Does Firewood Have Different BTU Content Depending On When It's Cut?
- Thread starter Philbert
- Start date
Help Support Arborist Forum:
brenndatomu
Hey you woodchucks, quit chucking my wood!
Ha! If a hen and a rooster both lay an egg on the peak of East/West facing barn roof, which way will the eggs roll???
Chris-PA
Where the Wild Things Are
Hint: don't eat the rooster eggs.
1project2many
ArboristSite Guru
Sap to/from leaves is controlled by cells at base of leaves, in abscission layer. Cells expand in fall, cut off sap, leaves die. Trees don't stop growing over winter, just slow down considerably. Growth rings do show large and small amounts of growth depending on season. Energy in stored sap is consumed to continue growth over winter. As said, energy = energy so once water is removed it's a wash.
Some people on here claim that standing dead elm with bark falling off burns better than bark on elm that is cut green then seasoned.
Hard to say.
But burn rate has something to do with the "perceived btu output" too. Tamarack has a btu rating of 19.5 but burns really fast. Red oak has a rating of 22.1 but burns slow. I know people who specifically search for or reject Tamarack based on its burn rate. But everyone wants red oak (unless you are spoiled and can get white oak, hedge, or ironwood.)
Hard to say.
But burn rate has something to do with the "perceived btu output" too. Tamarack has a btu rating of 19.5 but burns really fast. Red oak has a rating of 22.1 but burns slow. I know people who specifically search for or reject Tamarack based on its burn rate. But everyone wants red oak (unless you are spoiled and can get white oak, hedge, or ironwood.)
Deererainman
Firewood Hack
I've seen sap dripping out of Hedge Apple burn on hot coals like kerosene.
haveawoody
Addicted to ArboristSite
I can't imagine cutting wood at any time of the year will make any difference.
After seasoning before it can be burnt it will all be the same if it was cut green, standing dead, cut when the sap flows or when it's not.
Btu on dry wood will be identical.
After seasoning before it can be burnt it will all be the same if it was cut green, standing dead, cut when the sap flows or when it's not.
Btu on dry wood will be identical.
Yep, I'm one of those that say that... but I believe there's a bit more than "perception" involved.
The wood from standing-dead elm (standing-dead long enough for the bark to fall) is harder, denser and heavier than elm cut green and seasoned. I have a theory about why that is (and it's only my theory)... I believe the wood shrinks-up tighter as if "drys" on the stump.
I think of a dead wet wood like a wet sponge. If you hold the wet sponge by a corner and let the water run out of the pores, air displaces the water and the sponge shrinks very little. But if you seal the wet sponge in plastic (representing the bark of a tree), cut a small hole in one corner, hold the sponge by the opposite corner and let the water run out of the pores, air cannot displace the water and the sponge shrinks-up tight. If you've ever had an elm die in your yard where you can watch it, touch it every day I think this would make more sense. The bark stays tight to the trunk for some time, and then, about the time a few small branches begin to fall, the bark suddenly "goes loose" almost overnight. So loose there will be a full inch or more of "gap" between the bark and wood all the way round the trunk. Once dead, I believe the roots do allow water to drain from the tree... and they also allow water to be pulled up into the tree (like a sponge would do). I base this on the fact that standing-dead elm trunk will be dry all the way to the stump during "drought" years, but wet as high as 12-15 feet during "wet" years.
Now here's the thing...
The tree only contains so many pounds of wood. Whether it dies and drys on the stump or you cut it green and season it the total weight (mass) does not change... meaning the total (potential) BTU content does not change.
But what the "shrinkage" does change is the total volume of the tree, or the space it uses... meaning you'll have more pounds to the cord, but fewer cords from the same tree(s). You do end up with better firewood... just as oak is a better firewood than soft maple... because when you fill the firebox you can fit more pounds of it in there. On a BTU-per-pound scale, wood is wood; ten pounds of wood, any wood, contains the same BTU's... the only difference between oak and soft maple is the volume, or the space that ten pound consumes. So along that line of thinking (perception??); if oak is a "better" firewood than soft maple, than bark-off standing-dead elm is a "better" firewood than elm cut green and seasoned... correct??
