CTYank said:
At 20% MC dry-basis, you're typically just starting to lose the bound water, that which is intra-cellular. IMHO, that's far borderline acceptable for burning in an EPA stove. For such stove, IME, the very best would be 0% MC. Honest. This is a case where less is more- less moisture = mo-betta burning.
It's not like pass/fail, sat/unsat. There's a continuum, and the closer to 0 the better.
(My final stage of drying is indoors, stacked a foot or so from the stove (up to 180 deg F) for weeks until extremely dry. House can use the moisture in winter, and the wood burns great- lights/restarts quickly, can close door and set final draft quickly, and good night.)
Sorry, CT, but this is not quite accurate information. Most species of wood are still up near 28-30% dry-basis when they have given up all of their free water. This is called the "fiber saturation point" (FSP). This information is available from numerous sources on forestry, lumber, kiln-drying and wood technology. I like to use 28% as a general average because that is a figure used for general drying/shrinkage computations in Dr. Bruce Hoadley's book, "Understanding Wood". Some woods that have a very high extractive content are down in the lower 20% range, spruce is up around 30% and birch is way up near 35% MC (all dry-basis computations). You'd have to analyze each piece of wood to get the exact FSP, but it is usually much closer to 30% than it is to 20% MC.
It is simply not true that wood keeps improving the drier it gets. What scientific or combustion engineering sources can you cite that make that claim? Call any, and I mean
any modern major stove manufacturer and ask to speak to an engineer and ask if that is true about wood at 0% MC. You will be very disappointed in their answer.
Also, many scientific experiments over the last 30 years have definitively proven that softwoods need to be wetter than hardwoods in order to burn cleanly throughout the entire burn cycle. Softwood that are too dry initially give off their gases way too rapidly for complete secondary combustion to occur. You can defeat this tenancy to a degree by using very large splits or giving the fire lots of primary air (leading to a possible overfire situation), but the tendency in general is for super-dry softwoods to smoke excessively during the outgassing phase of combustion. Whatever smoke doesn't combine with oxygen in the secondaries ends up as wasted fuel going up your flue. This is not as significant in a catalytic design as it is in a secondary combustion design, but even with a cat stove you want to keep the MC above 12% for the cleanest and most efficient burn.
As far as finishing wood indoors, yes, you will get it dryer than you can possibly get it outside. I had a split sitting outside since the end of last winter's indoor wood drying experiment. I put it out there under shelter from any possible rain and just weighed it a few minutes ago. It weighed 7.44 pounds when I put it out there in March, it nows weighs 7.56 pounds, for a net
gain of 0.12 pounds. For this particular split that means it went from 11.06% MC dry-basis up to 12.85% MC as of today. I'm gonna put it back out there and check it again in the fall, I'll bet it is up to about 14% MC by the beginning of next burning season. Why? Because that's the normal late-autumn EMC of wood stored outside under cover in my area. Well, it may be a little lower because I haven't accounted for hysteresis effect, but that's another matter entirely that doesn't affect wood that is drying down from a higher MC, only wood that is rehydrating from a lower MC.
However, why someone would want to get their wood lower than is gets through long, natural seasoning outside is a mystery to me. Stoves are designed to be most efficient at the MC that occurs through natural seasoning and not lower than that. All you will do is decrease the total efficiency of your burn, whatever your instincts tell you to the contrary. As for all that evaporative loss that some folks seem to obsess over:
http://en.wikipedia.org/wiki/Wood_drying
When wood is burned, it is usually best to dry it first. Damage from shrinkage is not a problem here, and the drying may proceed more rapidly than in the case of drying for woodworking purposes. Moisture affects the burning process, with unburnt hydrocarbons going up the chimney. If a 50% wet log is burnt at high temperature, with good heat extraction from the exhaust gas leading to a 100°C exhaust temperature, about 5% of the energy of the log is wasted through evaporating and heating the water vapour. With condensers, the efficiency can be further increased; but, for the normal stove, the key to burning wet wood is to burn it very hot, perhaps starting [the] fire with dry wood.
Put your wood out in the open, preferably in a windy location, for at least a full year (yes, two is better, and loosely covered on top is better yet) before you burn it and it should be fine without having to have an anxiety attack about the amount of water in it. I have started to measure mine solely out of curiosity, but it goes into the stove when it burns correctly and smoke-free.
