[Greenbuilding] firewood moisture content - a question for Norbert perhaps
frank at livingsol.com
Sat Dec 17 18:46:45 PST 2011
/Does anyone on this list know whether wood burning best practice, as
outlined and no doubt practiced by several folks here, would 'work' in a
location where inversions occur? In other words is it conceivable that
even in 'the wrong' place it would be fine for everyone to burn wood if
it were just done right/very carefully?
I'd like to think so, but have so far not really found much to help me
Reuben, I'd like to think so, too; I really would.
I think there are lot of factors against it being fine for everyone to
burn wood, in any type of large village, town or city. Especially, in
an area of frequent inversions.
The main factor I believe is this:
Just about no-one is truly consistent about clean burning.
With the best of intentions, I have periods of burning time in which
there is a lot of unburnt wood gases, carbon particulates, and tars,
escaping up the flue pipe, to mix and dilute with the outdoor air.
During the initial fire start-up period is one common period of time.
Starting a fire in a cold firebox, (under 600degrees), guarantees that
some wood will convert to wood gas and be cooled too quickly to burn
into open flame. This unburnt wood gas then exits the flue into the
atmosphere, as a pollutant.
Even building a top-down fire gives off some particulate and unburnt
though this does shorten the unburnt gas productivity by a long, long shot.
Lighting a well-built fire from the top means that the chimney exhaust
will be clean, almost immediately, as the firebox begins to heat up,
since finely splintered kindling, shredded birch bark, and crumpled news
paper at the top of the fire will be burning cleanly with no dark coals
to smolder unburnt. This fairly well-carbureted gas mixture will burn
brightly and begin warming the flue, which increases the draft in the
chimney, while warming the firebox lining further. It's not perfect, and
there will be some black smoke, but the immediate heat of the fine
combustibles gets things going fairly cleanly.
Next, the upper layer of split wood begins to burn, while warming up the
thicker wood below it, and increasing the firebox lining's temperature
even more. Still burning pretty cleanly now, with plenty of combustion
air, and faster upward speed of the exhaust gases, in the now-warmed-up
As the lower layers of wood in the firebox are heated to the gassing off
temperature, the released gases are now ignited. They have better odds
of full ignition, as the firebox temperature rises to the flashpoint
temperature of the wood gas. When the firebox linings are heated to
this temperature, ignition is assured, as long as there is good
combustion air to mix with it. The object of the game is to burn only
fine split wood, until the temperature is high enough to ignite the
gasification of the larger wood pieces below the fine wood.
In most modern EPA stoves, the art of injecting the unburnt gases, back
into the hottest spot in the firebox, for a secondary burn, is the place
where recent technology has helped us woodburners come clean.
By and large though, a village of 1000 people, all burning wood, would
suffer poor air quality, some days, when there is no breeze to mix and
dilute the wood smoke. And how does one create the need in all these
different folks to learn and practice good combustion, which relies so
much on firewood moisture content, well designed appliances, well
maintained stoves and chimneys, and the human condition? A small
minority of those 1000 people in the village will be motivated and
curious enough to learn good burning skills, and to practice with
consistency. Meaning the majority of the fires will be burning poorly at
I think that the very interest in maintaining steady and even indoor air
temperatures is exactly the cause of poor burning. This comes about
through a generation of peoples' experiences living with central
heating, modern thermostats, and a variation of only a few degrees in
indoor air temperature, through out the whole day. This has been
socialized and normalized as a 'good thing'.
It is much easier to maintain a smoldering fire all day, giving off an
than it is to burn brightly, through the entire burning cycle, and then
let the fire go out.
For one thing, lighting two fires a day seems like a lot of work. And
another disadvantage of lighting twice a day, burning hot and clean, and
letting it go out, is that the temperature swings in many houses will
oscillate through the day, between ten degrees from high to low. This
sometimes requires donning a sweater for a few hours, then taking it
off, during the next burning cycle, and even having to open a window to
cool down, at times.
So one has to weigh and balance the pros and cons of consistently
burning cleanly, against the idea of having perfectly constant air
temperatures in the house, I think. Sometimes our lives are such that
we get what we get, and it's sometimes a smoldering, damped-down fire.
But I'd still like to think that educating a town full of woodburners
with keen enthusiasm for clean air could make it all more possible.
Living Sol ~ Building and Design
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