Sunday, December 27, 2015

handling firewood

Firewood will not be able to heat us all after the fossil fuels are gone, but with careful stewardship and fewer homes, it will be a way for many of us to keep warm in those places that get cold in the winter and have enough rainfall to grow trees.

For now though, I am lucky enough to be in a place where wood heat is feasible and sustainable for the local population density. The climate and terrain in Southwest Wisconsin are conducive to sustainable wood harvesting. 

I have projects in mind for improving the efficiency of our house, but in the mean time, we are burning around three cords of wood a winter. 

While splitting wood by hand, I often just kind of go blank, or "get in the zone", but the mind sometimes wanders. Can't tell you how many times I silently repeated that old saying "chopping your own wood warms you twice- once when chopping it, and once when burning it". Attribution is to many, but it's a pretty obvious conclusion to anyone who splits wood by hand.

But then I thought about how many times I touch the wood with my hand, ( and wearing out gloves) and began counting. After I drop a dead elm with my chainsaw ( not ready to use my crosscut on felling just yet, maybe next year) here are the steps to get in to the Russian furnace.
1. From the ground in to my truck
2. From truck to a pile next to the splitting station
3. From the ground on to the top of a big old oak round 
4. (smaller and more pieces now)From the ground to my wood shed
5. From my wood shed into a wheelbarrow
6. From the wheelbarrow in to a bin in our sunroom
7. From the bin in to the house and into the woodbox
8. From the woodbox in to the Russian furnace

and I guess we won't count the steps the ashes go through before they become soap or soil amendment.While the various handling steps don't wear you out like the actual splitting, they do make one appreciate how much time and labor we replace with fossil fuels these days. 

I was talking about firewood with a neighbor, and mentioned that I preferred not to use box elder because of the low btu content, and he stated that density and btu content are directly correlated, to which my immediate reaction was, gosh, that makes sense, but is it true? So I went looking for info to see for myself. Could find nothing on the internet that charted the correlation, so I found sources for various densities, and other sources for btu content, and graphed the two.

The chart below has density is in pounds (LBS) per cubic foot (FT3) and the heat content is in MBTU/cord. A cord of wood is a volume 4 feet by four feet by eight feet. Quite the mish mash of old units, but this doesn't really matter when looking for a correlation. 






In general, it is a pretty good linear relationship. Red cedar, apple, and shagbark hickory are the three that are a bit above the line, meaning that they have less btu content than one would extrapolate from their density. Shagbark hickory had the highest density, and also a very good btu content. Ironwood had the best btu content per unit volume, so it would be the best firewood to use, except it is less common, and does not grow to a large size. In my general area, oak is by far the most common wood for heating, as it has a good heat content, and is a common tree here. 

There are a few firewood or common trees for our area that I did not find numbers for, so this chart is of course not definitive, and I also can't vouch for the sources I used, but in general I think I've confirmed the  common sense notion that the density and heat content should have a linear relationship. Here is the data in tabular form. These are trees for the U.S., but I would imagine the relationship holds for other species.


alder
14.8
23.7
apple
21.6
46.5
black ash
17.9
32.8
green ash
21.1
38
white ash
21.6
39.8
aspen
13.7
25
basswood
13.7
24.8
american beech
22.7
40.8
blue beech
23.7
44.7
grey birch
19.5
34.4
yellow birch
22.1
41.7
white birch
20
37.4
butternut
13.2
25.2
redcedar
12.1
30.7
white cedar
11.6
19.6
black cherry
19.5
33.3
american elm
18.4
34.5
balsam fir
13.2
25.8
shagbark hickory
25.3
50.53
ironwood
26.4
47.5
black locust
23.2
44.2
honey locust
23.7
41.5
black maple
21.1
38.7
red maple
20
34
sugar maple
23.2
42.2
bur oak
22.7
41.8
red oak
22.1
41
white oak
24.7
44.3
jack pine
14.8
28.7
red pine
17.9
31.6
white pine
13.2
23.2

A final word for anyone heating, or contemplating heating with firewood- KEEP IT DRY! My pet peeve is heating with unseasoned or wet firewood. So much energy is wasted turning water to steam that it is a criminal waste, one might as well chuck half the wood you've worked so hard on into the nearest ravine. I see it all the time though, especially those who have the wood boilers.
The latest firewood shelter

I might rant on that another time, but for now, KEEP IT DRY!

