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[jreimer]

With the very low temperatures at this time of year, people see a lot of condensed water vapor coming from their chimney's and start to wonder if their wood has too high a moisture content.  While this may be true in some cases, it's important to know the chemistry of wood burning to understand your stove from what you see coming out of the chimney.

When you first load the stove with fresh wood, it is true that the water stored in the wood is being boiled off and contributes to the steam coming from the chimney.  However, due to the composition of the wood molecules, a good portion of the water vapor is generated from the actual chemical reaction of the wood burning.

Generally, wood is composed of the following:
H2O = 20%
C6-H10-O5 = 40% (This is the outer softer portion of the wood - floam)
C9-H10-O2 = 40% (This is the inner harder portion of the wood - core)

This is an over-simplification of the burning process of wood, but here goes...

So first in the burning sequence is the H2O gets evaporated.  That equals 20% of the wood.  This is the abundant steam you see coming from the chimney after a fresh load.
Next, the hydrogen atoms (the H10) get stripped (burned) from the molecules and combine with the oxygen (O2) in the air to form H2O, or the water vapor you see coming from the chimney.
Next, the carbon atoms (the C6 or C9) get stripped (burned) from the molecules and combine with the oxygen in the air to form CO2, or the carbon dioxide emissions.  This is invisible and is the heat waves you see coming from the chimney at the end of the burn when there are just coals left.
The ratio of hydrogen to carbon atoms in the molecules is higher for the softer outside portion of the wood, so this also contributes to more steam being generated towards the beginning of the burn cycle compared to the end.

Doing the chemical math analysis we see that 1 lb. of wood requires 1.36 lb. of oxygen to fully burn.  1 lb. of wood will create 0.66 lbs. of water and 1.70 lbs. of carbon dioxide.

So you can see that A LOT of water is being generated just by the normal process of burning the wood, and not just from the pre-moisture content in the wood.

Hopefully this can help clarify some misconceptions about the steam coming from your chimneys and help you become an expert at identifying the burning stage of your wood by watching the chimney from the comfort of your couch.

[mlappin]

natural gas and LP furnaces do the same out the vents, pretty sure both gasses have very little water in them, most likely its the moisture from the combustion air.

[jreimer]

Natural Gas is CH4, so you get 2 molecules of water and 1 molecule of carbon dioxide being produced for every natural gas molecule being burned.  LP is C3H8, so you get 4 molecules of water and 3 molecules of carbon dioxide being produced.  So the steam you see is the condensed water byproduct from the combustion process, not from any moisture in the gas itself or the incoming air.  You could have perfectly dry gas and air and see the same steam from the exhaust.

So the lower the ratio of carbon to hydrogen atoms in the fuel, the "cleaner" burning the fuel is classified as, because more water and less carbon dioxide is created during the burning process.