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How HOT is really too hot?

Post in 'Classic Wood Stove Forums (prior to approx. 1993)' started by mdocod, Dec 16, 2011.

  1. mdocod

    mdocod New Member

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    Our modified stove can run proper if and only if, we cherry the top of it. As in... when the lights are out, you can see a deep red glow coming from the top center of the stove. (have to shield your eyes from any light from the fire to see it).

    I realize that this is VERY hot surface temp, but I am wondering what is really considered "too" hot. Are we past that point?

    With the stove top just starting to cherry a touch like that, we can get enough heat up the stack to run clean (no visible condensate or smoke during a wood gas burn). The only way we can achieve this is with the door cracked opened slightly to feed the fire enough air to run really hot and vigorously. (not enough air supply through the supply holes to get there).

    We could make modifications, in order to increase air available with the doors closed. However, I don't want to do this unless I can feel "comfortable" with the idea of running the stove that hot unattended.

    Eric

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  2. raybonz

    raybonz Minister of Fire

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    I would call this overfiring and do not recommend doing this.. I would like to know how hot that surface is that's glowing red.. If you have an infrared handheld you'd have a better idea of if you are overfiring.. Any stove run this hot will burn clean and basically this is what EPA stoves do to create clean burns but do so in a safe manner..

    Ray
  3. coaly

    coaly Fisher Moderator Staff Member

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    From the owners manual; In all caps - QUOTE -
    Caution: If any part of the stove or chimney connector starts to glow, the heater is in an overfired condition. Stop adding fuel immediately and close the doors and draft controls completely until the glowing is eliminated.

    Doing this repeatedly can crystalize the metal leading to failure, cracking and fatigue.
  4. pen

    pen There are some who call me...mod. Staff Member

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    Sure what you are seeing is smoke and not steam?

    Close that door up please! It is not safe running w/ it cracked open and no screen. That is definately a recipe for an overfire and that is what you have going on.

    I'm surprised you say that you can't get enough air. How seasoned is your fuel? If the fuel is dry, then I am wondering if your mods may have reduced draft too much?

    pen
  5. Adios Pantalones

    Adios Pantalones Minister of Fire

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    Wet wood will mean that you have to run hotter to get secondaries. Make sure that it's not steam. Steam is normal in even the cleanest burns
  6. Agent

    Agent Member

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    Yes - overfiring is bad mmKay..
    It's what turned my stovepipe ash-gray. Metal does not like it, especially when done repeatedly.
    The problem with the older stoves is that there is not much you can do to reliably keep it from smoking.
    With my old Buck, even rocking an external flue temp of 800+, I'd still be belching out a hefty amount of smoke.
    The hotter it burns, the faster your wood off gasses. The faster it off gasses, the more air is required to burn cleanly. The more air you add, the hotter it burns. - And so this vicious cycle spirals upward until you run out of fuel, or give up and let it smoke some.
  7. oldspark

    oldspark Guest

    Ok, I have a question, I burnt my Nashua in the 250 to 450 range (external single wall) flue temp and never had any creosote to speak of, now was that a clean burn, the stove was no where near an over fire at those flue temps. Back then I did not ever check for smoke out the stack.
  8. Adios Pantalones

    Adios Pantalones Minister of Fire

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    No creosote in the pipe-you either burned clean or had a chimney fire :)
  9. oldspark

    oldspark Guest

    Why was he belching out smoke at 8oo degrees, start up may be?
  10. Adios Pantalones

    Adios Pantalones Minister of Fire

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    I hope it was just steam, rather than smoke. In much of CO it should be easy to get wood dry... maybe too dry? Just speculating
  11. firecracker_77

    firecracker_77 Minister of Fire

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    I think too hot is when you start smelling the chimney pipe paint burning off. That's when I know to let things simmer down.
  12. Agent

    Agent Member

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    Most likely my personal situation.
    Pine only doesn't help. Nor the fact that I could fit a small tree in that stove at a time.
    Plus, I'd bet some old stoves could burn almost as clean as some newer ones. I can tell you mine sure wouldn't.
  13. mdocod

    mdocod New Member

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    I should have put this in greater context. Yall have to realize, that we are sucking a ton of heat out over a series of baffles made from plates of welded together 1/4" steel. It takes a darn miracle to hit 600F surface temps on the stove pipe where it exits this baffle system. The stove pipe is not being hit with very extreme temps at all. Highest temp I have seen on the stove pipe (just beyond the connection to the stove) was 550F on the surface. The stove overall has been loaded down with a lot of extra steel with the baffle and various other modifications.

    So, the main consideration here is... How "bad" is it if the center of the top of the stove has some minor incandescence to it? No animals are being harmed in the making of this, I'm just trying to get a feel for the effects on the steel, and it's ability to hold up here. I am just trying to get a feel for the difference between "barely visible incandescence" and "it's going to melt into a pile of molten metal and burn your house down."

    Eric
  14. Kenster

    Kenster Minister of Fire

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    I think it's really going to be hard for any of us to give an accurate opinion on a Frankenstein type stove. Too many variables. If it's real heavy steel, maybe a wee bit of glow at one spot might not bee to bad. Long term? Who knows?

    The whole setup sounds downright scary to me, though. It's going to require way more wood than should be necessary. If you can't run a decent fire without nuking your stove stop and you have to always keep the door open to have any sort of draft you're design is major flawed. I'd say you're asking for trouble. Might be time to re-evaluate.
  15. coaly

    coaly Fisher Moderator Staff Member

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    Here's what he's got guys; Frankenbear

    Attached Files:

  16. oldspark

    oldspark Guest

    Aint that interesting, an over grown magic heat, where's pook?
  17. raybonz

    raybonz Minister of Fire

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    LMAO good one OS!

