# Heat loss due to wet wood



## Cowboy Billy (Dec 1, 2010)

I need some help here. We all know seasoned wood burns better than green wood. What I am trying to do is show a friend how much heat he looses by burning green wood. So I want to figure out how many btu's are in different woods. How many btu's it takes to boil off the water in it. And find out the net difference between a chunk of green wood and what it would be seasoned. 



> BTUs to Evaporate One Pound of Water
> Quick Answer: Somewhere around 1000BTU/lb
> 
> Long answer: It depends on the temperature of the water you start with. Before you can evaporate the water, you must heat it to it's boiling point. The warmer the water you start with the fewer BTUs will be needed to heat the water to its boiling point. Keep in mind the BTUs require to raise the water to its boiling point are very few compared to the BTUs required to change the water from a liquid to a gas.
> ...



So I'll start with poplar since I know it hold a lot of water. I believe I hear it was 60% water while growing.

A cord of poplar weighs 2168 lbs and has 15,000,000 btu. (it did not say if the it was green or seasoned) 

Shoot I don't have a scale whats a good sized split weigh? 5lbs?

15,000,000 divided by 2186 = 6861

So poplar has 6861 but's per pound (if I am doing that right) and say its  50% water thats .5 lbs of water and we need 575 btu's we have to take out of the poplars' total. Oops if it wet 50% I only have a half pound of wood so 3430.5 btu's minus 575 leaves me 2855.5 btu's net.

Ok I confused myself do you see where I am trying to go?

Billy


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## ISeeDeadBTUs (Dec 1, 2010)

Who 'burns' wet Poplar though?


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## Slow1 (Dec 1, 2010)

Interesting calculation.

I have found that I generally get about 800 splits/cord (average over the last couple years... scary that I count I know, but that is a different subject).  So, to use your poplar example:

1 cord = 2168 lbs / 800 splits = 2.71 lb/split
1 cord = 15,000,000 btu / 800 splits = 18,750 btu/split

Now to your question - is this dry or green?  My guess would be this is the dry based on the net weight/split that we end up with on this calculation.  Otherwise if we go with your 60% water (by weight) then that split would come down to somewhere around 1.5lbs once dry (20%ish) which I think is a bit too light based on my experience.  I'm going to go with the assumption that this is dry wood at 20% water by weight.

Now to take it up to 50% by weight, how much water do we add?  (digs through old HS math texts that are now being used by elementary kids with new terms..)

.2wa + .8wo = 2.71  Solve for wa,wo; wa = .542, wo = 2.168

Since we want wood and water to be the same next step is easy - wo and wa will be the same 2.168 lb so we have to add in 2.168-.542(wa) = 1.626lb of water.

Going with the calculation you have above assuming a room that is 80*f, it will take 1032 btu/lb or 1,678 BTU / split to boil off the water.  

Starting with the original 18,750 that would come to 8.95% of the BTUs.

Perhaps this doesn't seem all that bad, but then consider what the extra water does to the overall efficiency of the stove - when you have to open up the air to keep it burning while that water is boiling off and evacuating that steam which has a 1:1700 expansion ratio, you are putting a lot of heat up the chimney (higher volume of hot air flowing carries more BTUs out of the home); these losses are not reflected in this calculation.


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## CTYank (Dec 1, 2010)

For your calculations, you should know that wood moisture content is normally given as a percentage of DRY wood weight. That's absolutely dry.
Example: Some red oak is 80% moisture green; that means that the moisture content is 80% of that of the dry wood fiber.
Thus, you can have wood that's over 100% moisture content.

Good discussions of this in:
"Lumber Manufacturing", Ed M. Williston
"The Woodburner's Encyclopedia", Jay Shelton

Your friend must be a hard-headed one!


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## Adios Pantalones (Dec 1, 2010)

These calculations ignore a big fat piece of the loss: inefficient burning.  Not only do you have to heat and boil the water, but the wood will probably smoke and not burn like it would if dry.  That means that you don't have the same heat to begin with- much of the fuel is going up the stack unburnt (smoke)!

