Thursday, March 12, 2009


Wood and sun seems to be the logical choices for sustainable heating. My brother uses wood pellets and friends use chopped wood. Where I live local law requires any new house to either install solar water panels or a wood pellet furnace.


Roughly how much wood does one need to heat one year? I know there are many variables (size of home, how warm you like it, the type of wood, the type of wood burner, insulation, ...).

Friends use very efficient wood burners. They can be found in Holland at and in France at These wood burners are on the order of 90% efficient and produce about 2% of the pollution of a standard wood burner. My friends like the way they function better than a standard wood burner. First you make a very fast burning fire which lasts about 70 minutes. Then you close everything up, and they stay warm for 12-16 hours. Once the fire is out, the combustion chamber can be used for cooking (very easy in the French Finoven).

Wood burns at up to 600C. The air that goes out the pipe to the chimney is 180C in the optimal case. If it is hotter than 180C then it is less efficient as heat is lost. If it is below 180C a clean burn and a swift evacuation of the gases cannot be guaranteed. Some of the wood burners offered at the linked site (see above) are 800 to 960kg heavy. One might have to support the floor to support that weight. Some ovens due to the construction material are limited to a maximum number of fires per day (e.g. 3). More fires will result in the material cracking. Other models allow an unlimited number of fires. The size of the burning chamber indicates how often you will have to make a fire. The larger the size of the chamber the fewer fires you will have to make. The air usually circulates inside the wood burner in a strategic fashion. Some burners have side doors so once a year ash can be vacuumed from these circulation side vents. Once the flames are gone usually the vents are closed and optionally cooking can start. A single large wood burner like these can heat up to 50 square meter. Heating is done directly, i.e. warm air circulates throughout the home through open doors, walls absorb and pass on heat, etc.

In Spain I have seen other wood burners and even fireplaces that have a second "clean air" system. The "dirty air" from the flames goes through the metal pipe straight through the chimney. However, build around the burner or fireplace are air chambers with clean air. There are vents on the bottom, close to the burner. Here air from the room enters, it is heated through the warm bricks, rises and is carried in clean air pipes into neighboring rooms.

When thinking about wood burners and insulation. It is true that hot air rises, but hot walls carry the heat into all directions, also down. So, consider this when planning for your insulation.

A fire is 6-7 kg of wood which produces about as much heat as burning 2 kg of heating oil or about 20 Kwh. Heat loss is minimal because these wood burners do not get as hot as standard wood burners, so they do not heat the air as much (when you are 1 meter from a surface, the physiological temperature you feel is the average of the surface temperature and the air temperature, low temperature heating heats surfaces more than the air, so you have the same thermal comfort with a lower air temperature which reduces heat loss). This year my friends estimated they made 500-600 fires around 4000 kg wood / year or about 8.5-9 steres (cubic meters of stacked wood) or about the equivalent of 1100 liters of heating oil or 11,000 kwh/year or about 110 kwh/meter^2 year. (They do not heat the bedrooms, though they are insulated. They aim at about 17 degrees in their living room, but if the weather stays below 0C for a week, it gets cold in the house unless we spend the entire day making fires). The ADEME estimates that the French use about 400 Kwh / meter^2 year to heat their homes. This is enormous. According to a German plumber, a well insulated German house uses about 100 Kwh /meter^2 year to heat, a very well insulated German homes use about 70 Kwh/m^2 year to heat. When we moved into the house, I assumed that it would require more energy to heat (per meter^2) than a modern house. Now that I have studied the question a bit, I have found that it should take less heat than most modern houses because of the enormous thermal mass which enables us to store passive heat. Insulating the outside of a wall is much better than insulating the inside because the house can then store more passive heat. They still have more insulation projects ongoing to decrease heating requirements by about 15% per year and they plan on installing trombe walls next year.

Another model of wood burner is the Lopi Endeavor which is recommended in the the book Rural Renaissance by John Ivanko and Lisa Kivirist. The EPA has found the Lopi Endeavor non-catalytic stove to be the cleanest burning large stove ever tested at that time.


Roughly speaking, 4 square meters of solar panels for water heating should provide enough hot water to cover about 60% of the yearly requirements for a 4-person family.

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