Effect of Moisture on Wood Burning Efficiency, Revised – Installment 3

By John E. Laswell, Mechanical Engineer – Aug 2008

    Disclosure Statement – the information in this article is mainly the opinion of the author except where websites are shown.  There may be errors in the information and errors in any calculations.  So, the ideas presented herein should not be used to design a heating system for any home or building.  Application of any engineering formulas and principles may be in error.  This article is stickily for the reading enjoyment of the curiously minded individual!!

     Forward, Hello fellow DIYers.  My name is John L. and I live in the hills of scenic, southern Indiana.  I have burnt a wood furnace in my basement for 22 years.  The wood came from my 38 acre farm, mainly from dead or non-timber grade trees.  My wood was cut with a chain saw and hand split for the first nineteen years.  I now have a 27 ton hydraulic log splitter.  I mainly burn oak, hickory, ash, hard maple and dogwood.   For the last three years I have I have been heating one half of my home with a King, 1107J, Blaze King stove.  It is wonderful.  I have installed it in my dining room.  In my opinion this is the best wood stove.  If there is any interest, I would write an article about my experience with buying, installing, operating and the pure enjoyment of heating with the “king”. 

     I am so tired of products that have been “value-engineered” to the point of planned-failure-in-design.  How many times have you been asked when you purchase a fairly simple product if you wanted to buy an extended warranty plan?  Here is some of the modern philosophy: use it up, burn it up or throw it out when it stops working.  I recently bought a pair of trousers where it cost almost as much to alter the length as the cost of the trousers!  Something is wrong!  I know what it is? Products made in China and cost of labor in USA.

     This article comes in three installments:

     First – Introduction, Wood Stove Efficiency and Results

     Second – Determining The Percent Of Moisture In A Wood Sample, Conclusions,                               Energy to Cook-off Moisture, Operating the Wood Stove

     Third – What Is The Value of Burning Wood?, Creosote and Soot – Formation and                            Need for Removal, Chimney Fires, Combustion of Wood

What Is The Value of Burning Wood?

The answer to this question is, it depends on several factors.  In general it is cost effective to burn wood for heating purposes.  That said let us get into the details.  The value of burning wood should be determined by the cost of other forms of heating such as electricity, fuel oil, LP, natural gas, heat pump, geothermal heat pump, coal, corn, wood pellets, fireplaces, etc. I will only consider electricity, corn & LP for a comparison.

First factor is the cost of wood to you.  Second is your facilities for burning wood.  Third is your present facilities for heating.  Fourth is the design of the structure to be heated.  Fifth is the cost of other forms of fuel to you.  Six is your situation for seasoning / storing wood.  Let us now consider some basic engineering conversion factors.  Assume that dry wood contains 7,000 BTUs per pound.  One kilowatt-hour is equivalent to 3410 BTUs.  LP has about 91,000 BTUs per gallon.  And dry corn has 8,000 BTUs per pound.  A cord of wood is128 cubic feet of wood.  A rick (face cord) of wood is about one third of a cord of wood.  A cubic foot of seasoned oak weighs 45 pound.  Therefore, one board foot of seasoned oak weighs 3.75 pounds. Fifteen percent moisture corn weighs about 56 pounds per bushel.

The thermal efficiencies of the heating appliance must be considered.  Electric heat is almost 100 % effective in transferring electric energy to heat energy.  Efficiencies of wood stoves  range from 20 to over 80 percent depending on the design of the stove, the type of wood and the method of operation of the stove.  Modern LP heating furnaces are 75 – 85 percent efficient in extracting heat from the LP, but central air conditioning systems are not all that efficient.  In the days of “cheap” energy, central air conditioning was considered desirable, just heat the whole house!!!  But now, space heating, zoning, “tight” construction & well insulated houses must be considered.  Just a few years ago, outside wood boilers were considered “the way for wood heating”, but the stories that I have heard is they are expensive, very inefficient and very smokey, beware.  If you have lots and lots of cheap wood, no problem.

