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Clearances to Combustibles Theory

Post in 'The Hearth Room - Wood Stoves and Fireplaces' started by wkpoor, Dec 31, 2012.

  1. KaptJaq

    KaptJaq Minister of Fire

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    Fahrenheit 451 is a 1953 novel by Ray Bradbury. The novel presents a future American society where books are outlawed and firemen burn any house that contains them. The title is a reference to the temperature at which paper burns...

    I had to heat up the house quickly yesterday, high of 23° F and I was out until dinner time. Lit the old stove downstairs and watched it until the sides were approaching 850°, then shut down the air and kept it about 750-800 until the place was warm. Just by feeling the radiant heat from that stove I am sure that anything stacked next to it that was combustible would have approached 451° fairly quickly.

    KaptJaq

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  2. Joful

    Joful Minister of Fire

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    There have been many studies on this, and most are either inconclusive, or disagree with earlier established data. Similar to our oft-debated issue of what defines "burn time", there are debates over what constitutes "ignition". There was one recent study where wood was exposed to temperatures in the neighborhood of 150 - 250F for long periods, and indeed looked like it had been burned (was charred and blackened, looking like charcoal), but never spontaneously combusted or gave off flame. Did it reach its "ignition" point? The celulose had clearly started to pyrolyze, the normal definition of ignition, but no one ever observed the wood "burning".

    Here's one very basic, if not overly simplistic, description: http://virtual.vtt.fi/virtual/innofirewood/stateoftheart/database/burning/burning.html

    While more about ingnition temperature of wood not already exposed to heat for long durations (years), one of the most interesting, and telling, summaries on the subject of ignition temperatures is Table 1 on p.72 of this paper: http://www.doctorfire.com/wood_ign.pdf

    Note that here we're talking about autoignition, not piloted ignition, and this table shows that there is an enormous range and disagreement in the temperature required to cause autoignition. Temperatures reported in this table have been measured to be as low as 200C (392F) or as high as 714C (1317F). Clearly there was a large variation in species, sample size, and many other factors... but I would say the same for the construction of our various hearths and houses.
  3. SteveKG

    SteveKG Minister of Fire

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    Not in the mood right this moment to read every post so far in this thread, so if I am restating something, forgive me.

    But back some years, probably early 80s, Organic Gardening Magazine performed some experiments with this. I don't recall how they set it up, and unfortunately I did not save the article. They were doing a lot of these sorts of things and their methodology struck me as being solid. They ran tests to see whether the ignition point of wood lowered over time due to exposure to heat from stoves. The cellulose ignited, they found, at lower and lower temp's as time went on. In other words, wood that is heated over and over changes, physics--chemistry. It was an eye-opener. The practical aspect is that one can have a combustible surface [cellulose] which is perfectly safe in distance to the stove and gradually that distance is no longer safe. I will try to get to researching the subject online or something, as it is quite interesting. Obviously, we are not talking about a chair or piece of firewood that is left three feet from the stove once, by accident, for a few hours but, rather, something that is more permanent and near the stove all the time...one thinks of a wooden support post or some wall paneling or trim.
  4. rideau

    rideau Minister of Fire

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    !!!

    This kind of heat scares me.

    The day I see 850 degrees is the day I start heating with down...wearing lots of it.
  5. rideau

    rideau Minister of Fire

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    I bet moisture content of the wood has a lot to do with ignition temp...also density of a particular piece. Same species grown on different soil with different length of growth season has different density...
  6. BrotherBart

    BrotherBart Hearth.com LLC Mid-Atlantic Division Staff Member

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    The process is called pyrolysis.
  7. corey21

    corey21 Minister of Fire

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    Thank you for posting that. I see everyone's point and i am concerned not just about the wood stove but we have other things in our homes that can produce the heat to dry out wood over time. It would seem that hot water pipes could do this over time as well But i fully understand the point now.
  8. FyreBug

    FyreBug Minister of Fire

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    That's pretty much it. We build a fake room with plywood walls. The walls are lined up with heat sensors at approx. every 12 inches. All the sensors are fed into a computer. There are also sensor on the floor and in front of the stove.

