A Concise Guide To High Performance Woodburning

The information below re-printed with permission from: http://www.woodheat.org/

The challenge in burning wood efficiently is to burn off the smoke before it leaves the firebox. The rest of the suggestions in this fact sheet will help you get more heat from your wood and reduce creosote deposits and air pollution.

1. Understanding Combustion

As firewood burns, it goes through three phases:

Water

Up to half the weight of freshly cut logs is water. After proper seasoning only about 20% of the weight is water. As the wood is heated in the firebox, this water boils off, consuming heat energy in the process.

The wetter the wood, the more heat energy is consumed. That is why wet wood hisses and sizzles while seasoned wood ignites and burns easily.

Smoke (or flame)

As the wood heats up above the boiling point of water, it starts to smoke. The gases and tar droplets that make up the smoke are combustible and will burn if the temperature is high enough and oxygen is present. When the smoke burns, it makes the bright flames that are characteristic of wood combustion. If the smoke does not burn in the firebox, it may condense in the chimney, forming creosote.

Charcoal

As the fire progresses and most of the smoke and tars have vaporized, charcoal remains.

Charcoal is almost all carbon and burns with very little flame or smoke. Charcoal is a good fuel that burns easily and cleanly when enough oxygen is present.

Of the total energy content of the wood you burn, about half is in the form of smoke, and half is charcoal.

2. Preparing firewood

Firewood should be split and stacked in the early spring to be ready for burning in the fall. After drying in the summer sun and warm winds, the wood should be around 20% moisture content. A piece of dry firewood has cracks or checks in the end grain. Look for these when judging the quality of firewood. A range of piece sizes between 3 inches and 6 inches across the largest dimension makes it easy to build large fires or small fires to match heat demand. Hardwoods and softwoods are chemically similar – the difference is in density. Hardwoods, being more dense, produce longer-lasting fires. However, people who live in the north and west where hardwoods do not grow are able to heat their homes quite effectively with softwoods.

Burn dry wood
because . . .

it gives up to 25% higher efficiency
it produces less creosote
it ignites faster and smokes less
it is lighter to carry

It has been said that a long straight row of firewood standing in the yard in springtime is like money in the bank. It is indeed – as it dries in the summer sunshine, you’re collecting interest.

3. Starting or rekindling the fire

Use plenty of crumpled newspaper and dry kindling to start a fire. Never use glossy magazine paper or liquids. Crumple 5 to 10 sheets of newspaper, place them in the firebox and hold them down with at least ten pieces of finely split, dry kindling. Softwoods like cedar and spruce make the best kindling. Find out where the combustion air enters the firebox of your stove (usually at the front just inside the loading door) and light the paper there so the kindling fire will get plenty of air. Open the air inlet(s) fully to produce rapid combustion.

4. Fuel Load Geometry

Avoid loading only one or two pieces at a time on a coal bed – most often they will not burn completely because heat is lost from the pieces faster than it is produced by burning. A minimum of three pieces is needed to form a sheltered pocket of glowing coals that sustains the fire.

A loosely-stacked load of wood (in a crisscross arrangement) burns fast and small pieces of wood burn fast. So, if you want a quick fire to take the chill off the house in mild weather, use small pieces stacked loosely.

A tightly-packed load of wood burns more slowly and large pieces burn more slowly. So, if you want an over night burn, use larger pieces placed compactly in the firebox.

5. The firing cycle

Don’t expect perfectly steady heat output from your stove. Wood burns best in cycles. A firing cycle is the time between the ignition of a fresh load of wood and its consumption to a coal bed. Each firing cycle should provide between four and eight hours of heating. Plan the cycles to match your household routine. For example, if someone is home all day, two four-hour fires allow better control of house temperature than one eight hour burn. Adjust the amount of wood used in each load to match the amount of heat needed. For overnight burns, adjust the load so that just enough charcoal is left in the morning to kindle the next fire. Fire each load hot for a few minutes to heat the wood thoroughly and form a layer of charcoal on it.

6. The flash fire technique

To avoid overheating the space and smouldering the wood during mild weather, build a small fire and burn it quickly. Rake the coals into a pile at the front of the firebox and load at least three small pieces on and behind the coals. The pieces should be stacked loosely in a crisscross arrangement. Open the air inlet to produce a bright, hot fire. The air supply can be reduced slightly as the fire progresses, but never enough to extinguish the flames.

7. Building an Extended Fire

To achieve a longer-lasting fire, rake the coals towards the front of the firebox and use larger pieces of wood placed compactly against the rear of the firebox. Placing the pieces close together prevents the heat and flame from penetrating the load and saves the buried pieces for later in the burn cycle. Open the air inlet fully and leave it open until the surface of the wood has a thick layer of charcoal and is burning brightly. Then you can reduce the air setting so the flames slow down, but not enough to extinguish them.

8. Removing Ash

remove ash . . . . rake charcoal . . . . place new load

Remove ash from the firebox often so its build up does not interfere with the raking of charcoal and the placement of logs. If your stove is equipped with an ash pan, remember to empty it before it is full to avoid spilling ashes in the housing or on the floor. Once removed, wood ash should be stored in a covered metal pail away from combustible material outside or in the garage. You can sprinkle some of the ash on your flower gardens to reduce soil acidity, or you can put it in a hole dug in the corner of your yard, or you can put it out with other household waste going to a disposal site.