Is Wood Good? A Look at Burning Wood for Heat

This article by Clarke Snell originally appeared in the New Life Journal.

It’s winter, so I’m thinking a lot about wood. That’s because it’s a major strand in my life-line this time of year. I’m lucky enough to live in a house that is heated by a combination of direct sunlight and wood collected basically from my back yard. In a world that I am increasingly unable to grasp, it’s always a centering, empowering experience to warm myself with wood that I’ve cut, split, and hauled with my own hands.

A sensible question, though, is whether it’s environmentally responsible to burn wood for heat. The preamble to an answer to this question is that the “green” approach to heating has to start with a very well-insulated, weatherized building that maximizes the heating potential of the sun through passive and active solar design (see last month’s column for more info). These strategies will greatly reduce the amount of extra heat we’ll need in the building.

With that said, there are two basic environmental issues to consider: management of natural resources and pollution. As a resource, wood is both locally available and renewable. When you compare fighting wars over dwindling oil reserves to taking a chainsaw into your back yard, wood seems the clear winner. To my mind, IF (and this is a big IF) the wood is harvested in a sustainable fashion, then burning wood is a smart choice from the point of view of natural resource management.

That leaves the question of pollution. Is wood a clean burning fuel? The answer depends on how you burn it. To understand what I mean, we need a quick science lesson. (Full disclosure: I am not a combustion scientist, nor have I played one on TV.)

Combustion or “burning” is the chemical reaction between a heated substance (fuel) and oxygen. In the case of wood, there are three stages of combustion. In the first stage, wood heats up to the point that interior moisture turns to steam. This process actually consumes heat, so we are no closer to warming our feet at this point. With the water gone, though, the wood can get hotter and begins to vaporize releasing gases, better known as “smoke” to us laypeople. If the temperature is hot enough, this smoke will burn releasing heat in the process. After the gases have been released, what’s left is called “char” which is basically a pile of almost pure carbon. In the presence of enough heat and oxygen this carbon will combine with oxygen to produce carbon dioxide and release more heat.

Carbon dioxide?! Isn’t that a greenhouse gas that contributes to global warming? Yes, it is. The fact is that all types of combustion (whether coal, oil, gas, etc.) produce carbon dioxide. However, there is a difference. Trees take in carbon dioxide as they grow. When they die, fall, and decompose in the forest, they expel this carbon dioxide. If you burn wood, that same carbon dioxide is released, so theoretically there’s no global warming difference between letting wood decay and burning it. (The reality is, as usual, a bit more complicated, but, hey, remember I’m no combustion scientist.) On the other hand, the CO2 released from burning petroleum is CO2 that plants took in millions of years ago and has been sequestered all that time in the ground. Therefore, burning petroleum brings additional CO2 into the picture, and consequently contributes to human induced global warming.

If we accept this carbon dioxide argument, then how clean wood burns and how much of it’s potential energy is converted to heat is simply a function of how completely it combusts. If smoke is allowed to float away without combusting, the result is creosote and a bunch of nasty particulates that can pollute indoor and outdoor air. If the “char” doesn’t completely combust, the result is unburnt char and by-product carbon monoxide which as we all know is a poisonous gas. To avoid these environmental pitfalls, we need to (1) create enough heat and (2) contain the fuel.

Wet wood is the enemy of creating heat. A lot of energy is wasted burning off water which keeps temperatures low and therefore allows smoke to escape without burning.  The first step, then, regardless of stove type is to use well seasoned (dry) wood. Next, we need enough oxygen. Campfires burn well because they have access to plenty of oxygen. The problem is that the fire is on the bottom while vaporizing gases quickly rise away from the flames, therefore a lot of fuel just floats away unburned. We need to contain those gases in a heated environment so that they ignite after they rise away from the flames. Next, after the gases have burnt off, we need to keep burning the char hot enough to make sure minimal carbon monoxide is created. Finally, we have to find a way to store most of the heat off of this now incredibly hot, efficiently burning fire. If not we’ll quickly overheat our house.

Since we are geniuses, humans have created stove designs that solve these problems. In my opinion, the best of these are generically called “masonry wood heaters”. Though there are a number of variations, the basic idea is to burn wood hot and fast in a stove made of dense masonry materials (brick and stone) that then can absorb the heat and slowly release it into the house over many hours.

My favorite design is the “contraflow heater”. In this stove, wood is stacked log cabin style in a tall firebox made of firebrick with ample air intake basically creating an enclosed campfire. As smoke rises off the fire, it is confined and therefore compressed in a secondary combustion chamber where it ignites. Hot air from this combustion leaves the chamber at both ends and then travels back down the sides of the stove in channels created by another layer of brick that surrounds the stove. The air then enters the bottom of the chimney and moves out of the house. The brick soaks up much of the heat from the air as it travels up and then back down through the stove. Meanwhile back in the firebox, the char continues to combust in an environment of ample air and heat. After a couple of hours when combustion is complete, the stove damper and air intake is closed to prevent air movement out the chimney and the stove then slowly radiates its stored heat into the house for up to 24 hours. The result is an extremely efficient burn with very low particulates and a comfortable, even radiant heat.

As with all things, there are downsides. For one, masonry heaters are expensive and heavy, requiring a solid foundation to sit on. They also require a lifestyle adjustment over conventional heating sources. They heat up slowly, so you have to plan ahead if the stove has cooled down. In other words, no cranking up the thermostat when you get home from work.

Luckily, other wood stove technologies approximate the advantages of a masonry heater. New metal stoves have advanced catalytic combusters and/or ingenious air intake and injection strategies. Some metal stoves incorporate soapstone or other mass to allow for some amount of heat storage.

On the other hand, as I said earlier, there’s a bad way to burn wood. Metal stoves that are two large for the space or poorly designed require that you starve the fire to prevent overheating. Smoldering a log in an old damped down cast iron stove creates massive amounts of particulates and only turns about 25% of the wood into useful heat compared to 75% or more for a masonry stove. A roaring fire in an old-school fireplace is even worse, turning an estimated 0% (yes, you read that right) to 15% of the wood fuel into useful heat. If you’ve got an old metal stove, either modify it to include an air to air heat exchanger (here’s a link to show you how: http://www.aprovecho.org/), or get rid of it. Sell it for scrap or use it for yard art, but don’t pass it on to another user. If you are buying a metal wood stove, make sure that it is EPA approved (labeled EPA II).

To learn more about masonry wood stoves, check out the Masonry Heaters Association of North America. Their website is http://mha-net.org/html/library.htm. To further research metal wood stoves, first peruse this government website (http://www.epa.gov/woodstoves/index.html) so that you can impress your local wood stove dealer with some intelligent questions.

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