    I am not sure if the clearance test is the same as the safety test. For the safety test i've seen our lab tech shovel large QTY of charcoal in a ZC.

    I'll see if I can take a picture next time i'm in the lab.
  9. begreen

    begreen Mooderator Staff Member

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    Time and heat will change the chemical nature of the combustible, especially if it's a cellulose based product. I've read that paper exposed to high heat over time will start to yellow at 157F. It will combust at as low as 258F.

    One very important point they bring up in the paper listed below is that the ignition point depends a lot on the size of the specimen. Sawdust, wood chips, frayed drywall paper edges are much more vulnerable to issues. I know of a local fellow that lost part of his house because the ceiling support box was not scrupulously vacuumed out of wood shavings that fell from the roof hole cut. These shavings flash ignited at few years later. Fortunately they noticed the fire early and the fire dept. did a great job of putting the fire out. Still it was a mess and a very expensive repair to bring the house back into shape.

    Read this peer reviewed doc for some sobering info on pyrolysis:
    http://www.doctorfire.com/low_temp_wood1.pdf
    "In terms of safe design and safe practices for the installation of heat-producing devices adjacent to wood surfaces, it should not be a new or surprising piece of information that 250ºC would represent an extremely hazardous condition and that 77ºC (170F), in fact, must not be exceeded if the heating is prolonged. Already in 1959 UL17 issued this recommendation: “As a limitation on the temperature to which wood may be heated for long periods of time from a standpoint of fire prevention, many authorities indicate that 90ºF above room temperature (approximately 80ºF) normally prevailing in habitable spaces is a safe maximum and one which incorporates a reasonable margin of safety.” Since 80 + 90 = 170ºF, the temperature cited in the 1959 UL recommendation is identical to the one derived in the present study. But there is an important difference in that, in our study, the 77ºC (170F) value has a zero safety factor, whereas the UL value of 77ºC, based on much earlier research, was intended to include some positive, but unspecified, safety factor. In any efforts to establish an “allowable” temperature, when considering data on temperatures at which ignition can occur, a safety factor must be included to ascertain that ignition will not occur, rather than will-just-barely occur."
  10. wkpoor

    wkpoor Minister of Fire

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    Not sure if this explains why my wooden handles look in new condition that are clearly too close and daily are over 200 degrees. Maybe it will just take more time.
  11. begreen

    begreen Mooderator Staff Member

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    I can't answer that. Our little Jotul 602 has a wooden knob on the door handle. The stove is almost 30 yrs old and the knob still in good shape.
  12. Machria

    Machria Minister of Fire

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    FyreBug,
    I'd love to hear how the clearence tests, or at least how the clearance specs are developed for stoves. Do they fire up a stove, and move a sensor away from the stove in increments until it goes down under a certain temp? Or is is simply calculated based on the Max temp of the side or back... of the stove with some accepted formula? Or.... Does the manufacture do this, or some Guberment agency (like EPA...)? Is it a standard, or does each company do it their own way....?

    It's very suprising to me being new to stoves, it's not well known public knoweledge on how clearances are developed for each stove, and what they are based on. Any info you could provide would be helpful, thanks.
  13. corey21

    corey21 Minister of Fire

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    Thank you for posting that info. I can see how over time wood will become dry.
  14. Joful

    Joful Minister of Fire

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    What about clearances to combustable pets? Does the age or breed factor into it?

    ImageUploadedByTapatalk1357345097.385398.jpg

    ImageUploadedByTapatalk1357345147.450334.jpg
    tfdchief and FyreBug like this.
  15. begreen

    begreen Mooderator Staff Member

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    Yes, the smaller they are the more likely they are to spontaneously combust!
    Trilifter7 and Joful like this.
  16. BrotherBart

    BrotherBart Hearth.com LLC Mid-Atlantic Division Staff Member

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    If you insert the door handle rod in their posterior they will not burn according to wkpoor.
    Joful likes this.
  17. Joful

    Joful Minister of Fire

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    That's no small dog... it's just an enormous stove! :)

