Q: The installation specs for our stove notes that the clearance from the stove to back wall should be 2". Is that a typo? We just bought the house with
that stove in it, and the clearance is slightly more than that, yet we are concerned: after having the stove on for a short while the wallpapered lath
& plaster wall behind the stove gets quite hot. Anyhow, we were considering a protective shield. Any recommendations, or are we just paranoid?
Thanks for the inquiry! This is one of those situations where perception meets science, and you can rest assured that science has already been
employed to ensure your safety. Stove and fireplace clearances are determined in independent UL listed laboratories via an exhaustive testing
process. The clearance requirements that result from that process are WELL within a safe margin.
After a stove or fireplace receives its clearance listing and enters the marketplace, it is still subject to scrutiny. Even one case of fire or heat
damage caused by a stove installed according to its listed clearances would cause the UL approval to be removed, and halt all further sales of that
product. The fact is, there has never been even a single case among the hundreds of thousands of listed stoves currently in use where the listed
clearances have been found to be insufficient.
So, how can the test laboratory say that 2" is sufficient clearance, yet the wall behind your stove feels "quite hot" to the touch? Here's where the
science comes in:
Since the flames from the fire are contained within your stove, the only way any material outside the stove could ignite would be through the process
known as spontaneous ignition (or autoignition), which is defined as the lowest temperature at which a material, when heated in air, will
spontaneously ignite through heat oxidation in the absence of any other source of ignition energy (sparks or flames). The temperatures required to
cause spontaneous ignition are quite a bit higher than "hot to the touch".
Here are a couple of examples that might help put things into perspective and set your mind at ease:
1) Remember that time you stepped onto a wood deck in your bare feet on a sunny day and instantly launched into a frenzied rendition of the yi-yi
dance? You may recall that that deck was quite a bit hotter than the wall behind your stove. Ever wonder why you've never heard of a deck catching
fire from exposure to the sun? Even under scientifically controlled conditions, with all factors necessary to promote combustion present, the lowest
temperature at which wood has been made to spontaneously ignite from an external heat source is 482° F.
2) If you've ever scalded yourself while pouring tea, you're painfully aware of the fact that, to the 98.6° F human body, boiling water (which is only
212° F) is unbearably hot. If you can touch the wallpaper behind your stove without getting a blister, you know the temperature at the wall is well
below 200 degrees. The spontaneous combustion temperature of wallpaper is 451° F.
1/22/10: What about long-term exposure?
Q: I've always worried about how hot the tile floor under my woodstove feels. After reading the Q&A on the subject in your Sweep's Library, I was
feeling MUCH better, until a co-worker brought up the concept of pyrolysis: evidently, this is where constant or repeated exposure to temperatures
much lower than the ones you post will somehow change the wood over time, until eventually it will ignite much more readily. Any thoughts?
Also named Dean,
But in Klamath Falls, OR
Hi Dean II,
Your workmate is well schooled. Aside from autoignition due to direct exposure to high temperatures as described above, there is another
theoretical cause of spontaneous combustion of wood, first proposed in 1902 by German scientist Ernst Von Schwartz, based upon his study of a
previously published explanation for haystack fires. Von Schwartz's theory postulated that long term, low temperature heating of wood converts the
wood to pyrophoric carbon, and that this pyrophoric carbon is much more readily ignitable than virgin wood.
Von Schwartz's theory remained unproven well into the 1970's, when Dr. Fred Shafizadeh, considered the leading authority on wood pyrolysis of the
day, was able to create wood chars under anaerobic (oxygen-starved) conditions. He found that char created in the absence of oxygen provides an
environment that encourages a chemical bonding of oxygen to the char surface, which, he believed, could result in the generation of heat within the
charred wood itself, independent of the external source. Once begun, he theorized, this "self-heating" process could generate progressively higher
temperatures within the anaerobically charred wood, until the threshhold of spontaneous combustion was reached.
Shafizadeh began a series of experiments to prove the possibility that spontaneous ignition could result from self-heating due to long term, low
temperature heating of wood. Unfortunately, the professor died while his research was still in progress.
Much study has followed Shafizadeh's research, mostly in the laboratory but also at the very few sites of suspected "self heating" fires. Several
scientific papers have been published in support of Shafizadeh's theory, and a few in opposition. Here's a summary of the majority opinion:
Long term heating of wood to temperatures as low as 170° F could result in the oxidation of reactive char, which could result in self-heating, which in
turn could lead to spontaneous combustion. BUT ONLY IF:
a) The wood is subjected to long-term exposure to constant temperatures exceeding 170° F.
b) During this period, the wood is in an oxygen-starved environment.
c) The wood becomes charred.
d) The charred wood is subsequently exposed to a free supply of oxygen.
We should note here that, even when all four factors are present, incidents where spontaneous ignition of wood has been suspected to be a possible
cause for a fire are very rare. If any of the four factors are not present, it can't happen at all.
Nonetheless, modern hearth product clearance test labs acknowledge that Shafizadeh might have been onto something, and require that, even at
the highest possible firing rate, the listed clearances ensure that the surfaces surrounding the stove or fireplace being tested never reach 170° F.
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