- Principles of Combustion
Technically wood does not burn. It is the volatiles and charcoal that is created . That’s why wood will not catch fire immediately when you put a match to it. It has to undergo the chemical change that create the volatiles and a match does not create enough heat to activate this process. As kindling and paper evaporate the moisture in the wood, the wood absorbs heat and, at a certain point (the flashpoint: 250–300°C), the volatiles will burst into flame if sufficient oxygen is present. The volatiles give off more heat than charcoal which is why a fire with flames produces more heat than one that is all charcoal.
Since the volatiles are gasses and heat rises taking the gasses with it, it is very easy to create a situation in which the volatiles go up the chimney as soon as they are produced. This is what happens in a roaring open fire. One of the reasons ( but not the only one) that a wood stove produces more heat than an open fire is because the volatiles are contained in the firebox. A wood stove is designed to keep the volatiles in the firebox for longer. The longer they are in the firebox the more complete the burn. The more complete the burn the more heat is produced. That simple. The most efficient stoves are those that make sure that the volatiles are almost totally consumed reducing creosote build up to the minimum.
- Moisture and Combustion
Dry wood burns faster and better than green wood. The reason for this is that the heat cannot be produced until the moisture in the wood has been driven off. It takes time for fire to produce any usable heat. Wet wood will produce very little heat and a lot of creosote. Even if wood is seasoned correctly it will still contain approximately 20 percent moisture.
- How to test wood for dryness
There are two ways for a novice to test if wood is dry. If you look at a log that has not been split you will see cracks radiating from the centre. Another test would be to bang two logs together. Green wood will make a dull sort of sound and dry wood makes a nice crisp sound. It is best to order your wood supply during summer to make sure that when you need to use it will be nice and dry.
All about chimneys from www.woodheat.org, a really good site to get expert information on wood heating.
The chimney is the engine that drives a wood heat system.
No woodburning stove, fireplace or furnace can function properly without a good chimney. A good chimney is:
- the correct type for the appliance because there are a lot of options, some unsuitable;
- the correct size for the appliance, which is usually the size of the appliance outlet collar;
- properly located, meaning up through the heated space of the house; and
- properly installed following building code or manufacturer’s instructions exactly.
A good chimney and system design produces desirable performance characteristics:
- Fires are easy to light and draft builds quickly
- Smoke does not fill the room when you try to light a fire
- No smoke spillage when you open the door to tend the fire
- No foul odours or cold air from the hearth when it is not in use
When planning a woodburning system, the first thing you need is reliable advice on matching the appliance to the right type and size of chimney. Most wood heat retailers can guide you. Also, unless you have done it before, we strongly recommend having your chimney professionally installed by someone whose references you have checked. You never want to lie awake at night wondering if an incompetent chimney installation is putting your house and family at risk.