Inefficiency of burning wet wood

[fossil]

Yeah, " ...and a star to cow her by." just doesn't have the same ring to it. Rick

 

[Dix]

How about a COW pony?

Or, steer this

 

[Woods Dweller]

Does it make difference if you start with matches or a lighter?

 

[Apprentice_GM]

1)It's by weight. We typically use weight of water/total weight of wood + moisture.

2) you first need to multiply the mass of water in the wood x rise in temp to ignition x heat capacity of water (1 cal/g C).
Then add the product of the # moles water x molar enthalpy of vaporization.
Then add any additional heating of the gas (vapor heat capacity x g x temp change).
Convert to "per pound" and subtract from BTU/pound of wood (which is normally tabulated at something like 20% moisture- so only do the heat calc for the water amount OVER 20% mc in the wood).

A pound of water requires 1 BTU to raise its temperature 1 degree F...until it gets to its boiling (or saturation) point, which depends on the pressure it's under. Since we don't run our woodstoves pressurized, we're normally talking in the expected range of atmospheric pressures spanning our elevations...not a great deal of difference so far as water's concerned. Let's just say we're at sea level and normal atmospheric pressure, then the saturation (boiling) temperature is 212 F. We've expended 1 BTU per degree of temperature change to get the pound of water in our wood up to 212 F. Now, in order to turn that pound of water into steam at 212 F, we have to pump in the latent heat of vaporization, which is roughly 1000 BTU's, if I recall correctly, something in that magnitude, anyway. So, we've burned a bunch of wood to heat a bunch of water, then burned a bunch more wood to turn the hot water into steam, and now all that steam is going to go up our stovepipes, carrying all our heat with it, just itchin' to lose its latent heat and condense back into a liquid, depositing all sorts of nasty things with it (what we call creosote). Water...can't live with it, can't live without it. Water in beer = good. Water in firewood = bad. Rick

Thanks for the detailed reply and equations AP, and the simple explanation fossil . . . what I would like, and I'm sure a number of forum members including the original poster would benefit from, is a simple rule of thumb eg:

Heat going into home = efficiency of fireplace/woodstove * BTU's in firewood * (140% - (2 * moisture content))

or something similar

Of course I made up the last bit (140% - (2 * moisture content)) does anyone have a simple rule of thumb for this bit? There's gotta be a way of expressing that say burning seasoned wood at 20% moisture content yields 100% of the rated BTU's for that wood ('cause that's how they measure it) versus if you burnt it at 40% it would yield (say) 60% of the rated BTU output for that wood.

I know we all know that burning seasoned wood is way better than burning green wood but I'd like to be able to calulate it simply, or express it as a rule of thumb to friends etc without calc'ing moles and enthalpy (which I'm sure is correct and impressive but I can't do it in my head staring at a wood pile LOL).

C'mon geniuses, post and help a simpler bloke (me!) out

 

[fossil]

What would you do if you had such an expression? How precise a science do you think this really is? Would you decide whether or not to load another couple of splits before turning in for the night, or you've already gotten enough BTU's in there to see you through? Truly, I think what you're asking for is pretty much impossible to provide, given all the unknowns involved. I think we all burn qualitatively vs. quantitatively, because that's the reality of the process...too many variables to put it in a nice neat closed form equation. Believe me, as an engineer with more than my fair share of OCD, if I could burn by formula, I would. Rick

 

[Apprentice_GM]

What would you do if you had such an expression? How precise a science do you think this really is? Would you decide whether or not to load another couple of splits before turning in for the night, or you've already gotten enough BTU's in there to see you through? Truly, I think what you're asking for is pretty much impossible to provide, given all the unknowns involved. I think we all burn qualitatively vs. quantitatively, because that's the reality of the process...too many variables to put it in a nice neat closed form equation. Believe me, as an engineer with more than my fair share of OCD, if I could burn by formula, I would. Rick

Rick, what I am hoping it could do is a provide a rule of thumb like:

"Mate, I've measured your wood with my moisture meter and it's 30% MC. You'll need to burn 8 cords to get thru this winter . . . or if you leave it season for a year, it'll drop to 20%, and you'll need half that (or 5 cords or whatever). If you can leave it 2 years it'll drop to 10% and you'll only need to burn 3 cords . . ."

