but because of the mild temps I would close the air down too soon to keep the stove temps down. The kindling would burn down to coals and the main load was not burned in or charred well enough. Same situation as yours in that I would give it just a little air and it would take off like gangbusters. So the only time it happens with my stove is when trying to burn a full load extremely low and slow during mild weather. The remedy for this was to just stop trying to push it that far. If it is 20* or less I never have it happen. When 25* or warmer I just do one of these things. Load less than a full load, even what I would call a 1/2-3/4 load never does it. Load at least a reasonable amount of faster burning wood such as silver maple, pine, cottonwood, etc. as they never seem to be able to stall. Make sure I burn the load well enough before backing the air way down even if it gets the house too warm. Or finally if in doubt just leave the air open a little. Any one of these keeps it from happening. With mine it is just one of those things that when everything comes together just wrong it can happen with my exact setup and I have just had to accept that trying for those super long, low and slow, full load burns in mild weather is just not something I should do. If I were you the main thing I would be trying to do is to get better control on the air by fixing that air control. I could not burn my stove with having 20% slop in the air control. My normal full burning range except for start up is about 0-20%. Depending on the outside temp, say at 10-20*, 0% would run about 450-500 and 20% would run 700-750*. Different loads burn differently but with about 4" of total movement on my primary about every 1/4" raises the stove top around 75-100*. A 10% change could be the difference between 70-72* inside with a 10-12 hour burn and 80-90* with a 5-6 hour burn.:ZZZ
  6. begreen

    begreen Mooderator Staff Member

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    How is the other Jotul working out? Do you have this problem with it too? If not, as a test I would pull the cap on the chimney and connect up a 3-4' extension length of cheap 6" warm air pipe on a non-windy day. Then fire up the stove. If there is a marked improvement in stove performance then you have a clear answer on what the problem is. If it is the short chimney and it can't be altered, you may have to live with this inefficiency.
  7. Joful

    Joful Minister of Fire

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    Thanks, guys! I think you really nailed my situation, blwncrewchief. Lots to learn here.

    As to the air control, with the slop in it my best move is to shut down hard, and then crack it back open 5% - 10%. Similar to the way you run tables on an old milling machine without anti-backlash screws. The only trouble with that is Jotuls are notorious for the air control sticking when you shut them down hard, such that it's hard to crack it back open 5%, without overshooting to 50% when you pull it hard. Clearly I need to give that air control some attention this weekend, and thankfully I just made the cut for and installed the new air control access panel (a mod Jotul added to the stoves after the production date of this example), so I can get in there and fiddle with it.

    Begreen, the second Jotul is still in the garage, but my mason is at the house as I type this, chipping out old pointing and washing down the stonework with muriatic acid. I hope to have the new hearth floor done and repointing by Thanksgiving, and perhaps the stove installed and running a week or two after that. I really look forward to it, since I'll have two setups 100% identical, with the exception of the flue height (15 feet versus 26 feet), the new one being much taller than the one I'm running now.

    The short chimney could be altered, but it will not be cheap or easy. If it's determined I need a taller flue on that stove, I'd probably wait until spring to break down the cap on this chimney and build the masonry taller.
  8. Joful

    Joful Minister of Fire

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    Got called in from the garage tonight by the wife, who was upset that the whole house smelled like a woodstove. Although I didn't see the stove back-puffing, having only ever smelled that smell when it has in the past, I assume it was happening again. She had already tried bumping the air control open a bit, but then claimed that made it worse (likely imagination or insufficient time for things to clear played a part in that assessment).

    By the time I was able to stop what I was doing and come into the house a few minutes later, she had the air control open wide, as well as the bypass damper open. Not sure what she was thinking there, but she was frustrated. This problem is starting to turn her off to burning altogether.

    My trouble is that with the air control open wide enough to guarantee no backpuffing, the stove runs hot.... enough so that I don't want to leave it unattended if it's not shut down to minimum air. If we shut her down hard, I can keep stove top temps at or below 400F, but then there seems to be a good likelihood that backpuffing will occur.

    This time the wood supply was one big split of something white (pine?) low in the back, a cedar split low in the front, a half dozen small rounds of branchwood on top of the big split in back, and one small split of walnut on top of the cedar in the front. Not an ideal wood load, but it was the hodge-podge I had left in the indoor barrel after last night's burn and this evening's start-up. Odor occured around 10pm, which was about 2 hours after the most recent reload.