So do the water calculation, but then you need some way to estimate the total heat of combustion for the smolder as opposed to burning.  As someone that operates a large system that I tune/detune on purpose trying to get less efficient burning for much of the time- I can tell you that it's a big factor.


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## Cowboy Billy (Dec 1, 2010)

ISee

I don't think green poplar will burn at all. I have seen branches in the bonfire that barely char. But that is one that I remembered the % of water in as I have a lot of it and couldn't believe the difference in weight between dry and wet.

Thanks Slow1

Thats what I was looking for. But I had no idea how to set up the calculations up. I got my HS diploma but barely. And that was 26 years ago. Now I can tell him he has to do 9% more work for a fire that will be hard to light, keep going and creosote up his chimney. Hopefully by giving a number he can understand it. Actually I'll have to put it the other way. You can get 9% free heat if you CSS a year ahead.

Thanks CTYank

He is a really smart guy. But is cheep past the point of stupidity. He brought some logs over to cut up into lumber. I told him I would dump them in the woods I would be embarrassed to have someone see me cutting it into firewood. He said "well I will saw it up and see what happens" what happened was that a 2x6 he cut broke in three pieces when he and dad lifted it off the mill. My dad brother and I have been telling him to hook up his wood stove for the last 12 years. He finally did it last spring after seeing how well ours worked. He had been keeping his new house at 50 deg all winter and used a electric heater in his room. But now we got him to hook it up I'd feel really bad if he burnt the house down.

Thanks Adios

I would love it if someone could figure that out But if I give him too many options he'll try to figure the angles in a way to make it look like what he is doing is the right way.

Billy


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## Slow1 (Dec 1, 2010)

Cowboy Billy said:
			
		

> ISee
> 
> I don't think green poplar will burn at all. I have seen branches in the bonfire that barely char. But that is one that I remembered the % of water in as I have a lot of it and couldn't believe the difference in weight between dry and wet.
> 
> ...



I just crunched numbers.  Based on the feedback given I am willing to bet that the final 9% is way off - note that I did use a % by weight as my basis.  However, the assumption here was that we were going with green being 50% by weight.  I wonder if perhaps that is too low - the numbers could skew rather quickly if you keep going up.

However in any case - I think the best statement to make would be "you can get at LEAST 9% more heat for the work - likely much more" based on these calculations.  If anything I'll bet we have been rather conservative here.  So many other factors that can be worked in of course.  Bottom line though is that seeing is believing - once you burn really dry wood for a week or so you will never want to go back.


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## skyline (Dec 1, 2010)

Slow1 said:
			
		

> Interesting calculation.
> 
> I have found that I generally get about 800 splits/cord (average over the last couple years... scary that I count I know, but that is a different subject).  So, to use your poplar example:
> 
> ...



I can't comment on the counting number of splits/cord since often I am too busy weighing mine  

Similar to Slow1 I came up with the following 

From my notes I have:

1 kilocalorie = 3.96566683 btu

540kcal/kg is heat of vaporization for 1 kg of water

~2150 btu to boil 1 kg of water and 350 btu to raise 1 kg water from 60ºF to 212ºF together that equals approxamately 2500 btu / kg of water (I started my splits from 60 vs 80 °F )
So I figure for every 100g of moisture lost, I'm adding 250 btus to heat my house or 1136 btu / lb of wood for you die hard english units.

My kitchen scale reads out in grams so I stick with that and I commonly set some splits aside to weigh as they sit around the hearth to see how much water I lose. 

As others have pointed out, there are other factors wet wood adds to burning inefficiencies from lower secondary gas combustion, pipe cleaning and poor draft. 
But from my experience with a relatively leaky house, the biggest difference I notice with dry wood is good combustion at lower air flow rates which means I'm getting good heat output while having a lot less air going out the chimney that has to be replaced in my house by cold air leaking in. (I don't have a separate external air supply to the stove). 

