Does Firewood Have Different BTU Content Depending On When It's Cut?

[Philbert]

Theoretical question. Suppose you had a stand of oak, or maple, or whatever that you were planning on cutting for firewood. Would the BTU content (after seasoning) be different if the live trees were cut in the Spring, Summer, Winter, or Fall? Does sap content (once seasoned) add or subtract heat content?

Philbert

 

[mr.bear]

opcorn:

 

[zogger]

Very interesting! I know they dry quicker cut in the winter with the sap down. Hmm..sap is more than just water, so, it must have some calories/BTUs to it. I would pure guess cut when juicy and eventually dried, slightly higher BTUs.

BUT...available BTUs is another thing, they might not still burn as good as cut when the sap is down, even with the MC being equal..I don't know. So..hmmm

 

[Chris-PA]

Very interesting! I know they dry quicker cut in the winter with the sap down. Hmm..sap is more than just water, so, it must have some calories/BTUs to it. I would pure guess cut when juicy and eventually dried, slightly higher BTUs.

BUT...available BTUs is another thing, they might not still burn as good as cut when the sap is down, even with the MC being equal..I don't know. So..hmmm

That's along what I was thinking - interesting question. If I had to guess I'd bet the difference is negligible.

 

[Philbert]

Fatwood got me thinking of it ('Fatwood' would be a good A.S. user name, no?).
http://en.wikipedia.org/wiki/Fatwood

Maple trees have sugar sap, and sugar burns. Maybe it depend on the species of tree? And maybe, as Chris-PS suggests, the difference is negligible in a practical sense. Yet, I would never believe that they could get much oil out of squeezing corn kernels, and they fill rail cars with that stuff!

Philbert

 

[zogger]

Fatwood got me thinking of it ('Fatwood' would be a good A.S. user name, no?).
http://en.wikipedia.org/wiki/Fatwood

Maple trees have sugar sap, and sugar burns. Maybe it depend on the species of tree? And maybe, as Chris-PS suggests, the difference is negligible in a practical sense. Yet, I would never believe that they could get much oil out of squeezing corn kernels, and they fill rail cars with that stuff!

Philbert

Ya, just thinking of that. Pine would burn better dried with full sap, but it takes a long time to dry! I burn both here, stuff cut green is real dang juicy and sticky and hard to split. Standing dead and blow downs are much drier and easy to split.

Some of my cut green pine, split, dried, then re split into kindling went out as bundles last winter and sold better than my oak. THAT wood really burned fast and hot. I think because people ran out from the polar vortex and were trying to burn still half green hardwood...gonna do more this year for sure, got plenty of dry pine. Going by memory, I think pine burns better with the sap in it, ONCE it is really dry. And fatwood, geez loweez, I guess..like chunks of solid diesel, and that is mostly resin/solid sap.

Tree species are so widely varied, some are proly better sap in than without and vice versa.

 

[Chris-PA]

Ya, just thinking of that. Pine would burn better dried with full sap, but it takes a long time to dry! I burn both here, stuff cut green is real dang juicy and sticky and hard to split. Standing dead and blow downs are much drier and easy to split.

Some of my cut green pine, split, dried, then re split into kindling went out as bundles last winter and sold better than my oak. THAT wood really burned fast and hot. I think because people ran out from the polar vortex and were trying to burn still half green hardwood...gonna do more this year for sure, got plenty of dry pine. Going by memory, I think pine burns better with the sap in it, ONCE it is really dry. And fatwood, geez loweez, I guess..like chunks of solid diesel, and that is mostly resin/solid sap.

Tree species are so widely varied, some are proly better sap in than without and vice versa.

The fatwood is kinda like wood soaked in turpentine, but that might be a characteristic of that particular type of sap. My impression is that pine sap is used to make some different things than hardwood sap, and it's flammability may be different.

 

[Whitespider]

I doubt it makes any difference… because energy (mass) cannot be lost, it can only change form.

There are two kinds of sap in a tree.

The sap moving up from the roots through the center of the trunk/branches is mostly water and dissolved minerals/nutrients from the earth, it feeds the leaves. The leaves use photosynthesis to convert the minerals/nutrients into sugars, which flow back down just under the bark, feeding growing parts of the tree that cannot photosynthesize. Your firebox cannot burn water and minerals (the inner sap) so that means nothing to the recoverable BTU’s in wood. In fall/winter, when the roots stop feeding the leaves, causing them to shrivel and die, the sugars remaining in the outer sap is not wasted (that would be a stupid life design, don’t ya’ think?)… it is either stored (such as maple trees) to make new leaves in the spring and/or utilized by the tree to make more wood/bark/roots/etc. The (now) thicker (because of less water) sap also acts as a natural anti-freeze to protect the tree as it over-winters.

Really, the only thing that changes is the moisture (water) content of the remaining sap as the tree enters winter hibernation… simply because the tree doesn’t require as much moisture during hibernation. It may also dump any excess water and minerals/nutrients (that you can't burn) from the tree center back out through the roots, but the tree ain’t gonna’ dump any sugars it worked so hard to create… it just stops pulling in water and minerals your firebox can’t use anyway.
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[jrider]

I don't know but if you cut and burn in the same year, some people in here might just have a heart attack.

 

[taskswap]

... Pine would burn better dried with full sap, but it takes a long time to dry! I burn both here, stuff cut green is real dang juicy and sticky and hard to split. Standing dead and blow downs are much drier and easy to split.