    Actually, was headed up to bed after finally getting things running with less than well seasoned wood, and heard a very noticeable "thunk!" in the room with the stove. A big log had rolled into the glass, which made me think of my comments to Corey earlier in this thread. Thankfully, no (visible) breakage.
  18. corey21

    corey21 Minister of Fire

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    Lets just say i may not be burning E/W for a long while. Just loaded the bed time load all N/S About 15 minutes ago.
  19. Joful

    Joful Minister of Fire

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    Just got my IR thermo gun in the mail and checked my lintel with the stove running fairly hot (for us). 150F on the wood lintel, and 146F on the upper door jamb. This is the stove that was installed by the previous owner in 1993, with the lintel at 24" from the front top edge of the stove. Time to bend up a heat shield this week.
  20. Joful

    Joful Minister of Fire

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    Just read this paper. Yikes! This is an old house... was already 200 years old before the central heating was installed. We have plenty of hot water pipes feeding baseboards, which are bored directly thru joists, or long spans where they're strapped right to the under-side of our old wood flooring. Makes the floor feel nice and toasty warm, when you're in bare feet.

    Wouldn't it be ironic, if heating with wood stoves was actually safer than using the modern boiler, in our case?
  21. ddahlgren

    ddahlgren Feeling the Heat

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    I think one of the issues is air circulation between the heat shield and the material it is protecting. If there is no air circulation between the shield and material being protected the shield is doing very little to help you. it will be like putting the whole assembly in an oven and baking it. that is why there are different specs for a shield touching the material it is protecting and if there is a 1 inch air gap. I would think the air circulation in the cabin or bilge of a boat to be minimal, the same goes for a sealed fireplace opening with a stove inside it.
  22. wkpoor

    wkpoor Minister of Fire

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    Ok I read it. Here is my take. The 170F number has not been duplicated in the lab only documented in the field. That tells me there is more to the equation. One thing not mentioned in the article in the one documented case of hot water pipes being the cause of ignition, was that the pipes were going through OSB. OSB contains glues used to hold all the flakes together. Now there has to be hundreds of thousands of houses or more, heated with hot water heat. If this was a significant threat I would think it would be a pretty common event and insurance companies would be requesting owners to remove or upgrade systems to protect the structure.
    I will say the article explains why my handles still look new. Even though they are exposed to heat in the 200F range daily that is still not enough heat, based on the their size, for charring to occur.
    Now before people start mis quoting me. I'm sure its due diligence to keep combustibles at a safe distance and temperatures low. However, I'll probably win the lottery before I see wood burn at 170F.
  23. FyreBug

    FyreBug Minister of Fire

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    I thought I would provide an update on Wood Handles vs. clearance to combustibles. I asked our engineers and they said since a handle is part of the unit, it does not fall under the clearance to combustible safety testing.

    It has it's own safety testing. The thermocouple placed on the handle cannot reach higher than 300F + ambient. Coil handles and plastic handles have their own temp safety requirement.

    I find it odd myself since wood is wood and laws of physics are not suspended because a handle is part of the stove. But it's a UL thing...
  24. northernontario

    northernontario Member

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    Clearances to combustables is an interesting issue. My new stove (HT-2000), like my old stove, lists side clearances based on single-wall and double-wall pipe. I happen to have the stove in the basement, beside a 'landing'... so there is wood near the stove, but no where near the pipe. Now, do you follow single-wall clearances, or can you follow double-wall clearances?

    I have it installed at more than the minimum single-wall clearance... and lets just say that wood gets warm! (no IR gun to measure temps).

    IMG_0377.JPG IMG_0382.JPG

    Excuse the lack of finishing trim around the base, and the missing handle... this was immediately after install. My original installation (and then swapping this stove in) is also overkill for shielding... Didn't want to ever have to worry about that wall getting too hot. Also have ceiling protection with some sheet metal, to help reflect heat back down.
  25. FyreBug

    FyreBug Minister of Fire

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    Nice install. Nice to see folks using 45 elbows instead of 90's.

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