What may be more useful to your friend that I have reported here before in my "wood drying experiments" is that a small house fan on splits even in a unheated garage and high humidities (85-95%) can remove lots of moisture in just a few days.  On very wet Fir and Alder I lost over 21% and 41% moisture by weight respectively in 10 days. Do that in a warm dry place with a fan and you'll do even better.  
The logical next question is whether the cost of running the fan is equal or better than the btu's gained!

10 days of running my fan at 34 watts = 34 * 24 hrs * 10 days = 8160 watt hrs or 8.160 kw hrs at $.10 / kwhr = $0.81.6 
My fuel oil is currently $3.11/gal and contains 140000 btus/ gallon  
So if I calculate for 10 days running the fan I get:     $.816 * 140000 btu / $3.11 = 36733 btu's of oil equivalent that I'm spending on electricity. 
This divided by the 2500 btus required to heat 1 kg of water = 14.7 kg or 32 lbs of water that I need to remove from my stacks to come out ahead not counting any of the other gains I get from my stove running drier wood. 
 On the 5 pieces of wood (3 fir, 2 alder) in my fan experiment, I lost 8.7 lbs in 10 days. 2 of the fir and 1 alder were away from the fan but I could tell later that they were clearly benefitting from the air movement in the room as they gradually caught up to the pieces directly in front of the fan. See the graph pic below. So I figure I'll easily exceed the 32 lbs with a whole stack of wood that is wet but since I try not have wet wood in the garage it may be less of a slam dunk. For wet poplar it seems a no brainer if you have to use it.

But as the kind folks around here so often say, "season...season....season!"


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## Cowboy Billy (Dec 2, 2010)

Thanks again Slow1

I too thought there would be a bigger %. But as you and the others have said its just one factor. With the fire burning cooler it is not going to combust all the gasses from the wood etc. But it still gives me a good number to through at him. What I need to do is weigh some wet splits save them let them dry and see what the real difference is.


Howdy Skyline

Thanks. While I did not test it like you are. I have noticed a big difference using a fan. Last year I was burning marginal wood and id just didn't want to dry unless I put a fan on it. My stove is in the basement and I usually through two face cords down at a time. Also last year I threw down a additional face cord of wet maple and stacked it in the middle of the basement. It brought the moisture level of the house up to a comfortable level.

Billy


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## Backwoods Savage (Dec 2, 2010)

Billy, I fear you would baffle your friend with the numbers and reasoning. So why not try to keep it a bit simpler? Find out what type of wood he burns. Then find some green wood of that type or take some of his. Then find some of that type of wood that is dry and well seasoned. Now you and your friend should build two fires. One with the green wood and one with the dry wood. Note the difference in how it was getting the fire started and how the fire burned. Also take note of smoke from both fires. You might even try standing by both fires to see if there is any difference in the amount of heat both give. How about how long they burn? One could have lots of fun this way without trying to reason at all.


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## billb3 (Dec 2, 2010)

I should think a couple armfuls of nice dry  wood and a couple armfuls of horrible wet wood and two identical very  cold days  would be a  rather convincing  experiment.
Your friend is an accountant ?


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## billb3 (Dec 2, 2010)

Uh, oh, East Coasters have had thier first cup of coffee...


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## Wood Duck (Dec 2, 2010)

Here are my calcs: If you have 1 pound of dry wood, you have around 7000 BTUs of heat. This would be a small split of firewood. If you were to burn that in a 75% efficient stove, you'd get 5,250 BTUs into the house. If the same wood were at 20% moisture content, you'd be adding 0.20 pounds of water, which would require 0.20 x 1100 BTUs to evaporate, or 220 BTUs. That would mean you'd only get 5,250 - 220 =  5030 BTUs into the house, a loss of 220/5250 = 4.2%. If the same wood was at 50% moisture content, the 0.5 lbs of water would take 0.50 x 1100 BTUs to evaporate, or 550 BTUs, which is 550/5250 = 10.5% of the total heat that could have gotten into the house.

