Wood fuel quality, moisture content.

The moisture content is probably the most important quality aspect of wood fuels. In order to burn, the moisture in the fuel must be evaporated first and that takes useful energy out of the system. The water vapour usually disappears through the chimney and is lost.

Read more here:

http://woodenergy.ie/woodfuelsstovesandboilers/ moisture content
Calorific value of Irish woodfuel