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Clearances to Combustibles Theory

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

  1. jharkin

    jharkin Minister of Fire

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    That is suprising. Ive got Iron steam pipes in almost direct contact with wood where they pass through the floors and 80+ years of exposure to 215F iron hasn't caused any charring.

    But to address the overall point - I agree that clearances are not something even worth questioning. Just dont do it. Sure the clearances might be overkill ofr daily operation - but that's what a factor of safety is all about... In fact I might guess, and hope that the clearances are set to protect in case of an accidental overfire, not just daily proper operation.

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  2. firefighterjake

    firefighterjake Minister of Fire

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    Just a dumb firefighter here, but when the manufacturer tells me something needs to be X distance from combustibles I pay attention.

    Heard this morning about a fire up in Dover-Foxcroft where the Fire Chief said there were combustibles an inch away from the "Metalbestos" chimney that caught on fire. . . .
  3. wkpoor

    wkpoor Minister of Fire

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    This is really about time duration. I know there has been other threads or posts mentioning prolonged exposure can lower the ignition point. I'm not the only one to have tested the paper on the stove. I know 2 others besides myself that have purposely laid combustibles on a hot stove top to see if they would ignite. All 3 cases were in a shop environment were a small fire on the stove could have easy been delt with. Me nor the other 2 could get anything to burn. Not even fine saw dust.
  4. BrotherBart

    BrotherBart Hearth.com LLC Mid-Atlantic Division Staff Member

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    Yeah I checked today. I was thinking total temp. The standard is 90 over ambient for testing.
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  5. Joful

    Joful Minister of Fire

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    I have accute interest in this thread, because I'm in the process of installing a stove into a setup which is not even remotely covered by the clearances described in the manual. It is a ca.1995 stove in a ca.1773 fireplace, but that fireplace has many combustable components inside, such as a large wood lintel, several large timbers in the side walls, wood "bricks" to which door jambs are fastened, the door jambs themselves, and wood "lockers" (for lack of a better term) embedded high in the side walls. History tells me someone burned fires in this fireplace for cooking nearly around the clock for 100+ years, and continued to burn for ambience or heating purposes for at least another 140 years after that, so the likelihood of any of these components combusting is not high. However, putting 30" tall iron stove in this space is surely different than having a small fire down on the floor, and so it's wise to be concerned and/or cautious.

    I would be very interested to know how stove manufacturers determine the safe clearance for a combustable. How does 90F over ambient for a stove running some UL test load correspond to what the stove will impart on nearby combustables in a severe over-fire condition?
  6. corey21

    corey21 Minister of Fire

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    My wall behind the stove gets real warm but can always hold my hand on it and hold it there. I would rather be overkill when it comes to this topic then have house burn down.
  7. wkpoor

    wkpoor Minister of Fire

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    Quite a few people have wood stoves with no mfg recommendation on clearances which I'm aware is 36" in those cases.
    As for the Metalbestos, did maybe they have a chimney fire or a realy really hot fire that caused the pipe temp to exceed. I expect in many cases either a chimney fire or seriously overheated stove lead to what we read about.
  8. Machria

    Machria Minister of Fire

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    My stove, corner install is 12 1/2" from the walls on either side (from corner of stove to wal). The clearance is 12". The closest tile (1/2" polished slate, "Silver Slate") gets up to 180* when the stove is very hot and the fire is blasting. I called woodstock and they said it's fine. The tile has about 1/4" thinset behind it, and then 1/2" Next Gen dura rock, so it's a ways before you get to a combustible materal (studs), but it gets hot. I think the dark color of the slate attributes to the heat....?

    I'm going to try an experiement with a torch and an extra tile, and see how much heat goes thru the tile, and into the dura rock...

    But the moral of the story is, I would NOT get closer than any clearances, period.
  9. rideau

    rideau Minister of Fire

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    It seems like you just want everyone to tell you, "Yes, you're right, ignore clearance requirements, they don't mean anything. If anyone ever has a fire, it's because they did something stupid, not because they didn't follow recommended clearances. "

    I think everyone on the Forum but you has decided that this isn't even a topic for discussion, manufacturer's recommended clearances should be honored.
  10. Dune

    Dune Minister of Fire

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    Fact check.

    Kiln dried pine (but otherwise fresh) pine combusts at 600F.

    HOWEVER, THERE IS NO (NONE) (ZERO) DOES NOT EXIST) DATA AS TO HOW MUCH LOWER THE TEMP OF COMBUSTION CAN (DOES) GO
    AS WOOD IS HEATED AND DRIED OVER TIME.

    I shouldn't have to yell. We have covered this before. Manufacturer's minimums as MINIMUMS. They are not suggestions, they are not negotiable.

    I have personally seen three (3) fires where this phenomenon has come into play.

    All were old wood boats, wherein the clearances were enough for a long time, until they were not enough anymore.

    One was my father's boat. We used the same stove (oil) every winter for several decades, until the boat burned.

    The next was a boat I captained. The clearance to exhaust was sufficient until the wood had baked for long enough. (about 20 years)

    The next was a friend's boat. Same deal, only it took 30 years to bake the wood enough to lower the temperature of combustion to the danger level.

    Do not let these seemingly long periods of time fool you. Proximity and temperature are just two of the many variables which make determining the actual time needed to sufficiently lower the temperature of combustion (as well as the actual temp) impossible to calculate.

    I might add that old wood has already traveled a ways down this road, WITHOUT being heated by proximity to a heat source.
  11. Dune

    Dune Minister of Fire

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    Umm, no.