Simple calculations will show that one gallon of LP is equivalent to 13 pounds of wood.  One gallon of LP is equivalent to about 11.38 pounds of corn.  And that about 2 kilowatt-hours equals one pound of wood or 2.35 kilowatt-hours is equivalent to one pound of corn.  Now specifically, wood is fairly cheap in southern Indiana at about $35-40 per good rick which is about 1300 pounds for seasoned, well split oak.  My last LP cost me $1.79 per gallon for summer fill.  So, now it is about $2.20 per gallon.  A rough calculation is 1300 pounds of oak is equivalent to 1300 divided by 13 or 100 gallons of LP which would cost you $220.  At first glance the price of wood looks like a real winner.  But, appliance efficiencies must be considered.   How many effective BTUs of heating am I getting in each of the following examples: LP – 100 gal times 91,000 BUTs per gallon times 75 % efficiency equals 6.83 million BTUs which is $32.21 per one million BTUs.  Now, for wood consider this matrix of variables:

Table 2

               % moisture,  great stove,    good stove,    old stove

                                  15            .68%              .59%               .51%

Million BTUs                           6.19               5.37                 4.64

Cost per million BTUs           $6.64             $7.45               $8.62


                                  30            .53%              .46%               .32%

Million BTUs                          4.82                4.19                 2.91

Cost per million BTUs          $8.30              $9.55             $13.75

As you can see wood that costs $40 per rick clearly has the cost advantage over LP, even the old, inefficient stove while using 30 percent moisture wood.  But, $80 per rick wood would not have near the cost advantage as $40 per rick wood, but by burning well season wood, all three stoves still have a good advantage over LP costing $2.20 per gallon.

Now what about electric heaters?  The calculations are similar.  I  pay about 16 cents per kilowatt-hour, wow.  Two KWHs per pound of wood times 1300 pounds per rick equals 2600 plus KWHs per rick of wood.  Then 2600 KWHs times $0.16 equals $416 of electricity and is equivalent to one good rick of oak wood.  The equivalent number of 2600 KWH to BTUs is about 9 million BTUs or a cost of $46.22 per million BTUs at 100 % efficient heating.  Table 2 still applies to the electric heat situation, so you can see that heating with electric heaters can be much more costly than LP.  Wood can be 6-7 times more cost effective than electric heaters.

Calculations for corn heaters are similar to LP and electric heating.  One good rick of oak is equivalent to1135.5 pounds of corn (20.3 bushels) or a cost of $101.56 for $5 per bushel corn.  Thus the cost for one million BTUs for corn heating with an efficiency of 60% would be 101.56 divided by 9 million times 60 percent equals $18.81 per million BTUs.  Again comparing to the wood cost in Table 2, the wood stoves with well seasoned wood have a significant cost advantage over corn heat.  Remember this comparison is based on $40 per rick wood and $5 per bushel corn.  Two dollar corn would have been very competitive to $40 per rick wood but back when wood could be bought in southern Indiana for $30 per rick.

One final aspect of the value of wood for burning is the amount a timberman will offer to pay for standing timber?  Lets assume that there is a nice, red oak tree that has 40 feet of logs and is 20 inches at the butt.  The tree has a taper of the diameter that decreases two inches ever ten feet.  So, the timberman is planning to cut four, ten foot logs that are 1) 20 in dia & 18 in dia, 2) 18 in dia & 16 in dia, 3) 16 in dia & 14 in dia, 4) 14 in dia & 12 in dia.  The timberman will pay you for board feet based on the small end of the log, but the total weight (wood to be burned) of the logs is almost twice the weight of the lumber that is sawed out! 

The calculations show that the four logs will saw-out about 383 total board feet of lumber and a weight of 1,437.5 pounds; however, the total weight of the logs that were sawed would be 2,562 pounds or 17.94 million BTUs which is equivalent to 199.3 gallons of LP or $438.46.  Thus, the logs of the sawed-out board feet of lumber would be worth $438.46 of LP divided by 383 board feet which equals to $1.14 of LP!!  The timberman might offer you one half of the$1.14 per board foot but more likely, one third.   