*
1project2many
ArboristSite Guru
Once combustion starts the reaction wants to complete quickly. If one wood burns slower think of it in terms of what is inhibiting fire. Density of fuel has some to do with it but it's primarily because of moisture content. Many seasoned woodburners will complain that Oak stored over 4 years will burn too fast.
Whitespider, how does that theory "stack up" with the observation that the bottom of a standing dead Elm contains more water than the top? Does the bottom of the tree shrink once its dried?
It took at least three years of CS&S for green cut American Elm to begin to burn like standing dead AE. That wood really seems to hold onto water. I watch small trees here die and it usually takes a couple of years. I'd be curious if there's a decreasing moisture content during that time that shortcuts some of the time needed for seasoning.
haveawoody
Addicted to ArboristSite
1project2many,
American elm is an odd beast.
I have found that cut green and split green it never becomes like standing dead or cut into rounds and left to dry a while before splitting.
It's like total different firewood for burn times and burn quality.
I'm also in the American Elm shrinkage boat.
Unlike most other wood American Elm tends to shrink inward and the bark drop is a pretty good indication of it.
Once split that tends to stop most of the inward shrinkage, split green American elm holds it's bark, split after round drying or as standing dead the bark is very loose.
I think what we are seeing is a semi dense wood become a very dense wood and the reason for very different burn qualities cut and split as green or dead.
Top or bottom of a standing dead American Elm I can't say much, all my American elm sits as rounds format for a month or two before I split them so I can't give any useful input on it.
I think though once the bark has lifted the shrinkage is done so top to bottom will just be a dryer top than bottom thing with little or no impact after a short seasoning.
American elm is an odd beast.
I have found that cut green and split green it never becomes like standing dead or cut into rounds and left to dry a while before splitting.
It's like total different firewood for burn times and burn quality.
I'm also in the American Elm shrinkage boat.
Unlike most other wood American Elm tends to shrink inward and the bark drop is a pretty good indication of it.
Once split that tends to stop most of the inward shrinkage, split green American elm holds it's bark, split after round drying or as standing dead the bark is very loose.
I think what we are seeing is a semi dense wood become a very dense wood and the reason for very different burn qualities cut and split as green or dead.
Top or bottom of a standing dead American Elm I can't say much, all my American elm sits as rounds format for a month or two before I split them so I can't give any useful input on it.
I think though once the bark has lifted the shrinkage is done so top to bottom will just be a dryer top than bottom thing with little or no impact after a short seasoning.
Well, first I'll answer your question by asking another... does the bottom of a standing-dead elm always contain more water than the top??
Two years ago when I was cutting in September/October during the drought I found them to be dry all the way to the ground... cut, split and tossed them directly in the basement. I do most of my cutting during winter, and rarely do I find a standing-dead elm wet more than 3 feet up from the ground. What few I do cut in spring/early summer will be wet higher up, depending on where they're standing... those standing in low, wet areas will have more water than those standing on high areas.
Now, with that said, the elms attacked by DED in my woodlot tend to die in late summer/early fall and then spend a winter (when it seems they are the driest, or more water drains from them??) before the bark "goes loose" in spring, and then the bark starts dropping 'round mid-summer... they're "ready" for harvest by late fall and winter. But, to answer your question... does the lower wet part of the tree not shrink as much (or get as dense)?? Well, that could be, I know it tends to be the stringiest part to split... but I usually toss the wet stuff on the outside stacks to dry, and toss the dry stuff directly in the basement to burn. By the time the wet stuff ends up in the basement it's mixed up with everything else that's spent seasoning time outside... I guess, after the lower wet stuff has dried, I've never made a direct comparison to the upper dry stuff. Really, because it a relatively small portion of the tree, and mixed in with most anything, I've never payed any attention. (shrug)
Still, even if the lower wet part doesn't "shrink-up" like the rest of the tree, it only represents... what?... 10%, maybe 15% of the total tree.