    :lol:

    Ray
  18. begreen

    begreen Mooderator Staff Member

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    As mentioned in the previous thread, you are robbing too much heat from the stack. Get rid of the top hat and the stove should burn with a cleaner stack at a more reasonable temperature.
  19. Adabiviak

    Adabiviak Feeling the Heat

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    Is that custom made? That series of baffles, the 90° bend, and the horizontal run out combined with the hamster trail of apparently an outside air intake seems like putting the brakes on any real draft. I imagine the heat needed to push air through that contraption would be as high as you describe? Watch yourself.
  20. mdocod

    mdocod New Member

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    Yep, all the baffling creates a problem. Our only saving grace to making the system work at all is the tall insulated SS stack IMO.

    Having thought about this with some of coaly's input, I think one of the problems here is simply not enough air inlet cross section on the stove itself to get into a nice self-sustaining mode with the doors shut.

    It didn't help that a lot of our prior problems with buildup were probably related to our inability to force ourselves to have good stove use technique to begin with. Being new to the whole thing, we were trying to use it as a way to provide constant heat on a demand style basis, meaning... smaller loads, cooler burns, etc etc. Granted we INTENDED not to do this, but it is hard not to go over and poke at a new toy, and throw a log in once and awhile.

    The last couple days, I have been trying to stick to a very strict regiment and things are definitely working better. When I build A fire, I build it with a very large pile of kindling, and a couple large logs under that to act as the grate (a mixed bag of upside-down build technique and log cabin/teepee/mess standard). I burn through most of that kindling very fast and hot with the door cracked and then, about 10-20 minutes after the kindling has been working, add as many larger pieces as I can fit in with loose stacking arrangement in order to keep air flow up, but also, maximize the use of the box space. After this load, I continue to leave the door cracked a bit, for about another 10-25 minutes (depending on how fast the fire wants to build due to variables). When the stove temp has reached about the peak I am looking for, most of the "clinking" sounds from the baffle contraption expanding will slow down and, around that time, the center top of the stove may have a very slight incandescent glow if viewed with the lights completely off and blocking your eyes from the light of the fire. At that point I close the doors, but leave the damper full open (don't even have a damper on the exhaust, only on the intake).

    After doing that, the best thing I can do for the stove seems to be to leave it alone. Allow the burn cycle to commence unattended and undisturbed, under the natural amount of dampening we have in place as a result of the mods. The center top of the stove will stay some-what steady leaning towards a decline in temps steadily for about an hour or more at this point, running very hot, but not getting any hotter. The exhaust goes clear, and the stove provides a couple hours of very high heat output, followed by 2-3 hours of diminishing heat as the fire is reduced to coals and most of the coals quiet down towards the end. After about 4-5 hours from when the initial fire was started, I can rake coals forward into a pile. The blast effect of air gets them excited again, and this provides a continuation of lower heat output for about another hour or so. At which point, it's either time to let it die completely (if it's day time then and not enough heat demand to call for another cycle), or a pile of kindling and large pieces and start all over again.

    This endeavor has me wondering about rocket stove designs. I've done a lot of research on them. I realize this forum doesn't really seem to placate to them. I suspect they probably have a lot of problems that are not very well disclosed by all the burlap sack folks pushing them on youtube. That said, I have to admit that with our particular stove design, I have found that I can get a much cleaner burn, earlier in the burn, if I use the stove in a more "rocket stove" manner.. In that, rather than loading new fuel on top of coals at the front of the stove, if after utilizing those coals a bit at the front, I then move them to the BACK, then load fuel, the result is a remarkably clean burning "self fueling" fire that works it's way against the flow of air though the stove. The wood stacked in the stove acts as a series of "jets" to fuel the fire in the back of the stove with "forced" air. The baffle system heats up very fast this way, the exhaust then is almost entirely steam even early on once a "fire" is going. This sort of burning method would never be a good idea in a stove of this design if it drafted directly into a regular stove pipe (would probably melt the pipe), however, it works pretty well in this stove design, and when the cap is clean, the drafting effort is more than enough to make a deep "roaring" sound as the air feeds the intense fire. I think I am going to make a point of building fires "backwards" more often now as well in this stove. I think it's the key to making it work right. I realize that giving the stove air through the door to burn properly at certain times is generally considered a big "no-no" in stoves, however, I have to imagine that in our case, doing so is not counterproductive as it is in the case of most stove designs, since- sending more heat/air up the back of the stove, simply means that we will heat that baffle system up more, which can move that heat to the house. In fact, we "HAVE" to put more heat up the back of the unit if we want enough EGTs to keep the chimney clean.

    Thank you all for the constructive and useful critiquing and information. This is a great forum, lots of great people here.

    Thanks,
    Eric
  21. begreen

    begreen Mooderator Staff Member

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    Yes, when I saw your design I thought of the heavily baffled designs like Russian fireplaces and some kachelovens. It could be that your stove will run most efficiently with one or two hot fires a day, letting the mass of steel absorb the heat, then slowly radiate it back into the room. Maybe clad it in soapstone? :)

    Rocket stoves are awesome. There are some very good folks perfecting this design for third world use. Properly designed they use a lot less fuel and produce a lot less smoke.

    http://www.rocketstoves.com/
    http://www.rocketstove.org/index.php?option=com_frontpage&Itemid=1
    http://www.burndesignlab.org/
    Good site for reference docs:
    http://repp.org/discussiongroups/index.html

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