Creosote and Soot – Formation and Need for Removal (www.supervent.com)

Wood-burning stoves may require a great deal of chimney maintenance.  How you burn wood in your stove directly affects the formation of creosote.  Smaller, hotter fires are better than large, smoldering ones.  Fast, effective start-ups are important, as is the moisture content of the wood being burned.  Ideally, you should use seasoned wood with a moisture content of 15 % or less.  If your wood is not completely seasoned, use more dry kindling and paper first to warm up the chimney system to a temperature of 350 to 500 degrees F.  A good investment in assisting you in monitoring your heating system is a surface thermometer for single wall stove pipe or a probe thermometer for double wall stove pipe.

When wood is burned slowly, it produces tar and organic vapors as smoke, which combine with expelled moisture to form creosote.  The creosote vapors condense in a relatively cool chimney of a slow-burning fire.  As a result, creosote residue accumulates on the flue lining.  When ignited, the creosote makes an extremely hot fire.

With a new chimney and stove installation, the chimney should be inspected every couple of weeks to determine the rate of creosote buildup.  When familiar with the stove and chimney, it should be checked every 2 months during the heating season.  If creosote is building up, it must be removed to prevent the risk of a chimney fire. 

Chimney Fires (www.supervent.com)

 Chimneys are not designed to be used as combustion chambers.  It is very easy to over-fire your stove with kindling, scrap lumber, or any fast burning fuel, with the result that flames and high temperatures are produced all the way up the chimney and the result may be chimney damage.  If you see the stove pipe glowing red, then you are risking chimney damage or a fire.  The creosote in the chimney may be on fire.  Flames may be coming out of the top of the chimney.   If you suspect a chimney fire, first, immediately close all draft doors / dampers.  Second, alert all occupants of the danger of an overheated flue.  Third, inspect your stove and chimney surroundings for damage and if in doubt call your fire department.  Fourth, do not use the stove or chimney until a thorough inspection of the total installation is made.  Fifth, after a chimney fire, when it is safe to do so, check the internal locations such as attic and under the roof and keep watching for a few hours for delayed, smoldering and any subsequent ignition, even if the fire inside the chimney has been controlled.   

Combustion of Wood (http://www.rise.org/au/res/wood/index.html )

Wood is grown by the process of photosynthesis of carbon dioxide, water and trace minerals.  Such that,  CO(2) + 2H(2)O = ([CH(2)O + H(2)]) + O(2)   which is a reversible process with sunlight required for the left side of the equation and energy (heat) is released on the right side of the equation.  CH(2)O is a carbohydrate.  Less than one percent of the Sun’s energy is converted into biomass (grass, trees, shrubs, etc.).  Wood is composed of hemi-cellulose and lignin.  It also contains water, sulphur, nitrogen and some inorganic compounds that remain as ash after combustion is complete.  For dry wood the stoichiometric equation for the combustion of wood was found to be:

C(4.17) H(6.5) O(2.71) + 4.44 O(2)  = 4.17 CO(2) + 3.25 H(2) O

From the above equation, one pound of dry wood requires1.42 pounds of oxygen ( about 11 cu ft or 6.13 pounds of dry air which is about 56 cu ft measured at Standard Temperature and Pressure (STP) of  32 deg F and 14.7 psi).  And the combustion products are 1.83 pounds of CO(2) (10 cu ft) and 0.59 pounds of water vapor (8 cu ft) at STP.  About 0.5 % of the combustion products of the original weight of the wood are ash.

 You might ask, “Where is the creosote?”  The creosote comes from the incomplete combustion of wood!  The stoichiometric equation above represents experimental combustion with the different elements and compounds being controlled and measured.  The smoke you see from the chimney is the condensed, unburnt volatiles.  If there were complete combustion of gases coming from the chimney, then you normally would see nothing or in colder weather you would see a white, stream of water vapor that is condensing.