That's the kind of rule-of-thumb stuff I'd like to do with it.

It's the only thing I don't have a rough rule of thumb for so far - the missing element. We know how many BTU's in firewood by mass depending on species - that's measured and listed. We know how many cords per weight per species, and even how many cords from rounds or standing trees. Moisture meter's allow "reasonably" accurate measurements of MC, simple weighing can also tell you seasoning progress. It won't take me long to work out how long different species and different sizes of my typical firewood species season in my average climate (am doing that experiment soon) depending on wood stacking technique (Holz Hausen vs normal rows, am doing that experiment soon). Fireplaces / woodstoves have efficiency ratings.

I know it's not an exact science but neither is newtonian mathematics yet it serves a pretty good purpose

I really would like to know the amount of usable heat lost to moisture content as a % of rated BTU's (measured at 20% MC) over 20% per percentage. Even rough. Rule of Thumb. Ball park.

I mean something else that seems to go missing, is people get upset on these forums when they order and pay for a cord and get less. Rightly so. But usually they order a cord of seasoned wood, which should be down to <20% MC. If it is delivered at 25% for example, and that delivers 30% less usable heat (I'm making this number up, I don't know, it's what I'm seeking a simple rule for) then it's like they got 30% less than their cord as well. Just an example.

What's OCD? Off-the-cuff design?

 

[Cluttermagnet]

What's OCD? Off-the-cuff design?

Obsessive Compulsive Disorder ;-)

I'm not an expert, but I do understand intuitively that the seasoning process is highly variable. The rate of wood moisture loss is tied to many external factors including RH (relative humidity) over time, amount of sun and wind exposure, amount of heating from all sources, mainly the sun if outside, wood type, etc.

I couldn't agree more, I'd also love to have a good, strong rule of thumb which was even a fairly rough estimator. But I think there are just too many variables in the process. The only thing we know for sure is that most all process rates are dependent on differences, i.e. if the wood moisture is quite high, it outgasses at a much higher rate, right after splitting, than we will see at 1 week, and in turn, still less at 1 month than at 1 week, etc. We should probably expect that it is a steeply falling curve at the beginning, and it begins to level out fairly soon. When that curve becomes dead flat, the wood moisture would be in equilibrium with the RH. The absolute difference between the wood moisture and the RH is constantly decreasing, and it is the magnitude of that difference that drives the rate of change.

But RH goes up and down kind of randomly as the weather changes. And our wood gets rained on and 'backslides' a little. We get more or less sun and the rate of outgassing bounces around. Just ask the guys in the US Northeast this year- it rained endlessly up there. Probably a 'bad' year for wood seasoning. I'd think a rule of thumb would only be possible 'per species' (one formula per tree type), and probably only in a predictable, kiln-like environment for months on end.

 

[potter]

Wow! Just think how smart A.P. would have been if they had not kicked him out of school for drawing cartoons!

Yes that was a very educated answer. Im guessing AP is a chemist or an engineer of some sort by profession?

Naaa- he just stayed in a Holiday Inn Express last night.

Nah, he just got his paint brush stuck to the pallet and decided to become a rocket scientist

Remind me never to get into a war of wits with a Potter :cheese:

Just t let you know, your comments are being monitored by card carrying member of the P.A.L. (Pottery Anti-defamation League). :coolhmm:

 

[Apprentice_GM]

What's OCD? Off-the-cuff design?

Obsessive Compulsive Disorder ;-)

I'm not an expert, but I do understand intuitively that the seasoning process is highly variable. The rate of wood moisture loss is tied to many external factors including RH (relative humidity) over time, amount of sun and wind exposure, amount of heating from all sources, mainly the sun if outside, wood type, etc.