I agree with Cowboy's earlier comment- a 10% difference is much smaller than I would have expected. Perhaps the bigger change is due to the way you have to burn wet wood. If the wood is wet, additional air is needed, which would reduce the stove efficiency. Old 'smoke dragon' stoves are supposed to be around 40% efficient, so if burning wet wood caused you to operate the stove differently and get only smoke dragon efficiency, you get only 40% x 7000 BTUs = 2,800 BTUs into the house. This would mean a loss of (5,250 - 2,800)/5,250 = 46.7% loss of heat value. There is your answer! I am not sure it is the right answer, but a 47% loss is a lot more dramatic than a 10% loss of heat.


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## Battenkiller (Dec 2, 2010)

Adios Pantalones said:
			
		

> As someone that operates a large system that I tune/detune on purpose trying to get less efficient burning for much of the time- I can tell you that it's a big factor.



As someone who operates a large wood burning system and deliberately uses green wood to induce a slow, smoldering and very smokey burn, you are no better than any of the old timers who are constantly castigated by the doyens of wood burning for doing the same thing in their wood stoves.  Should a kiln get a pass on this?  We should all remember that the EPA regs were put in place to keep the air cleaner, not to save you wood.  Not finger pointing, my good man, I've burned plenty o' green in my day.  Just trying to introduce a reality check for those whose only concern seems to be using less wood and cleaning their chimneys less often.


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## Battenkiller (Dec 2, 2010)

skyline said:
			
		

> What may be more useful to your friend that I have reported here before in my "wood drying experiments" is that a small house fan on splits even in a unheated garage and high humidities (85-95%) can remove lots of moisture in just a few days.



Sky, I've been waiting all season for the results of your "experiment".  Surely this deserves its own thread, eh?  I have many questions, but don't want to hijack this thread about heat loss.  What say you?


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## Adios Pantalones (Dec 2, 2010)

Battenkiller said:
			
		

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1) I have passive dampers that add air after the chamber as an afterburner system for most of the inefficient burning period
2) The periods of actual smokey burning are in the 1- 1.5 hour range every 3 months or so, not 12 hours a day for 6 months as with old woodstoves
3) My kiln chimney is the cleanest chimney you ever saw- bar none- after my high-fire period with flames blowing out the top
4) I'm not using green wood per se, but cutting down air or using dampers
5) This is not a manufactured device, but hand-built.  If they regulated fires in fire pits in my town, then maybe we could talk on it.
6) If I lived in an area where they allowed no brush burning for air quality reasons, then it might be a better analogy.

I'll add that in some areas, kilns are regulated, and there are laws in many places to ensure low-smoke operation- even when operated inefficiently (as in- smoke in the ware chamber).  Many designers use passive dampers or active post-combustion techniques along with taller chimneys as secondary combustion methods.  Dry wood often gives better inefficient burning because it gasifies so quickly, though water does do some other interesting chemistry for you.

I talked with a high school teacher from a private school in Dedham, MA last weekend about a wood kiln at the school where she teaches (I was blown away that they had a wood kiln there).  She told me the designer came over from Japan to build it with the main intent to reduce smoke during firing.


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## Cascade Failure (Dec 2, 2010)

skyline said:
			
		

> Slow1 said:
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Now, just to point out one other little factor, is the friend is storing the wood outside and bring it in just before or very shortly prior to burning? If so, we will need to redo the calculations from let's say 31F to 60F. My guesstimate would be another 150-200 BTU. This might be the reason I keep have a cord near the stove.


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## Adios Pantalones (Dec 2, 2010)

That wood still needs to be heated up- whether the fire does it before it gets in the stove or after- that's energy loss- so just use the outside temp.


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## Wood Duck (Dec 2, 2010)

Adios Pantalones said:
			
		

> That wood still needs to be heated up- whether the fire does it before it gets in the stove or after- that's energy loss- so just use the outside temp.



