Why wood burns, how a good stove is made

To raise the heat efficiency of burning wood, you have to know what happens in the combustion process. The first chemical change is the release of moisture from the burning wood to drive out the moisture.

As the moisture escapes, solid wood begins to burn, releasing gases that are a complex mixture of organic chemicals known as volatiles. The energy stored in wood comes from these volatiles, and they must be burned for maximum efficiency. If not, they contribute to creosote buildup in the flue and chimney.

The problem, then, is to burn off these volatiles. Stovemakers over the years have designed stoves to achieve a secondary combustion to eliminate the volatiles. Most stoves today supply a second stream of air (oxygen), a secondary draft to burn these gases together with heat from the burning wood for the required ignition temperature (see drawing).

The idea of a proper draft is extremely important. The regulated supply combustion air determines how long and how hot a given amount of wood will burn.

You can see that the lack of a closely regulated draft is why fireplaces and open stoves are not efficient. Closed stoves, or airtight stoves, are efficient , however. These designed stoves permit air to pass in or out of the stove only through the flue and draft passages. The draft in an airtight stove can be set so that a load of wood can burn all night. And a bimetallic-strip thermostat can regulate the draft automatically for heating needs.

Manufacturers offer a wide range of wood-burning appliances such as open stoves, closed stoves and heaters, circulating heaters, and furnaces.

The open stoves, such as the traditional Franklin, are less efficient. Some newer designs -- the Defiant and European Combi-fires -- give a closed-stove operation and the pleasure of a visible fire. Closed stoves heat their spaces by radiating infrared rays, offer wide design range of materials and size, and average about 50 percent efficiency in burning wood.

A good example of the closed-stove box, maximizing secondary combustion, is the Norwegian Jotul. Circulating heaters heat and move air by convection current or by an electric blower. Their trim wood-burning cabinets provide about 60 percent efficiency.

Stoves are made of cast iron or sheet metal. Small steel stoves heat quickly but cool fast. Cast-iron stoves retain heat over a longer period of time but take longer to produce usable heat.

Durability is different, too. If a stove is overfired, cast iron may crack; steel will warp. Both types may have a liner of metal or firebrick to help hold heat and maintain high firebox temperatures.

The use of wood-burning stoves is rising, then, just about as sharply as the cost of oil and gas. Some architects and designers of wood-burning systems claim that burning wood can cut energy bills in half. But a lot depends on where you live, how available wood is, how much it costs, and the cost of other fuels.

A heating stove can cost anywhere from $200 to $300 or more and an extra chimney another several hundred dollars.