    The likelihood of any of these components is actually MUCH greater than new wood.

    Your's is a PERFECT example of this issue.
  12. corey21

    corey21 Minister of Fire

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    I may be wrong but houses are made different then boats.
  13. pen

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

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    :( You are missing the point

    pen
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  14. WellSeasoned

    WellSeasoned Guest

    It's exactly like a piece of charcloth in survival. Put a piece of cloth in a tin and into the fire, the cloth will dry out or char, but will not ignite unless left in the fire long enough. When it's ready for later use, it takes no more than a spark to ignite.
  15. BrotherBart

    BrotherBart Hearth.com LLC Mid-Atlantic Division Staff Member

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    Wish there was a video somewhere. In the tests the stove is packed with "firebrands". They are like four by fours of the driest stuff you can find. And they overfire the crap out of the stove measuring the temps on the walls of the test chamber. They get that stove hotter than anybody is gonna get it.
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  16. corey21

    corey21 Minister of Fire

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    I just thought that the wood on boats was a little different but i check my walls a lot to make sure the temps are safe i use my hand and the IR gun. Better to be safe then sorry.
  17. Joful

    Joful Minister of Fire

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    Corey, I think the point being made, and why I am also concerned with this issue for my own install, is that you're only checking the walls under normal operating conditions. When a burning log rolls into and breaks the glass in the middle of the night while you're sleeping upstairs, and that stove overfires at 950F, check your walls.
  18. jharkin

    jharkin Minister of Fire

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    Joful, can you use heat shields to satisfy you clearance problems? My stove is in a 19th century fireplace and they fabricated sheet metal shields to protect the mantle. It would not have passed the fire inspection otherwise.
  19. schlot

    schlot Minister of Fire

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    I agree that clearances should be maintained. Those are set by the manufacturer not just based on science, but as with anything designed and sold ( especially to the general public) also included liability exposure. That's why engineers use safety factors. It's typical for a design to include a safety factor of 2 or 3. That factor is all about covering your a#@, and that is a direct result of exposure to liability.

    That being said, anything being designed is done in a "perfectly controlled world". Meaning that the operator will do exactly what the engineer assumes he should do. Placing a combustible box of kindling right next to the stove is not what the engineer had in mind.

    So in summary, are the combustible clearances specified conservative? In my opinion....absolutely.

    Should we ignore them and stick stuff next to the stove? Well, I bought some lottery tickets a while back and didn't win anything so I'm going to assume I'm that unlucky with my stove too.

    Just my 2 cents.
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  20. Dune

    Dune Minister of Fire

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    I still remember the night my father's boat burned. It was very cold. It is possible (I guess) that the draft of the stove pipe increased, causing something of an overfire, even though the oil flow was regulated. The stove was a "Shipmate" known for exceptionally high quality. In the summer we used it to cook, in the winter, it kept the engine and batteries warm and the bilge from freezing.

    Interestingly, the wood was protected by actual asbestos with the normal 1" gap.

    The problem was over many years the wood had baked, thus lowering the temp of combustion.
  21. BrotherBart

    BrotherBart Hearth.com LLC Mid-Atlantic Division Staff Member

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  22. Joful

    Joful Minister of Fire

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    Well, I didn't want to hijack this thread, but seeing as this is in the vein of the original question... Yes I can, to some degree. I can and will protect any combustables closer than the defined or default NFPA clerances allow with heat shields. The bigger question is the combustables further up inside the fireplace, which are more than 36" from the stove, but in a confined space (alcove?) above.

    The lintel in both fireplaces is a no-brainer. No clearance is specified for something above and in front of the stove, so I am told we must assume 36" for that. I have 24" to the one in my avatar (which has been in continuous use since 1993, and 31" on the other fireplace where I'm doing the new install. Both will get heat shields in the next week or two.

    den_fireplace_Jotul12_stovepipe_clearance.jpg

    Door jambs are more questionable, as they're off the front corners of the stove, and again no clearance is given for front corners in the manual. If I use the rear corner clearance (12") I'd be fine on the new install, but if I use the safer side clearance (21") I fail. I can make heat shields for these door jambs, although I'm really going to hate looking at them, and it's not even totall clear how they could be attached, air gap, etc.

    den_fireplace_plan_view.jpg

    The bigger question is some of the wood timbers and wood boxes built into the walls further up inside the fireplace, in the area where one would call the "smoke chamber" in a fireplace. These features exceed the default clearance of 36" for any combustable not otherwise specified in the manual, but they're in a confined space above the stove, not in open space. Some of these components would be very difficult, if not impossible to shield. There is some debate in my mind as to whether this qualifies as an "alcove" or not, and the few times that question has been posted here on the forum, I've received varied answers.

    My plan? Install, continue to research (I have several good papers on the subject already), and monitor. If I see any of these wood components up in there getting warm enough for even the most remote concern, I'll be looking for a stove where the clearances are better defined. The cost of the stove is no issue for me, but finding a good catalytic stove that doesn't look hideous or require side-only loading is.
  23. firefighterjake

    firefighterjake Minister of Fire

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    Answer: Yes . . . doing so will keep Hat, Steve, myself and a few others here gainfully employed. ;)
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  24. corey21

    corey21 Minister of Fire

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    OK i never considered that till now that drives the point home. I would say the wall would be real hot. I am a lot more concerned then i was last night.
  25. corey21

    corey21 Minister of Fire

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    I see the point but i am wondering how low the combustion point can get.

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