So as far as "stacking up" with my theory... well... I don't see that relatively small part of the tree as "stacking it down" either.
But hey... it's just my theory, what do I know??
*
On that note I cut some standing dead, bark off sugar maple. That stuff was rock hard, heavy, and sparks were flying from the chain as I cut it. So I'm thinking this may have a similar shrinkage/compaction factor.Yep, I'm one of those that say that... but I believe there's a bit more than "perception" involved.
The wood from standing-dead elm (standing-dead long enough for the bark to fall) is harder, denser and heavier than elm cut green and seasoned. I have a theory about why that is (and it's only my theory)... I believe the wood shrinks-up tighter as if "drys" on the stump.
I think of a dead wet wood like a wet sponge. If you hold the wet sponge by a corner and let the water run out of the pores, air displaces the water and the sponge shrinks very little. But if you seal the wet sponge in plastic (representing the bark of a tree), cut a small hole in one corner, hold the sponge by the opposite corner and let the water run out of the pores, air cannot displace the water and the sponge shrinks-up tight. If you've ever had an elm die in your yard where you can watch it, touch it every day I think this would make more sense. The bark stays tight to the trunk for some time, and then, about the time a few small branches begin to fall, the bark suddenly "goes loose" almost overnight. So loose there will be a full inch or more of "gap" between the bark and wood all the way round the trunk. Once dead, I believe the roots do allow water to drain from the tree... and they also allow water to be pulled up into the tree (like a sponge would do). I base this on the fact that standing-dead elm trunk will be dry all the way to the stump during "drought" years, but wet as high as 12-15 feet during "wet" years.
Now here's the thing...
The tree only contains so many pounds of wood. Whether it dies and drys on the stump or you cut it green and season it the total weight (mass) does not change... meaning the total (potential) BTU content does not change.
But what the "shrinkage" does change is the total volume of the tree, or the space it uses... meaning you'll have more pounds to the cord, but fewer cords from the same tree(s). You do end up with better firewood... just as oak is a better firewood than soft maple... because when you fill the firebox you can fit more pounds of it in there. On a BTU-per-pound scale, wood is wood; ten pounds of wood, any wood, contains the same BTU's... the only difference between oak and soft maple is the volume, or the space that ten pound consumes. So along that line of thinking (perception??); if oak is a "better" firewood than soft maple, than bark-off standing-dead elm is a "better" firewood than elm cut green and seasoned... correct??
*
1project2many
ArboristSite Guru
Interesting... to both posts. I'm first to admit plenty of people have more experience with AE than I. I managed to find one large, live tree before I knew what it was and it left me emotionally scarred. Most of what I get here is small, 10" diameter or less thanks to DED. The dead trees here seem to be all wet or all dry. Weather, wind and sun exposure, and length of time dead all seem to be contributors. I missed out on a dead monster a few years back that would have given me something to compare with you folks. I think the small ones just aren't the same.
I know that rounds from the large green tree lost almost all bark fairly quickly whether split or not. The wood was piled outside uncovered and stayed wet. Orange mushrooms started growing from just under the bark, a slimy layer developed, and the bark could be pulled off the round or log with little effort. I chalked it up to cambium decaying, not to dimensional changes in wood. This makes me want to lay out some Elm cut in various stages of life / death to watch what happens.
One thing, I found it interesting how DED kills an Elm. It's the Elm's response to the fungus that actually kills the tree. The tree plugs its xylem effectively cutting off its own water supply. I wonder if this causes the tree to consume its own moisture in an attempt to survive.
For reference, some of my 1800s industrial manuals give recommended times to allow wood to "temper" before using in different applications. I'd never put "wood" and "temper" together except that day I was trying to hand split the big, green Elm.
