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I know from personal experience that greenwood (freshly cut) does not provide as much heat energy as seasoned wood (has been down for 6-12 months). When burning it spends a lot of it's energy boiling the water out of the wood. It takes more green wood to provide the same amount of heat as you would get from seasoned wood.

Assuming I let my firewood season for 12 months, if I burn seasoned fire wood and my neighbor burns green wood, how much more fire wood will he burn in season?

According to the Wikipedia article Heating value of firewood there are a lot of variables. I am just looking for general rule of thumb. I always burn seasoned wood, but my neighbor just moved into his homestead and will need to burn greenwood this winter. How much more wood should he expect to need, compared to my normal usage?

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TLDR

You need about 15 to 20% more green wood to achieve the same amount of heating as with seasoned wood.

Calculation

According to the linked Wikipedia article green wood weighs 70-100% more than seasoned wood and seasoned wood has an energy content of 4.5kWh/kg = 16.2MJ/kg. This obviously depends on the wood, but we are just looking for an estimate. Therefore any equal sign used here is meant as approximately equal.

As wood is usually measured and sold in volume (cord, stere, cubic meters, ...) we neglect the additional weight of the water and just take into account the energy lost due to "boiling" it. Water has a specific heat of 4181J/(kg K) and vaporization heat of 2257kJ/kg. Assuming an initial wood temperature of 0degC green wood thus has an energy content of (16.2 - (2.257 + 0.004181 * 100) * 0.7 to 1)MJ/kg = 14.3 to 14.9MJ/kg.

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    I have been looking around for answers, and this is the most scientific sounding answer I have found. I can't validate the math but the logic sounds good. It is also the only answer I have found with a specific value. It has been a long time since I was dependent on wood heat, in my memory it seems like the value would be more like 50% to 100% more green than seasoned wood, but that might be because green wood releases it's heat slower, so when you start a greenwood fire in a cold house it takes longer to warm the house. You remember those long spans of being cold waiting for it to get warm.
    – James Jenkins
    Sep 2 '16 at 16:20
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    I hope some of the upvoters actually looked at the maths, it is always entirely possible that I did a stupid mistake. But I think it should be sound.
    – imsodin
    Sep 7 '16 at 13:50
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    If the water vapor leaves the fire at a temperature above 100°C (it probably does) then this value is low, because it only covers raising the temperature to 100°C and converting to vapor. I'm not sure how much the impact is, because I don't know whether the final temperature of the released vapor is 110°C, 1500°C, or anywhere in between :)
    – hobbs
    Dec 10 '18 at 20:30
  • I guess with the given math this answer gives us a reasonable upper bound for greenwood efficiency, with other effects (e.g. what @hobbs mentioned) potentially further lowering it.
    – fgysin
    Oct 11 '21 at 5:53

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