2. Fuel - effects of varying moisture content on calorific value.

Different species of tree retain different levels of sap during growth, and of course many conifers produce resins which can be quite flammable. For this reason, green or unseasoned fuelwoods vary greatly in their burning properties though ash has always been admired for the heat it produces in almost any state – hence the old rhyme that ends "But ash green, or ash old is fit for a queen with a crown of gold." In an earlier era it was normal practice to burn wood while it was still green, since dry logs burned too rapidly in an open hearth. Thus people became keen observers of how different types of unseasoned wood burned.
XXThe interesting thing is that these observed differences all but disappear when logs are fully seasoned. And proper seasoning is what we must achieve when we run a modern closed appliance, since moisture degrades heat output severely and also plays havoc with the chimney. (See the chimney section for more information on this.)
XXThe chart below is rather academic in that the column on the left represents a 100% dry sample (ie one that has been oven dried) and the ones to the right include theoretical deductions for the energy lost when varying proportions of moisture are boiled off. In practice, burning wet wood in a stove always produces much worse results than the figures suggest for two reasons;-

• Combustion efficiency is degraded. A fire must reach a certain minimum temperature in order to burn cleanly and efficiently. When excessive moisture is present the fire is 'quenched' and produces a dense pall of smoke containing unburned tars and creosotes.
• Stove efficiency is degraded. The normal human response to an under-performing fuel is to over-run the stove. This significantly increases heat-loss to the chimney.

You will incurr several penalties if you persist in burning wet wood over a lengthy period of time. First, you burn far more fuel. Second, you foul up your chimney and probably lay up a fire-hazard. Third, over-running a stove shortens its life by subjecting it to excessive heat along the flue path. Fourth, you lose control because it tends to sulk or go out unless you run it continuously at 'full throttle.'
XXOne useful check on the moisture content of your fuel is to run the stove for a short period on some demolition timber, or wood reclaimed from a dry discarded pallet. If this tranforms its performance you will know that you have more work to do to get your main wood pile properly seasoned!