    Any new ideas on what I could be doing wrong? Maybe I need to just accept this stove wants to run at 550F thru the whole burn, whereas I'm trying to always run it much lower. Sure hoping I don't need to run the other Firelight at like this to avoid back-puffing, when I get it installed... my plan was to run that stove really damped down most of the time.
  9. pring7

    pring7 Member

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    I was having similar problems last year and early this year too. I decided that the cat must be clogged, so I decided to take it apart. The problem was that the screws were seized and I couldn’t get into it without getting a bit and a cordless drill. I didn’t want to mess with it because it was late so I simply sprayed out the cat really good. It kicked up some dust in the room, but fixed the problem for now. We’ve had about a dozen fires since I sprayed it out good and have had no puffing problems.
    Joful likes this.
  10. pring7

    pring7 Member

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    I used an air compressor to spray it by the way. I think that there may be ash and sediment that builds up in that area. I don’t know why I decided to try this, but it worked pretty well. My stove is about a 1982 model, so it is sometimes a little quirky.
  11. Joful

    Joful Minister of Fire

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    Good for you! In my case, this is a new SteelCat from Condar. However, now that you mention it possibly being related to the cat, there may be a relation here. I never saw this problem before switching from the original Jotul ceramic cat to the Condar SteelCat, so it's possible the SteelCat is simply a little more restrictive than the ceramic cat. It would appear by looking at them that the opposite is true, but who knows?

    I say I never had this problem with the original Jotul ceramic cat, but I really only burned 2 - 3 months on that cat, and mostly smaller loads.
  12. jharkin

    jharkin Minister of Fire

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    Hey Joful..... My stove does this sometimes, and I posted a thread earlier this fall I was having problems with the smoky smell on long low burns. I agree with you that we have to get a handle on this, my wife was threatening to ban the stove completely over here!

    A couple things I noticed -
    #1 my door wasn't passing the dollar bill test 100%. I re gasketed now so tight it will rip the bill in half and it seems to help slightly.

    #2 The colder weather seems to help a lot. I tend to get this every fall, and once the temps dip to the 30s and below I can run fully shut down without problems.


    BTW, I never heard of a back puff blowing a latched door open. Thats crazy.
    Joful likes this.
  13. Jags

    Jags Moderate Moderator Staff Member

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    The higher operating temp of the ceramic cat (when it goes active) vs. the lower ignition point of the Steelcat could have something to do with it. At your desired 350-400 with the ceramic, the cat would have been no where near as active as the same temp with the steelcat. Just trying to throw ideas out.
    Joful likes this.
  14. mellow

    mellow Resident Stove Connoisseur

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    I am toying with mine loading the wood N/S on the bottom with top pieces being E/W to see if that helps, if I fully close my air down I get minor blow backs towards the end of the burn once the wood has fully off gassed and is going into the coaling stage, once I loose that little bit of flame in the box it seems to build up gasses and relights from the hot cat sending smoke out the air ports.
    Joful likes this.
  15. Joful

    Joful Minister of Fire

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    I'm surprised a leaky door could cause this, as it really only seems to occur when I shut the draft down all the way. But in either case, I re-gasketed all the doors and windows this summer, with the exception of the ash pan door (which already looked good). Like you, mine's tight on the dollar bill.

    I'm hoping cold weather helps more, but at the moment this occurred last night we were already hovering around freezing outside. Our typical day-time highs all winter are just around (or even above) freezing. Ironically, when temps go into the 40's or 50's and draft is the worst, is when I want to run the stove the lowest.

    Maybe I'm just aiming to run too slow and low on this particular setup. My procedure has been to get the stove top to 500F, engage cat (cat probe usually shoots up to ~800F), and then work the air control down to fully shut over the following 20 minutes. After this, I can get the stovetop to settle down to 350F and cruise for about 24 hours, with good cat temps (700 - 1200 F) for much of the burn cycle, albeit some likelihood of back-puffing.

    On the other hand, the stove seems to run just fine with no backpuffing with air maybe 20% open, although the stove top usually peaks above 600F early in the burn and then holds 450 - 500F for the next several hours. We had anticipated being able to run a cat stove much cooler than this, and clearly can, if we can find a solution for the back-puffing.

    As begreen has already stated a few times, the second Firelight 12 I'm installing on the much taller chimney will tell me most of what I need to know, so I guess I'm just griping at this point. :(
  16. jharkin

    jharkin Minister of Fire

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    I think Joful mentioned his stove is a downdraft like the Vermont Castings. In these designs the stove top temp doesn't have as much of a relationship with the cat temp as a BK or Woodstock. Last night I had a full load in with outside temp around 25. Air was clamped down and I had about 375F on the griddle and a reading of 1550F on the catalyst probe in the back.

