Outdoor Wood Boilers and Furnaces – Why Do They Smoke So Much?

With energy prices continuing to reach an all time high, more and more people are turning to alternate fuel sources. One of those fuels is wood.

Although people have burned wood for years, why is there so much buzz about outdoor wood burning boilers (or furnaces as they are most commonly referred to).

Well, first off, lets talk about what these are. If you live in a neighborhood where one has been installed, then I am sure you know what they are! In general they are a self-contained building that looks something like a shed. They consist of a firebox and an area that contains water.

Wood is added to the firebox and the heat that the wood produces is then passed to the water, which in turn enters the house or building and is used for heating and domestic hot water.

Sounds like a great thing right? It is. Or is it? The biggest problem that most people have with outdoor wood furnaces is most produce a ton of smoke at start up. This smoke can also last for a long time during operation (heat cycle) due to the large wood load stored in the firebox.

So why do they smoke so much? There are several reasons. The first reason is most manufacturers do not build these to be very efficient. As the fire burns, the firebox rarely gets hot enough to “gasify” the wood gases driven out of the burning wood. These gases consist of many compounds. The most common compounds are hydrogen gas, carbon monoxide and methane gas. Because these compounds don’t get hot enough to ignite and combust, they escape up the chimney in the form of dense smoke.

Another leading cause of dense smoke comes from burning “green wood”. Green wood is wet wood. In other words it has not been “seasoned” or left out to dry for a year or two.

As the green wood burns it gives off water vapor or steam from the water trapped inside the wood. This leads to heavy smoking. This problem is a very simple one to fix. Don’t burn green wood! In fact, even know some makers of outdoor furnaces and wood stoves say that you can burn green wood, you shouldn’t. You will consume more wood if you burn green wood, and thus loose even more efficiency. Some of the heat energy from the existing fire needs to be used to drive the moisture out of the wood instead of being used to heat the water.

A responsible owner of one of these devices, such as myself, can vary the way you “fire the furnace”. Instead of filling the firebox to capacity every time, just add less wood as well as the time of day. I fire my stove at night so the smoke doesn’t bother my neighbors during the day. And by adding less wood at each loading, it cuts down on the amount of smoke output.

For the most part, most outdoor wood furnaces are not regulated by the EPA. However, that is changing. Due to the overwhelming popularity of these things, the EPA is now starting to address this problem. Most manufactures of outdoor furnaces are now building models that conform to smoke emission criteria. This is nothing more than a normal path of evolution for these manufactures if they want to continue to sell their products into the future.

The bulk of manufactures are achieving this cleaner burning stove through the use of more modern technology. Although there are many different ways of doing this, the basic principle is the same. That is you need to increase the “combustion temperature” inside the firebox.

In most cases this is achieved by adding a secondary combustion chamber. And in some cases introduces “secondary air”. Regardless of the method used, the goal is to get the exhaust gases / gases of combustion, hot enough to gasify, or ignite.

By getting the gases to ignite in the secondary combustion chamber several things are achieved. First, the gases are being burned at a very high temperature and therefore are achieving “complete combustion” (as much as possible in this type of situation) which in turn produces more heat.

The more heat that is produced, the more heat is transferred to the water, thus making the stove more efficient. Because we are able to gasify and burn the gases trapped in the smoke, the overall smoke output is greatly reduced.

The idea of gasification is not new. In fact, the extraction of hydrogen gases from wood was used during World War Two by the military to run jeeps and alike.

One of the attractions towards burning wood is that it is a renewable source of energy.

And, of course reduces our need for oil from others countries.

Despite what some well-intentioned folks may tell you, burning wood is no more harmful then some fossil fuels from a carbon dioxide standpoint. In short, here’s why.

Wood contains carbon dioxide as part of the natural growing process. It’s part of nature.

Of course all fossil fuels contain this gas as well. This is one of the big problems with burning fossil fuels.

So, why is burning wood different? Because trees contain carbon dioxide that will naturally escape from the wood in time anyway. That is to say, as a tree dies and decays it will emit carbon dioxide.

Fossil fuels on the other hand, only emit carbon dioxide when burned. So the thought here by most, is if the fossil fuels were never burned and left underground there would be no carbon dioxide emission.

Remember, this is not to say that burning wood is any cleaner, it’s just that burning wood adds less NET carbon dioxide to the environment. (So long as we continue to burn fossil fuels)

So with the help of the EPA and stove manufactures, we will soon have a wood burning outdoor furnace that will smoke less. Hopefully this will reach some middle ground between those who are upset over smoke and those who STILL have a right to burn wood.