ChimneySweepOnline mentions this in one of their FAQs about burning pine for newbies. They say it's where the myth about that being terrible came from - not that burning pine is bad because once it's dried it's just like anything else. It's just that because it HAS the resin, it's easier to burn it green (which you shouldn't do) than it is to burn other species green. The point being that just because you can light it doesn't make it good wood stove material (yet).

So by reverse logic that sap is fuel, once dried... which confirms your point.

 

[Whitespider]

So by reverse logic that sap is fuel, once dried... which confirms your point.

Yes... but...
Going back to the original post/question...
Does the total weight (mass) of the burnable stuff in sap, in a living tree, change with the seasons??
And if it does... have those BTU's been lost, or has that (usable when dried... i.e., moisture removed) mass simply been converted into another form with the same potential BTU's??
I say, the difference in potential BTU's contained in a tree will not change with the seasons. No living thing will, through natural process, simply waste energy by dumping it... especially something it had to use energy to convert (into sugars) in the first place.
Heck, even your body converts potential energy and stores it as fat rather than waste it...
Biology... the original question is a biology question... the answer was taught in your second grade science class.
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[zogger]

Yes... but...
Going back to the original post/question...
Does the total weight (mass) of the burnable stuff in sap, in a living tree, change with the seasons??
And if it does... have those BTU's been lost, or has that (usable when dried... i.e., moisture removed) mass simply been converted into another form with the same potential BTU's??
I say, the difference in potential BTU's contained in a tree will not change with the seasons. No living thing will, through natural process, simply waste energy by dumping it... especially something it had to use energy to convert (into sugars) in the first place.
Heck, even your body converts potential energy and stores it as fat rather than waste it...
Biology... the original question is a biology question... the answer was taught in your second grade science class.
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When it goes back down in the roots in the winter (if this happens, I think it does), then some of that is lost where you aren't getting it. Obviously as the tree is growing and makes new cells they use some of what is in the sap to build the cells. Once the tree is cut, cell division most likely stops fairly soon, so..the sap is still there, eventually it dries, whatever burnables were there are still there, unless there are some volatile organics that evaporate.

 

[mallardman]

The sap doesn't really go back down to the roots. It was used by the tree to grow make leaves etc. The leaves also finely drip it.


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[zogger]

The sap doesn't really go back down to the roots. It was used by the tree to grow make leaves etc. The leaves also finely drip it.


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So, in the fall it just stops flowing, and as the leaves wither, it (water content of the sap_ transpirates out/drips?

 

[Whitespider]

When it goes back down in the roots in the winter (if this happens, I think it does), then some of that is lost where you aren't getting it.

If it does, it would only be the sap contained in the heartwood... just water with dissolved minerals/nutrients from the soil, nothing you could burn in a wood stove. Only after that heartwood sap is delivered to the leafs can the sun's energy be used to make sugars (the burnable stuff)... which is sent back down the tree just under the bark to the tree parts that are growing. Heartwood does not grow in the sense it reproduces itself, sapwood becomes heartwood as new sapwood is created... sapwood is the living, replicating tissue That sap, the outer sap, the stuff with the sugars in it, ain't gonna' be dumped back into the soil by the tree... that would be a stupid life design.

So, in the fall it just stops flowing, and as the leaves wither, it (water content of the sap_ transpirates out/drips?

When the tree stops delivering fresh heartwood sap to the leafs, they're still alive and suck out as much moisture/minerals/nutrients as possible until they succumb. And, with few exceptions, a tree's bark, buds and whatnot is not 100% air tight... after the flow of new water from the soil stops, some remaining moisture will be lost to evaporation, transpiration, and such (but the sugars, the burnable stuff, the stuff containing potential BTU's for the wood burner, is not lost... it is stored by the tree for next year's awakening).

Biology 101... the potential BTU's, as firewood, contained in a living tree does not change with the seasons... but water/mineral content may depending on species and conditions.
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[Whitespider]

Oh... there are a few exceptions zogger,
One I know of is Birch, which has a pretty air-tight bark. Birch trees have unique roots that can allow water flow in both directions... but will only allow the flow of dissolved minerals coming in, not out. Because of it's relatively air-tight bark it can't easily get rid of excess moisture and does, in fact, dump water (just water) back into the soil as winter approaches... some of the outer (sugar carrying) sap is also allowed to dump moisture (just water) back into the soil. In the case of Birch, some sugars (burnable stuff) is stored in the roots where it would be lost to the wood burning fool. But birch is the exception... not the rule.
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[066blaster]

When I cut standing Dead oak it seems denser and heavier than if I cut green oak and season it. Maybe some of the sugars stay in the wood instead of evaporating??? Also noticed white oak is rated higher than burr oak. Is it just because burr oak has a much thicker bark and there will be a higher percentage of bark in a cord.

 

[stihly dan]

I too have noticed this.

 

[dancan]

A pound of dry wood or a pound of any dry wood has the same BTUs

 

[Wood Doctor]

Wood that is rotting is losing BTU content. Before it rots, the heat content is likely no different. When green (unseasoned or wet) it just burns more slowly and boils off more water, which has no heat content at all. If you could measure the moisture content of all the wood species and all samples had the same moisture content (say 10%) the species of wood with the highest density at that point (lb per cu ft) would likely also have the highest heat content (BTU/lb).

The only possible discrepancy to this experiment would be if the moisture for some reason set off an exothermic chemical reaction in a few of the species and that reaction produced heat as the burning progressed. That is not impossible.