I know that rounds from the large green tree lost almost all bark fairly quickly whether split or not. The wood was piled outside uncovered and stayed wet. Orange mushrooms started growing from just under the bark, a slimy layer developed, and the bark could be pulled off the round or log with little effort. I chalked it up to cambium decaying, not to dimensional changes in wood. This makes me want to lay out some Elm cut in various stages of life / death to watch what happens.
One thing, I found it interesting how DED kills an Elm. It's the Elm's response to the fungus that actually kills the tree. The tree plugs its xylem effectively cutting off its own water supply. I wonder if this causes the tree to consume its own moisture in an attempt to survive.
For reference, some of my 1800s industrial manuals give recommended times to allow wood to "temper" before using in different applications. I'd never put "wood" and "temper" together except that day I was trying to hand split the big, green Elm.
haveawoody
Addicted to ArboristSite
,
Got to love the fun job of cutting up standing dead Sugar Maple.
Makes for some serious long burn wood though but when cutting you have to keep reminding yourself of that LOL
Worse sugar maple I ever cut was a semi dead just over 3' and had about 1" deep of dirty bark, serious chain duller, that and the hollows sections filled with dirt made for a very long day.
A few sections with no bark at all made for tough sawing but the stuff with bark/dirt was gawd awful.
Every time you opened a hollow section up on a cut huge white bug larvae and dirt dropped to the ground then it was time to sharpen.
Got to love the fun job of cutting up standing dead Sugar Maple.
Makes for some serious long burn wood though but when cutting you have to keep reminding yourself of that LOL
Worse sugar maple I ever cut was a semi dead just over 3' and had about 1" deep of dirty bark, serious chain duller, that and the hollows sections filled with dirt made for a very long day.
A few sections with no bark at all made for tough sawing but the stuff with bark/dirt was gawd awful.
Every time you opened a hollow section up on a cut huge white bug larvae and dirt dropped to the ground then it was time to sharpen.
haveawoody
Addicted to ArboristSite
1project2many,
A fun experiment with American Elm is to cut two very similar size pieces of green wood.
Split one straight away and leave the other un split, put the splits and the un split round in a similar drying location and come back one month later.
At that time have a look at bark separation difference, wood shrinkage and give each a rap with a hammer and listen to the difference in pitch of identical wood drying in identical locations.
Get a marker and write G on the green split and D for the stuff you split at that one month, have a small fire with each and see the difference when they are both fully seasoned.
A fun experiment with American Elm is to cut two very similar size pieces of green wood.
Split one straight away and leave the other un split, put the splits and the un split round in a similar drying location and come back one month later.
At that time have a look at bark separation difference, wood shrinkage and give each a rap with a hammer and listen to the difference in pitch of identical wood drying in identical locations.
Get a marker and write G on the green split and D for the stuff you split at that one month, have a small fire with each and see the difference when they are both fully seasoned.
mainewoods
Addicted to ArboristSite
Seems reasonable that a piece of firewood cut in the spring and dried to 20%mc, has the same BTU rating as a piece of firewood cut in the winter and dried to 20%mc. They are both the same species, and they are both at 20%mc. If identical tree's, one with a 75% mc and the other with a 50% mc, are both seasoned down to 20%, why would the BTU output be any different?
A sawyer may be able to validate this. I know that wood in the round dries differently than when it is split or sawn. Could this "compaction" while drying in the round acutually cause the wood to be more dense than green cut wood?
I'll bet that someone at the USFS has studied this. I thought that there might of been some more 'rule of thumb' type information available. Thanks for all the input!
Philbert
Philbert
Download this...
http://www.nhla.com/assets/1603/wood_handbook.pdf
... then read chapter four, "Moisture Relations and Physical Properties of Wood".
Just about anything you'd ever want to know about moisture in wood, and its affect on wood will be found there.
*
ppkgmsy
ArboristSite Operative
Very informative, stuff I didn't know. Thanks.