I couldn't agree more, I'd also love to have a good, strong rule of thumb which was even a fairly rough estimator. But I think there are just too many variables in the process. The only thing we know for sure is that most all process rates are dependent on differences, i.e. if the wood moisture is quite high, it outgasses at a much higher rate, right after splitting, than we will see at 1 week, and in turn, still less at 1 month than at 1 week, etc. We should probably expect that it is a steeply falling curve at the beginning, and it begins to level out fairly soon. When that curve becomes dead flat, the wood moisture would be in equilibrium with the RH. The absolute difference between the wood moisture and the RH is constantly decreasing, and it is the magnitude of that difference that drives the rate of change.

But RH goes up and down kind of randomly as the weather changes. And our wood gets rained on and 'backslides' a little. We get more or less sun and the rate of outgassing bounces around. Just ask the guys in the US Northeast this year- it rained endlessly up there. Probably a 'bad' year for wood seasoning. I'd think a rule of thumb would only be possible 'per species' (one formula per tree type), and probably only in a predictable, kiln-like environment for months on end.

Well I definitely suffer OCD then!

But all I was after was a rule of thumb for how moisture content (MC) affects usable BTU's, as opposed to a rule of thumb for MC in any given wood pile given all the variables as you said.

I'm happy for example to tell my friends that their new green wood pile is >40% MC now and in a year will be down around 20%, which few of them will really care about. When I extrapolate though and tell them that they only need to burn 50%, or 70% or whatever after a year for the same usable heat output, they ALL want to know that - it directly relates to how much effort and $$ they put in. So I was mainly really curious to know if I can say something like "every year of seasoning halves the MC in wood from time it was split, and increases heat output by 50%".

It was the second bit of that, not the first, I was really interested in and seems measurable and achievable, as opposed to the first bit, which yeah I understand so many variables too hard for a good rule of thumb (easier to just say "season it for 1 or better 2 years).

 

[fossil]

...Rick, what I am hoping it could do is a provide a rule of thumb like:

"Mate, I've measured your wood with my moisture meter and it's 30% MC. You'll need to burn 8 cords to get thru this winter . . . or if you leave it season for a year, it'll drop to 20%, and you'll need half that (or 5 cords or whatever). If you can leave it 2 years it'll drop to 10% and you'll only need to burn 3 cords . . ."

Even if you were able to quantify all the other variables involved to your satisfaction, you still wouldn't be able to make a statement like that with any high degree of confidence for the simple reason that it's impossible to know ahead of time just what sort of winter it's going to be. I'd like the Universe to be neat & tidy, and to understand all the subtle relationships between everything and everything else...to be able to derive from simple first principles the answers to questions which occur to me. But things just aren't simple. As you pointed out, even Isaac can take us only so far...then we need Albert to take us beyond Isaac's inherent limitations. I understand your "burning desire" for a reliable and consistent rule of thumb relating MC, seasoning time, fuel consumption and heat released. I just think it's unattainable beyond a SWAG. (Scientific Wild-Arse Guess). (BTW, I gotta tell ya I love Australia...at least what little of it I've seen. Visited Perth a couple of times during my Navy career.) G'day! Rick

 

[Adios Pantalones]

manoman- I go camping and look what happens. Burned lots of high moisture content (read "scavenged") wood this weekend. Kept warm by a good fire despite my tequila consumption.

 

[BJ64]

manoman- I go camping and look what happens. Burned lots of high moisture content (read "scavenged") wood this weekend. Kept warm by a good fire despite my tequila consumption.

LOL. I was wondering if you was gonna show up for this or not, after all we got on a roll because of your post

 

[JPapiPE]

manoman- I go camping and look what happens. Burned lots of high moisture content (read "scavenged") wood this weekend. Kept warm by a good fire despite my tequila consumption.

Yeah what A.P doesnt tell you is he bought a case of those Lincoln Logs for fireplace use and left them out in the rain after imbibing a case of tequilla and calls that wet wood.HA

 

[Todd]

[

I really would like to know the amount of usable heat lost to moisture content as a % of rated BTU's (measured at 20% MC) over 20% per percentage. Even rough. Rule of Thumb. Ball park.

Try a search for firewood BTU's and you will come up with many different tables. There are lots out there and they usually go by million BTU per cord at a certain m/c. Look for the fine print that will tell you the moisture content for the given numbers. If you can find a few different tables compare the BTU's and moisture content and maybe you can figure something out.