    The living room was 76F :)
  17. Woody Stover

    Woody Stover Minister of Fire

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    I would definitely try the old ceramic combustor if it's still functional, along with the other suggestions mentioned here. Could very well eliminate the problem. I noticed that my steel combustor will ignite gasses in front of/beneath it...but that may be a function of the new stainless combustor scoop with its fine-mesh screen...
    You might also experiment with the way you build your loads. You could try top-down from a cold start, or rake the coals forward and put small stuff in the front, larger splits in the bottom/back. That might slow down off-gassing of the load, or control it more as the burn progresses...
    They recommend against using compressed air to clean a combustor, saying that you can blow the catalyst right off the substrate... _g
  18. jharkin

    jharkin Minister of Fire

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    It probably was nothing but coincidence. Most likely its the colder weather making the difference.

    I have found through trial and error that I just cant burn above 45F or so... maybe 50 if its overcast and windy. Above those temps the house holds heat to well and we would overheat. At those marginal temps I run small loads and just let the stove die out during the day. Its surprising how long you can let it coast and then coax a few coals to life to kickstart a new fire if they are buried in the ash bed.

    I have the same Condar steelcat . I find that if I don't get he probe to shoot up to 1000+ within a few minutes of engaging I know its stalled. Sometimes on a big load it will light up at 500 then sluggishly climb to 700-800 then sit there even while there are lively flames in the box. When it does there will be billowing smoke out of the chimney. If I leave it like that with the air half open, sometimes an hour later it will suddenly catch and go thermonuclear (1700+) without warning.

    The solution for me when this happens is to go back on bypass and get the firebox hotter. I have found that the larger the load the hotter you need to get the box to take off. On a 3-4 split load I can sometimes engage at a griddle temp as low as 400. On a stuffed box I often need to get it up to 600+.

    In general it seems like the cat temps you see are lower than what I experience with similar operation habits.. Its probably just the differnces in design, bu I wonder... Does your stove have a secondary air feed to the cat chamber and is it adjustable or maybe clogged? Generally if Im buring hot with a lot of flame in the box I will see anywhere from 1200 to 1650 on the probe, and If I shut it all the way down for a long burn it usually settles in at 1100-1300 depending on the load size. Once it drops below 1000 on closed air I know its down to the coaling stage.
    Joful likes this.
  19. Joful

    Joful Minister of Fire

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    Thanks for all the great info! To answer your last question, my stove has two 2" air inlet holes on the bottom near the back corners, which feed a plenum that (I think) is the entire bottom of the stove. From this plenum, air is fed separately to the cat chamber and the primary air control (which then feeds the firebox). I do not believe it's clogged, as I had the entire cat chamber out last year, but I suppose anything is possible. However, I do sometimes see cat temps as high as 1700F. When I'm trying to heat the house up, say on a Saturday morning when I'm going to be home, I will have cat temps in the 1100 - 1700 F range. However, when the stove is shut down for a slow cruise, my cat will run more like 700F - 800F. It seems to be doing its job at any temp above 500F (no smoke out the chimney!). The general rule I've always heard is that if you need 1000F to get reburn, your catalyst is likely depleted or poisoned, as you can get reburn at 1100F with no catalyst involved.
  20. mellow

    mellow Resident Stove Connoisseur

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    Do you fully open your damper when you need to get the firebox hotter or just open it enough to get a good draft going?
  21. jharkin

    jharkin Minister of Fire

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    Ok so it sounds like you main air control affect air feed both to the firebox and cat. The VC deisgn is different, as it has a seperate air feed to the cat with a fixed thermostatically controlled air inlet. On VC stoves cat problems are sometimes caused by that inlet being out of adjustment (you set it to 1/8" open cold and then as the stove heats a bimetallic coil closes it down).

    The situations where I have seen smoke below 1000F is on a cat stall. I see this when I engage and the proble shows 500 and then climbs slowly to only 800ish over 5 minutes or so. When it lights off properly you will see it go 500 - 550 -600 -650 - all the way up to 1000 almost as fast as I am typing this. Definitely in a minute or less. I imagine that as soon as the catalyst sparks ignition the temperature of the flames inside the cat chamber naturally drive the temps up that fast and then the burn becomes self sustaining.

    On the trailing end of a burn cycle as the temps drop down I wont see smoke even if I let it die down to 600 in the cat.

    Maybe my catalyst is on the way out as evidenced by the big load stalls... but its only got ~ 6 cords of wood through it and I never have a problem lighting it off on small and medium loads.

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