MinnesotaGuy said:
Outside of losing some BTU's and adding some creosote, should I be worried if I burn the occasional 27% oak or black cherry split in my pretty new stove?
I think you are over thinking this whole issue.
For one thing, you should realize that moisture meters are calibrated to the way moisture content is assessed by the wood products industry, a method that is not very intuitive to us, but works for the industry just fine. From a previous thread:
...take the difference between the wet weight and the dry weight (which is the total amount of water removed), then divide this difference by the oven-dry weight to get the final ratio of water removed to oven-dry weight. Multiply by 100 to get the starting moisture content of the wood as it is most commonly expressed.
All MMs are set to use this method as the basis for their internal calculations. Otherwise, they would all disagree with each other.
For example, a chunk of wood that weighs 127 pounds and reads 27% MC on the meter actually only contains 27 pounds of water. 127 minus the weight of the water is 100 pounds. 27 pounds divided by the fully dried weight (100 pounds) times 100 = 27% MC on the MM. But the actually amount of water in that 127 pound block of wet wood represents only 27/127, or .21 or 21% water by weight.
A similar block of wood that weighs 120 pounds and reads 20% MC on the meter actually only contains 20 pounds of water. 120 minus the weight of the water is 100 pounds. 20 pounds divided by the fully dried weight (100 pounds) times 100 = 20% MC on the MM. But the actual amount of water in that 120 pound block of wet wood represents only 20/120, or .17, or 17% water by weight. Don't trust me, see below (in blue type):
(broken link removed to http://extension.missouri.edu/publications/DisplayPub.aspx?P=G5450)
How wood burns
When wood burns, three things happen:
* Water is removed by evaporation
* Chemically, the wood breaks down into charcoal, gas and volatile liquids, with carbon dioxide and water being the chief end products
* The charcoal burns, forming carbon dioxide either directly or with an intermediate conversion to carbon monoxide.
One pound of very dry (zero moisture content) wood of any species has a calorific value of approximately 8,600 Btu (British thermal unit, which equals the amount of heat required to raise the temperature of one pound of water one degree F). Any moisture in the wood reduces the recoverable heat by carrying heat up the chimney during vaporization. Each pound of water vaporized uses about 1,200 Btu.
Additional Btu are lost through the formation of volatile liquids and gases during combustion, but these vary by the type of heating unit and should be considered part of the efficiency factor of the heating unit.
A pound of wood with a 20-percent moisture content contains 0.17 pound of water and 0.83 pound of completely dry wood and has a heat value of about 7,000 Btu. This is the base figure used in the heating comparisons made throughout this publication
So, for a 10-pound split, there is really only 0.4 pounds of extra water in a split that measures 27% MC compared to one that measures 20%. That is a total difference of about 6.4 ounces of water in a 10-pound split. This much water represents a heat loss of only about 390 BTUs out of a potential 70,000 BTUs of sensible heat in the split. Pretty small difference IMHO. And as far as all that extra water going up the stack and condensing on your flue walls? Well, a 10-pound split of wood that is at 20% MC and, therefore, contains 8.3 pounds of wood is creating
4.5 pounds of water (steam) as a product of combustion. That is
huge compared to the 0.4 pound difference between woods at 20% and 27% MCs. And if you don't recover any of the latent heat in that steam (which you
really don't want to do) by condensing it back into a liquid, you always lose 4365 BTU for every 10-pound split at 20% MC that you burn. Does that 390 BTU difference seem all that bad in comparison?
Folks should take it easy on themselves and stop obsessing over the finer points of moisture control and do as Dennis and many others here recommend: Take any wood that you feel is marginal (whether you are using the "heft", seasoning time, or MM method of assessment) and mix it in here and there with the good stuff during a hot fire. Your stove will be just as happy, and you won't be able to tell the difference.