Semi dry vs. 20% ish

James02 Posted By James02, Nov 2, 2011 at 12:21 AM

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  1. oldspark

    Guest 2.

    Been burning wood (dry wood by the way) for over 30 years and never gave it much thought, BK makes me think and thats good at my age. :lol:
  2. joefrompa

    Minister of Fire 2.

    Sep 7, 2010
    SE PA
    One thing I haven't seen at all discussed here is absorption of heat in the stove (versus going up the flue). I don't disagree at all with Battenkiller's numbers, but that's a given set of BTUs.

    Now, those BTUs have 3 choices: boil water, go into the stove, or go up the flue.

    As moisture content increases, it seems to exponentially increase the amount of air input needed to the stove to combust the wood. And, therefore, the rate of loss of heat up the flue seems to exponentially increase. This makes sense when you realize that for a bone dry (12% MC) piece of wood, you can maybe meter in (making a number) 5 cubic centimeter of fresh air every second where as for a 30% MC piece of wood you need 4x that amount (a far more open air intake).

    As far as I can tell, the problem with high MC wood isn't that it has so much less BTUs, or that it takes so many BTUs to boil the water's that good combustion of the wood requires so much more air that the loss-rate/efficient rate of those BTUs gets so much worse for the wood burner.
  3. red oak

    red oak
    Minister of Fire 2.

    Sep 7, 2011
    northwest Virginia
    Agreed! I never knew so much analysis could go into this topic!
  4. Battenkiller

    Minister of Fire 2.

    Nov 26, 2009
    Just Outside the Blue Line
    All of these things are accounted for in the stack-loss method of measuring overall efficiency. An array of sensors measure all of the gases that leave the flue, the volume of them, and the temperature at various points along the column. What you are suggesting simply does not occur in real life - in the lab, or in the stove at home.

    You are using terms like "seems" instead of actually measuring the heat flow up and out of the stack. Just because you open the air up to an opening 4X as wide doesn't mean 4X as much heat is lost, all it really means is that there is less resistance to air flow at the intake. The stack is what determines the mass flow of the entire system. Once a good draft is well established, the intensity of the fire will actually increase as the intake opening is decreased. This is because the same amount of air is entering the stove, but at a higher velocity. Higher velocities cause turbulent air flow, and turbulence is the best and most efficient gas/air mixer there is. If you can burn it all here in the primary fire zone, there will be nothing left for the secondary air to combine with. This doesn't happen, but you can get close with proper stove design and good air control practices.

    Here is a photo of the air intake of my stove taken while burning a load of very green wood last year. The stove was at 750º, the flue was at 600º, and the basement was about 85º. As you can see, the air is shut down pretty far, about 1/8 or so of the max opening. I had shut the draft down in increments, just like I would do with dry wood. I do this by ear, not by the numbers. It was probably open more than I might leave it with dry wood, but not by a bunch. Once I got the burn well established, the stove was handling that wet load just fine and was producing massive amounts of heat in the living space where I want it. A bit too much heat, in fact, but I was doing it to prove a point to myself, so I pushed it as far as I could.

    BTW, that particular load lasted a very long time, and burned smoke-free even when I closed the bypass and sent the thing into a nice secondary combustion. The reason why is that I initially got the stove very hot and kept it that way throughout the burn by regulating the air rather than just opening it all the way.

    Attached Files:

  5. River19

    Member 2.

    Nov 19, 2008
    Southern Ma & Northern VT
    I am amazed at how many people end up with extremely smokey fires in the north country where I burn. I usually am burning between 350-500 degrees with wood at about 10-15% moisture (by digital for what it is worth) and have little smoke.

    I drive around and see these smoke balls coming from the woods and from chimneys with smoke just billowing out of the top of the pipe etc.

    I think most people burn what they can up here and I would guess a lot is old and wet......
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