For the same reason, an Outside Air Kit doesn't change overall efficiency. One way or another, cold air from outside your house is heated to combustion temperature before it takes part in combustion. That can happen in the stove or in the living room, but it still happens and absorbs some energy. It isn't a lot of energy, neither is warming up the wood. There are far bigger uncertainties in the calculation - such as moisture content of the wood, type of wood and stove efficiency - to make warming up the wood or the air in the stove a critical part of the calculation unless you are a scientist.


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## Cascade Failure (Dec 2, 2010)

Adios Pantalones said:
			
		

> That wood still needs to be heated up- whether the fire does it before it gets in the stove or after- that's energy loss- so just use the outside temp.



Unless I was lucky enough to have brought the wood in prior to freezing or during a thaw. Then Mother Nature needed to use her BTU's, not me. 

Just making an point, perhaps an extreme one, to support Skyline's basic chemistry argument.


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## Adios Pantalones (Dec 2, 2010)

WD- Yup.  I don't have an OAK, but I think what it does do for you is to put that cold air in the stove, in the case that negative pressure would otherwise cause little drafts in the house, and it may solve negative pressure probs for people with draft (as in chimney draw) issues.

I am a scientist, and the question of taking water at one temp, and turning it to steam is actually one of those things you are taught in basic chem.  I haven't gone through anyone's calculatuions though- mostly because it looks more like work


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## Adios Pantalones (Dec 2, 2010)

Cascade Failure said:
			
		

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Just commenting on your calculations as they are- don't change the scenario.  We are assuming an outside temperature of 31F, I thought, for purposes of example.


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## Cascade Failure (Dec 2, 2010)

Adios Pantalones said:
			
		

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Fine...I should have been more specific in framing my argument.


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## Wood Duck (Dec 2, 2010)

Adios Pantalones said:
			
		

> I am a scientist, and the question of taking water at one temp, and turning it to steam is actually one of those things you are taught in basic chem.  I haven't gone through anyone's calculatuions though- mostly because it looks more like work



I am an engineer, and when we do calcs we leave out anything we don't think matters. We don't always choose the right things to leave out, but it makes the work a lot easier.


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## Slow1 (Dec 2, 2010)

Adios Pantalones said:
			
		

> WD- Yup.  I don't have an OAK, but I think what it does do for you is to put that cold air in the stove, in the case that negative pressure would otherwise cause little drafts in the house, and it may solve negative pressure probs for people with draft (as in chimney draw) issues.
> 
> I am a scientist, and the question of taking water at one temp, and turning it to steam is actually one of those things you are taught in basic chem.  I haven't gone through anyone's calculatuions though- mostly because it looks more like work



That is the basic calculation that started this whole thread.  I believe the key question (variable) that may be throwing the raw calculation off is exactly how much excess water may be in a given split.  The second question (and perhaps more interesting) is how to put an objective measure on all the other factors related to the effects of burning the wetter wood.  I'm not sure of how material some parts of the argument are (such as heating room air), but certainly the way that the boil-off affects the burn is material beyond the energy consumed converting water to steam.


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## Adios Pantalones (Dec 2, 2010)

Wood Duck said:
			
		

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We do too for most scenarios, but someone else postulated that the starting temp was different.  Fact is- water has a high heat capacity, and it's not a bad idea to see if it's on the order of magnitude to cause a difference.



			
				Slow1 said:
			
		

> That is the basic calculation that started this whole thread.  I believe the key question (variable) that may be throwing the raw calculation off is exactly how much excess water may be in a given split.  The second question (and perhaps more interesting) is how to put an objective measure on all the other factors related to the effects of burning the wetter wood.  I'm not sure of how material some parts of the argument are (such as heating room air), but certainly the way that the boil-off affects the burn is material beyond the energy consumed converting water to steam.



That was my first post- that the wood isn't burning the same, and I think (my educated guess, and no more) that this is a really big effect in overall efficiency.  There's still too many variables.  If I know that my wood is wet (sideways rain or slush, etc) in my woodstove- I mix it in with dry or leave the air open more to compensate.  I get a clean burn that way, but do sacrifice efficiency.


