[Greenbuilding] firewood moisture content - a question for Norbert perhaps

Norbert Senf mheat at mha-net.org
Sun Dec 18 15:12:08 PST 2011

At 09:49 AM 12/17/2011 -0800, Reuben Deumling wrote:

>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 understand this.

Just to get some numbers on it, let's look at the 
extremes, which occur during low heat demand: a 
well functioning pellet stove, vs. an old 
technology outdoor boiler in low output (smoldering) mode.

The pellet stove will emit roughly 1 gram of 
particulate (PM) per kg. of fuel. The outdoor 
boiler can emit as high as 100 grams of PM/kg or 
more. Furthermore, the pellet stove PM will be 
soot, which is less harmful to health than tar, 
which is what the outdoor boiler emits. The 
boiler efficiency will be considerably lower, so 
you'll need to burn about 50% more wood at low 
output for the same amount of heat as the pellet 
stove. Notice that at 100 g/kg PM,  10% of the 
weight of your wood is leaving the chimney as tar, which is unburned fuel.

With even an old tech stove, if you burn it hot, 
it is fairly easy to burn at 1 g/kg. That would 
be best practice, for emissions. However, you 
could easily get it up to 30 g/kg or so by 
smoldering it, which you would be forced to do 
during low output unless you are willing to split 
your wood down to matchsticks and reload every 5 
minutes. With a newer EPA stove, your upper limit 
might be down around 5 - 10 g/kg, depending on 
the model. The EPA limit is 7.5 g/kg, which is 
averaged over 4 burn rates - the hardest is to burn clean at a low burn rate.

Masonry heaters are around 1 g/kg also. It is 
really no big trick, since you simply have to use 
dry wood, properly sized, and burn it fast 
(enough), and be a little careful about the 
start. You get around the heat output issue by 
doing a daily batch burn and storing it for 
release over 24 hours. If you operate it badly, you will be around 4 g/kg.

If you have a big old drafty farmhouse and need 
to burn a lot of wood, it is easier to burn 
cleanly, although your particulates will still 
add up because of how much wood you have to burn. 
A masonry heater would be a bad match, because it 
can't do high output. Ideally, Reuben's "wrong 
place" will consist of low energy houses, that 
don't need to burn a lot of wood to stay warm.


Norbert Senf---------- mheat(at)heatkit.com
Masonry Stove Builders
25 Brouse Rd.
RR 5, Shawville------- www.heatkit.com
Québec J0X 2Y0-------- fax:-----819.647.6082
---------------------- voice:---819.647.5092

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