Efficient Heating

In the middle of winter in the north land, we sit around the stove and think of planting in spring. It is true that burning with wood heats you up several times: 1) to cut the wood, split it and stack it for drying, 2) Moving the dry wood staging it for ready consumption, and 3) to be heated with the fire of your wood burning appliance.

But how efficient is that wood burning appliance? Are you sending a lot of heat up into the winter air, and therefore having to gather a lot more each winter than if you had a more efficient burn process? Well, I guess that is a simple formula to agree to: the more efficient, the less wood you have to burn. Burn efficiency is really determined by your stove design. One you have the stove there are some things you can do to help. I will review the things we all can do to improve any stove, and then I would like to speculate some on the most deficient stove designs that could be created.

Principles for any stove to work its best

The type of wood you have available will determine how much heat or thermal energy that wood can release when burned. Hard woods have more internal energy than soft wood. There are interesting wood thermal charts like this one. Considering the situation where you are heating your home with the wood you have on your land, you may not have a choice between selecting hard woods for your fire (say if you are in the far north or west, and have only soft woods available.) If you have a choice, hard woods will give more heat per unit volume of wood, and produce less ash. Some soft woods have so much pitch, they are hardly worth using for firewood.

What we can all do is make sure our wood has been allowed to dry well before use. The key to getting wood to dry is splitting it (opening up the grain) and stacking it where it has good air flow through the stack. Keeping rain out of and off of the wood pile is also very important. I prefer to do most of my wood cutting in winter and early spring while it is still cool outside. This would give the wood 9 to 14 months to dry before use which is ideal.

Another factor is that wood burns more efficiently if split into smaller pieces. A portion of a round will burn more cleanly than a large and intact round. The balance of this is the effort it takes to split the round, so take this for what it is worth in your situation.

Principles for making an efficient stove

Most of us can not go into the stove creation business, but I must admit, I want to build my own wood burning stove. There are options for the adventurous. Have you every heard of a "thermal mass heater"? These are large (massive) stone or cob structures built into a home through which the exhaust from a fire passes. Most all of the exhaust energy is absorbed by the mass of the structure, and by the time the gasses exit from the chimney the air is warm but no longer hot. This makes for a very efficient stove, considering efficiency being measured by how much heat is captured in the home compared to the total released. Many of these stoves work best with a smaller hot fire to heat the mass, which then radiates this heat throughout the following hours maintaining a comfortable room temperature.

For an efficient fire burn, you need small sized fuel. You also want the burn chamber to be hot. So hot that the volatile gasses from the wood completely burn. For this to happen the temperature of the burn chamber must be around 800 degrees F. One idea is to have the combustion chamber enclosed in an insulative material. You can also preheat the burn air by the intake air passing through the coals (to warm it) and then provide the air of active combustion above. An insulated chimney directly above the burn chamber gives a place for the volatile gasses to mix with oxygen and any remaining gasses to burn. This also improves the air draw and air flow through the combustion chamber. This design is commonly called a "rocket stove" which holds a lot of interest.

Putting these two ideas together, and you have a "rocket stove mass heater".

Before I get too busy with spring planting, I think I will play with some designs, and see what I can come up with. I will add photos to this post as I go along. (so check back for this article).

Stay warm!


Resources:

• Rocket Stove Design (pdf)
• Work Shop Rocket Stove (video)
• Rocket Stove Principles (web)
• Rocket Stoves in Latin America (video)
• Designing Efficient Wood Heating Stoves (pdf)