Wood, no matter how it is produced, contains levels of moisture.
With kiln dried wood the moisture content can be as low as 8%, or up to as high as 20%, depending on the initial degree of drying and how much it has been exposed to damp since it was kiln dried.
Green wood, i.e. that which has been recently cut and hasn’t been seasoned or kiln dried, such as wood from the garden or obtained from a landscape gardener, can easily have a moisture content of in excess of 30% and in some exceptional cases more than double the weight of the wood alone can be water.
Seasoned wood is that which has been allowed to dry out naturally under shelter. As a guideline, it takes a about year for each inch thickness of wood for it to season properly. In Britain, where we have an oceanic climate, the humidity of the air is rarely low enough to season wood naturally to a moisture content of less than about 16%, and usually it is quite a bit more than this.
The higher the moisture level, the more of the heat energy of the fire is wasted on burning off the moisture, so if the moisture content is 20% for example, then it is likely that 20% of the heat energy of the fire will be used up just in burning off the moisture. Whilst this is going on the heat output of the fire will be lower, so the point at which the smoke has cooled enough for the moisture to condense out can under some circumstances be actually within the chimney rather that out in the air above the chimney pot. This can result in tars and creosote forming inside the chimney, which can set hard like resin glue and be almost impossible to remove. It is flammable however, and can result in a chimney fire, so this needs to be avoided.
For this reason your choice of wood to burn can be quite important.
Getting it from a good supplier who does not include any softwoods (the trees of the pine family for example, including Cedar) is a very good first step, followed by arranging your wood storage and when you buy it so you can give it a chance to dry out as much as possible before you need to use it.
Certain woods are better than others. Ash, Sweet Chestnut, Birch and Willow are all good coppice woods that burn well and so are ideal carbon neutral fuels. There’s quite a lot of Oak available from time to time due to the need to keep old coppice woodland in good order by thinning out trees that have grown too large, and this is an excellent wood that gives great heat when seasoned. Fruit woods like Apple and Plum need to be well seasoned to burn well, but give a good heat and a pleasant scent.
Measuring the level of moisture in wood before you purchase is quick and simple with a Moisture Meter.