From the above equation and associated calculations you can understand one reason for the inherent inefficiencies of the wood stove.  For each pound of wood burnt, the “perfect” stove requires 6.13 pounds of air that is taken into the stove at about 70 deg F and exhausted up the flue at 250 to 400 deg F.  In actual use, the EPA Phase II stove probably uses two times that amount, which would be over 12 pounds of air for each pound of dry wood.  In colder weather you might be using 5-6 pounds of wood per hour, which translates into 60-70 pounds of air per hour.  Other inefficiencies of the wood stove come from having to keeping the flue hot and the flue’s associated heat losses.  The stove must be heated and kept at operating temperature.  The ash absorbs energy which is lost when it is cooled and removed from the stove.  The ash itself (0.5 % of charge) is part of the stove’s inefficiency because it can not be burned. 

 You can now understand the engineering-value of a catalytic combustor that is incorporated into the design of some wood stoves and is capable of burning the volatile gases and smoke particles.  These clean burning stoves are probably as clean or cleaner burning than many diesel trucks or oil fired furnaces.   Now, hear the conclusion of the matter about wood stoves and my parting advice is to season your wood folks, season your wood! Finis


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5 Responses to Effect of Moisture on Wood Burning Efficiency, Revised – Installment 3

  1. George B. says:

    As I read John’s conclusions, I recall sitting in on a conversation about Solar panels and their use here in the Great NW. A younger public school teacher was quick to point out that it was not about efficiency or receiving a return on investment that she and her school teacher husband were concerned about. For them it was about doing the responsible thing and setting a good example. I looked across the table into the eyes of one of the best Boeing electronics techs I ever met, he glanced up at the ceiling and I saw that look of astonhment. It’s not that he was too polite to comment, it was that he understood it would be a total waste of time to try and alter her opinion. Later we compared notes, the same thought had past through our minds. There is a great oppurtunity to sell people Solar Systems at a huge cost savings >IF< performance and a return on investment is NOT a factor. In Seattle, it might take 20 years for the owners of non functional solar installs to catch on, and the environment is rich with people who would never bother to pick up a pencil, why would they?, algore has already done all the thinking for them...

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  3. Woodstove says:

    Great article! Wood should be properly dried before burning though to achieve maximum efficiency. Many people try to burn green wood, which will be smoky and inefficient with burning.

    • theascen says:

      True, but I may have found a way to speed up the season process, I call it active kiln. The process works by burning seasoned hard wood with a bank of 2 or 4 logs of unseasoned hardwood set around the seasoned wood away from the flames, as I have room available for this. The seasoned wood will heat the chamber and will drive the water out of the logs before being used on the next full cycle. This roughly gets my moisture level down from 70-60% to at least 35-25% before the once unseasoned wood gets burned. I find this technique to be unfounded and based on a personal educated guess. So far so good, and I have had interesting affects trying this. It seems to me that the heat not only expands the wood to breath but it also compacts the wood fibers tightly from 1 to 3 inches from the bark inward. I believe this is making a longer more efficient burn with less wood. I have air and damper control to about 3/4 damper closed to about 1 inch air control for oak, hickory, and beech. The coal stays red to yellow hot, with rich blue flames tipped with thin yellow licks. The over all flame body is a smooth liquid burning effect that rolls around the stove in hunt for the sweet spot. With 4 to 5, 18″x6″ split logs. I average 6 to 7 hours at at an even 71-73 degree burn for a 1500 sg ft log cabin. (main level) That’s roughly a 3/4 split log a hour. I’m guessing each stick weighs under 10 lbs “active kiln” which comes to about 7.5 lbs of wood burnt an hr in cold weather with 14.7 psi and 14.7 air to wood ratio. I have an older Buckstove, and I can only guess what it’s efficiency is. However, if I rely on mid way efficiency I am doing well with “Active kiln”. I am doing better this year with consumption than last year by at least 30% more efficiency.

  4. George B. says:

    Reply to Woodstove, John did a great job of covering green VS dry in installment one. As I reflect on John’s article, I think it’s a wonderful example of how we should analyze any investment in energy. My comment about investing in Solar….the intent may not have been a clear, my thought about energy investment… is it should yeild a return on investment or why do it?

    John’s article forces me to look at my own wood stove more closely, and to consider an investment in a new and more efficient unit.

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