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## skyline (Dec 2, 2010)

Battenkiller said:
			
		

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Battenkiller, you have guilted me into it. My wood splits are in the oven so I can get a final dry weight to calculate their real moisture contents and not just how much water they lost. 
I'll start a new thread when I finish the results.

As Wood Duck pointed out, the ~10.5% difference from just the btu calculation is not that great and many of us suspect, greater differences may be due to efficiencies in how our stoves burn dry vs. wet wood. But that is partly why the early fan results were exciting (this is a relative term as only us wood burners know!). The fact that a fan in an unheated garage with high relative humidities can lower the moisture content of really wet wood in a couple of weeks by a huge % means that the btu calculation between this "blown" wood and "properly seasoned" and may turn out to be just a few 1-2%. Mostly I thought it might be helpful to all those folks (not me, of course) that aren't quite as far ahead on their wood seasoning as they should be. The other plus is that I think a fan in 24 hours will remove most of the "surface moisture" from wood that has been well seasoned but suffered from more recent wind blown rain/snow wetting.

Now back to the aroma of baking Fir!


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## Cowboy Billy (Dec 3, 2010)

Backwoods Savage said:
			
		

> Billy, I fear you would baffle your friend with the numbers and reasoning. So why not try to keep it a bit simpler? Find out what type of wood he burns. Then find some green wood of that type or take some of his. Then find some of that type of wood that is dry and well seasoned. Now you and your friend should build two fires. One with the green wood and one with the dry wood. Note the difference in how it was getting the fire started and how the fire burned. Also take note of smoke from both fires. You might even try standing by both fires to see if there is any difference in the amount of heat both give. How about how long they burn? One could have lots of fun this way without trying to reason at all.



Hi Dennis

   He's a retired H.S. auto shop teacher And is used to telling everyone what to do but doesn't listen much.  And feels everything he is told is a exaggeration. For him a hard number will work. Actually he's my Uncle but he still doesn't listen well. And I am sorry to say he would be to busy talking to pay any attention to a wet/dry wood comparison. 

Billy


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## Cowboy Billy (Dec 3, 2010)

Wood Duck said:
			
		

> Here are my calcs: If you have 1 pound of dry wood, you have around 7000 BTUs of heat. This would be a small split of firewood. If you were to burn that in a 75% efficient stove, you'd get 5,250 BTUs into the house. If the same wood were at 20% moisture content, you'd be adding 0.20 pounds of water, which would require 0.20 x 1100 BTUs to evaporate, or 220 BTUs. That would mean you'd only get 5,250 - 220 =  5030 BTUs into the house, a loss of 220/5250 = 4.2%. If the same wood was at 50% moisture content, the 0.5 lbs of water would take 0.50 x 1100 BTUs to evaporate, or 550 BTUs, which is 550/5250 = 10.5% of the total heat that could have gotten into the house.
> 
> I agree with Cowboy's earlier comment- a 10% difference is much smaller than I would have expected. Perhaps the bigger change is due to the way you have to burn wet wood. If the wood is wet, additional air is needed, which would reduce the stove efficiency. Old 'smoke dragon' stoves are supposed to be around 40% efficient, so if burning wet wood caused you to operate the stove differently and get only smoke dragon efficiency, you get only 40% x 7000 BTUs = 2,800 BTUs into the house. This would mean a loss of (5,250 - 2,800)/5,250 = 46.7% loss of heat value. There is your answer! I am not sure it is the right answer, but a 47% loss is a lot more dramatic than a 10% loss of heat.




  Ah Ha!! efficiency of the stove was something I did not think about at all And that makes a lot of sense Wood Duck!! Like me he is running a Johnsons Energy Systems wood burning furnace.  