 

[Todd]

I looked at two different tables for Red Oak. One table was 20% mc, it had 24 million btu's per cord and the other was 12% mc and had 27 million btu's per cord. So if some math wiz out there can figure out the difference you may get a rough idea on loss or gain of BTU's by moisture content.

 

[JPapiPE]

You guys are crazy...most seasoned cord wood for a year is only in the 30% range, and 30-35% is acceptable in Maine for seasoned wood. Board lumber at lumber yards, which is kiln dryed measues about 15%M.C. this time of year. While Master Cabinet Makers will settle for 10-12% ...And Master Furniture Makers will usually have their own stash air drying for 2 years or more. At any rate furniture wood will be about 6-8% ...which is ideal. If you guys are worrying about 30-35% M.C. in your wood pile...DON'T ! you are right on target. I am a Master Joiner/ Finish Stair Builder and have dealt with these values for many years. So don't sweat the small stuff.

 

[Todd]

You guys are crazy...most seasoned cord wood for a year is only in the 30% range, and 30-35% is acceptable in Maine for seasoned wood. Board lumber at lumber yards, which is kiln dryed measues about 15%M.C. this time of year. While Master Cabinet Makers will settle for 10-12% ...And Master Furniture Makers will usually have their own stash air drying for 2 years or more. At any rate furniture wood will be about 6-8% ...which is ideal. If you guys are worrying about 30-35% M.C. in your wood pile...DON'T ! you are right on target. I am a Master Joiner/ Finish Stair Builder and have dealt with these values for many years. So don't sweat the small stuff.

30-35% is too wet. 15-25% is what you want to shoot for. Most firewood will reach this after 1 year sitting out in the sun and wind. Anything over 30% will sizzle in your stove, waste BTU's, and most importantly could cause creosote build up and chimney fires.

Here is my latest MC readings taken from the middle of a split piece,
2 year old Black Locust 15-20%
1 year old Black Locust 25-28%
3 month old Boxelder 35%
3 month old Red Oak 38%
6 month old Pine 21%
1 year old Spruce kept inside for last 3 months 8%! Wow, I didn't think it could go that low without a kiln. Maybe the dehumidifier in the room sucked all the moisture out?
Of course these reading were taken with a cheapo $20 meter, but it's a good enough guide for me.

 

[JPapiPE]

You guys are crazy...most seasoned cord wood for a year is only in the 30% range, and 30-35% is acceptable in Maine for seasoned wood. Board lumber at lumber yards, which is kiln dryed measues about 15%M.C. this time of year. While Master Cabinet Makers will settle for 10-12% ...And Master Furniture Makers will usually have their own stash air drying for 2 years or more. At any rate furniture wood will be about 6-8% ...which is ideal. If you guys are worrying about 30-35% M.C. in your wood pile...DON'T ! you are right on target. I am a Master Joiner/ Finish Stair Builder and have dealt with these values for many years. So don't sweat the small stuff.

30-35% is too wet. 15-25% is what you want to shoot for. Most firewood will reach this after 1 year sitting out in the sun and wind. Anything over 30% will sizzle in your stove, waste BTU's, and most importantly could cause creosote build up and chimney fires.

If you wait around for 15% You might not be burning wood for the next 2 years...A luxury most of us can't afford!

 

[Todd]

You guys are crazy...most seasoned cord wood for a year is only in the 30% range, and 30-35% is acceptable in Maine for seasoned wood. Board lumber at lumber yards, which is kiln dryed measues about 15%M.C. this time of year. While Master Cabinet Makers will settle for 10-12% ...And Master Furniture Makers will usually have their own stash air drying for 2 years or more. At any rate furniture wood will be about 6-8% ...which is ideal. If you guys are worrying about 30-35% M.C. in your wood pile...DON'T ! you are right on target. I am a Master Joiner/ Finish Stair Builder and have dealt with these values for many years. So don't sweat the small stuff.