5 comments:

  1. Great post!

    Question about the graph: are the wood "BTU content" values per weight or per volume? If they're per-volume I would expect a strong correlation with the density, whereas if they're are per weight as well, that is _very_ interesting (and I don't know quite what it means)...

    Your post reminded me of this lovely old poem that I read years ago:
    http://www.eudesign.com/mnems/burnwood.htm

    ps. love your wood shelter -- I'm part way through building a very similar wood and bicycle shelter myself (we burn a lot less wood in a very mild climate ;-)

    Best wishes for 2016,
    -Angus

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  2. Hi Angus- Interesting question. The BTU content is on a volume basis, but it would be interesting to figure out what the comparison is on a weight basis. I might try to figure out a plot that would determine that, but I'm in no rush. I'm a bit rusty at algebra now.

    Great poem. Lots of practical experience speaking there. I take it this is a mnemonic from England, and wonder if their elm is the same as ours. I have been burning a lot of elm this winter ( dutch elm disease killed a lot of them last year) and it burns pretty well. Not as good as oak, but quite serviceable, and better than churchyard mould.

    Thanks for the comment, and best wishes for 2016 to you to.

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  3. Hi Steve,

    Great shelter for the firewood. I'm assuming that you bring the firewood inside and keep it somewhere dry over the winter, or are your winters very low in humidity so it doesn't matter? Also just out of interest, how long do you have to season your firewood and does it differ depending on the species.

    I had to do some metric conversions to understand your post so 3 cords is 10.8 cubic metres.

    Also you may be interested to know that the local species of timber here has a density of 650kg/m3 which equals 40.58 pounds/cubic foot and there is an awful lot of it, but it must be seasoned for at least 24 months prior to use or as you say it is worse than useless. Burning unseasoned or wet timber is a total disaster as you quite rightly point out. The high acidity of the smoke also rapidly destroys the steel in your firebox too. It is just very wrong.

    Firewood is my main heating source for about half the year. You may be interested to know that over the past day I've just completed converting my old chicken shed into an all weather dual steel lined fire wood shed. The winters here are very humid so the firewood would be very wet and damp in your arrangement and it just will not burn. I'll put some photos up on the next blog and hopefully fill the shed at some point over the next week.

    I reckon locally sourced firewood is about as sustainable as it gets – and no I haven’t considered the cross cut saw either. But I do use an electric log splitter which is powered off the solar power system – it is very good.

    Cheers

    Chris

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  4. Oh yeah forgot to mention the best timber here is river red gum and the trees live for many hundreds upon hundreds of years (unlike my lot which is probably about 400 years max - but my lot grow to huge heights in less than 30 years) and that is apparently about 750kg/m3 which is 46.8 pounds/foot3

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  5. Chris- sorry I got lazy on including metric conversions for cords. Your comment got me curious about river red gum, and found this site that is more tuned to your neck of the woods. Looks like river red gum rates pretty well.
    http://herfoswoodshed.com.au/herfosshop/index.php?main_page=page&id=3

    About storage- Our winters are low humidity, so I mostly just need to keep rain off the wood. Having said that, I might put some partial walls on my shed, as driving rain happens on occasion, getting some of the wood wet. I've always been told that plenty of moving air and sunlight are important for seasoning, thus the open sides. I also don't need to worry about brush fires like you do.

    I've not been burning wood that long, so am still using up the dead elms here. Since they are standing deadwood that has been dead for at least two years when I cut them, they are well seasoned. I still cut and split a year ahead.

    The good practice norm around here is to season for a year, but I hope to season for longer when I have to start using other wood. I'll learn about the necessary seasoning durations for various species as this happens. Thanks for the comment.

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