   I knew if I got this out and all of us kicking it around we would get something figured out! As my friend Rudi says "Confusion breeds Discussion which breeds Knowledge which breeds Confidence which breeds Friendship"

Billy


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## CountryBoy19 (Dec 3, 2010)

Sorry, I didn't read the whole thread yet, but the most simple way to figure this is to find a wood chart that has weight/cord both green and seasoned. I like this one: http://firewoodresource.com/firewood-btu-ratings/

So lets use cottonwood for example:

You have 2225 lbs of seasoned wood, or 3475 lbs of green wood. That means there is 1250 lbs of extra water that needs to be evaporated out of the wood. Because we're actually superheating the steam (heating it above boiling) and we have to heat the water before it evaporates as well, I'm going to guess that it takes 1200 BTU/lb (I'm too lazy to dig out my referrence books and look). So it's going to take 1.5 MBtu. So the 16.8 MBtu drops to 15.3MBtu.

Just do that calculation for whatever wood you want. As mentioned already though, that doesn't take into account the inefficient burning as well.


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## CountryBoy19 (Dec 3, 2010)

Wood Duck said:
			
		

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Actually your logic on that is flawed, you have to take into account the heat has to transfer out of the stove.

Let me ask you this, does a 40 degree temperature decrease make more difference in your room temperature or in the stove temperature? IE, would you prefer your stove temp go from 550 to 510, or your room temp go from 75 to 35?

That is putting it in really simple terms, but that demonstrates that temperature difference is not equally comparable inside and outside of the systems.


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## Battenkiller (Dec 3, 2010)

Cowboy Billy said:
			
		

> So I'll start with poplar since I know it hold a lot of water. I believe I hear it was 60% water while growing.



You have to be very careful with those moisture content figures.  Most all of the available data on moisture content in green wood was gathered by the U.S. Forest Service, and so is expressed as a percentage of the oven-dry weight of a given piece of wood.  Wood heat values OTOH are computed using another formula, based on the actual weight of the water in the wood as a percentage of a green piece.  Check out the table at the bottom to convert between the two methods.  This table was taken from a book mentioned in a post above, "The Woodburner's Encyclopedia" by Dr. Jay Shelton.  From the table, you can clearly see that wood that is quoted by the industry as being at 60% MC actually only has 37.5% water by weight when green.




> So poplar has 6861 but's per pound (if I am doing that right) and say its  50% water thats .5 lbs of water and we need 575 btu's we have to take out of the poplars' total.



All hardwood has a heat value of about 8600 BTU per oven-dry pound.  All computations must begin with this figure, with heat losses computed and subtracted from this number.  Doing anything else will give wildly inaccurate results.



> Oops if it wet 50% I only have a half pound of wood so 3430.5 btu's minus 575 leaves me 2855.5 btu's net.



Even if that split was really 50% water by weight, you are getting twice as much weight in a green cord, so it's a moot point.  You will still have the same amount of wood fiber in that cord as in that same cord that has been oven-dried to 0% MC (and now weighs half what the green cord weighs), and the same amount of potential chemical energy will be locked into that wood.

Because of the incredible number of variables, it is pretty near impossible to put a figure on the heat loss.  You can get a very close estimate of the _theoretical_ loss due to heating up and evaporating the contained water, but you can't extrapolate this onto heating efficiencies in general.  All you can be sure of is that you will lose _at least_ all of the heat required to raise the water to 212º, plus the heat it takes to evaporate that same amount of  water, plus all of the latent heat of condensation from the water that is formed as a product of wood combustion (this is a lot of heat loss that cannot be avoided no matter how dry your wood is).  

Efficiencies of 75% are based on total combustion efficiency ("high heat value"), not heat transfer efficiency.  All of the water that leaves as steam will carry away about 1000 BTU/pound, whether it was in the wood in the first place or if it was formed by combustion itself.  This lowers the burn efficiency closer to the low 60% range (63% is the EPA estimate for non-cat stoves), with even further loses due to heat going up the stack.

These is absolutely no reason to use computations for this info, there are already industry-established tables that will give this info (called "low heat value") for any water content you will encounter.  Below is a nice chart (from the same book mentioned above) that presents this info in an easy to see graphical format.  As you can see, there is only a very small amount of additional heat loss due to burning wood at 60% MC (37.5% water by weight) compare to wood burned at 20% MC (16.7% water by weight).


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