30-35% is too wet. 15-25% is what you want to shoot for. Most firewood will reach this after 1 year sitting out in the sun and wind. Anything over 30% will sizzle in your stove, waste BTU's, and most importantly could cause creosote build up and chimney fires.

If you wait around for 15% You might not be burning wood for the next 2 years...A luxury most of us can't afford!

That's why I always recommend getting 1-2 years ahead.

 

[Apprentice_GM]

Try a search for firewood BTU’s and you will come up with many different tables. There are lots out there and they usually go by million BTU per cord at a certain m/c. Look for the fine print that will tell you the moisture content for the given numbers. If you can find a few different tables compare the BTU’s and moisture content and maybe you can figure something out.

I looked at two different tables for Red Oak. One table was 20% mc, it had 24 million btu's per cord and the other was 12% mc and had 27 million btu's per cord. So if some math wiz out there can figure out the difference you may get a rough idea on loss or gain of BTU's by moisture content.

That's getting almost good enough for me! On that single example it works out around 1.5% heat gain per 1% MC lost. That's a good rule of thumb. Of course as you said I'd have to check a few different species and tables and do some averaging. But it's a start.

Extrapolating that seasoning wood from 40% green to 20% typical gains the woodburner 30% heat. Nice. Or 45% to 10% (probably 2 to 3 years depending on all those variables) gives a 50% heat gain. Beauty! I'm off to find some tables . . .

 

[RedRanger]

Wow-do I hear an echo in here?/ If you can`t afford to burn dry wood, then yes, you can muddle thru. but you are playing with fire=creosote fire!! Okay, then be prepared to clean your chimney at least every 6 weeks, and don`t bother complaining to the manufacturer of your stove that is isn`t producing the heat that you expect!!

Very, very simple people, dry wood equals nice heat and very little cresote. Don`t follow the rules-then don`t expect the best results. Enough said, that`s it period!! :roll:

And maybe it`s time for some of you that don`t have that nice dry wood to burn to "think ahead to next burning season"??

 

[fossil]

...Beauty! I'm off to find some tables . . .

This from the guy who had to ask what OCD is. :lol: More power to ya, Mate! Let us know what you figure out. Rick

 

[JPapiPE]

Some of us started later than others in our decision to burn wood...or were forced into it by super- high price increases in heating fuel. We are what we are and we will burn what we have. There is no competition here. We burn what we have... Maybe next year we will be a little ahead of the game, but right now we are stuck... Any wood is better than a gallon of heating fuel... So we don't need be told how bad off we are... We'll make do.
We always do, one way or another...so relax all you excited wood burners with less than perfect wood...we all will make it. Let's let someone else panic, but not us...we are a brotherhood of wood burners and we will survive...by hook or by crook...i did spend all my emergency money on the stove/wood setup...so I am broke just like a lot of you. Don't fret we will persevere and come out on top. Warm and happy

 

[RedRanger]

Some of us started later than others in our decision to burn wood...or were forced into it by super- high price increases in heating fuel. We are what we are and we will burn what we have. There is no competition here. We burn what we have... Maybe next year we will be a little ahead of the game, but right now we are stuck... Any wood is better than a gallon of heating fuel... So we don't need be told how bad off we are... We'll make do.
We always do, one way or another...so relax all you excited wood burners with less than perfect wood...we all will make it. Let's let someone else panic, but not us...we are a brotherhood of wood burners and we will survive...by hook or by crook...i did spend all my emergency money on the stove/wood setup...so I am broke just like a lot of you. Don't fret we will persevere and come out on top. Warm and happy

Yes, you will for sure.. Just saying that what ever dimes or whatever you can scape together? It is for sure a good idea to think ahead to next year for your wood. This is the one game that you want to stay ahead of. Get to where you are going to be burning nice dry-seasoned stuff, year after year. And the farther ahead you can get yourself , the more money in the bank.

Or should I say, the more btu`s in the bank

 

[Rockey]

I think almost everyone here can empathize with the position your in we all had to deal with it when we first started. The problem I have is that you are actually endorsing it. Remember there are many first time lurkers out there that read everything on here and the last thing you want them to think is that it is ok to burn unseasoned wood.