Cleaning up our wood fires

The more we try to slow a fire down the more it is bound to crash into a smoulder.


Cleaning up our wood fires

Much as I appreciate the CVRD’s iniative to help people to swap out old woodstoves for newer high efficiency ones, this does not solve the problem of woodstove pollution. Although the newer stoves with secondary combustion technology can burn cleaner, this only occurs when they are being run hot enough. Whenever there is blue or dark grey smoke exiting a chimney the fire is smouldering, and operating way below the stove’s touted 70 per cent efficiency. In my rural neighbourhood four out of five new high efficiency woodstoves are frequently smouldering in this manner.

The more we try to slow a fire down the more it is bound to crash into a smoulder. Smoke/gas is released from the wood at above 150 C, but ignition into flame only occurs above 300 C. When a woodstove’s reburn process is operating in between these temperatures, too much smoke/gas is passing through too little flame to achieve full combustion, and the wasted fuel is going straight up the chimney to become atmospheric pollution.

The most common contributing factors to this kind of mis-fire are:

• burning wet/green wood;

• not enough kindling and mid size pieces on start up and reload;

• ramping the draft down too soon after start up;

• not ramping the drarft up long enough after reload.

Back in the days of drafty cabins and poorly insulated houses we had reason to run our stoves hotter. But the better insulated our houses become, the slower we try to burn, the more often our fire crashes into smoulder. Burning slow is a flawed paradigm, and there is only so much reburn technology can do to compensate. As long as common operator errors are at play, woodsmoke air pollution is a reality.

The only sure-fire way to acheive clean combustion is to burn as hot as possible. Once the fire is over 600 C all of the volatile woodgases burn completely. The question then is: what to do with all of this rapidly generated heat? Some people have tried water jackets and tanks, but this is dangerous due to water’s explosive expansion pressures at over 100 C. Also, water is the universal solvent, constantly working to get out of whatever container we put it in (and eventually it will).

The safest and simplest way to store post-combustion heat is in earthen mass such as brick, stone, ceramics, or cobb (clay-sand). These are the materials used in the construction of masonry heaters, with high-mass bodies that absorb the thermal energy from a hot fire for gradual delivery into the living space. Such heaters have been around for thousands of years, and are still quite common in Northern Europe and other cooler regions of the world. For more information about this see:

If you are not feeling ready to let go of your old (or new) metal box stove, or run it hotter, but you would like to improve its combustion efficiency, then you might want to consider burning from the top down. This involves loading larger splits on the bottom, placing smaller pieces above them, kindling above that, and lighting from the top. This way, all the smoke rises upwards into the flame path (rather than above it). It may seem upside down at first, but when you think of how a candle functions, ignition of fuel is more dependent upon radiant heat than flame contact. When we locate the flame above the fuel the base of the flame sends heat downward into the fuel below.

Loading a fire box to burn this way may take some getting used to. The wood must be well seasoned, dry, and a fair amount of kindling is required. But once the kindling catches, the fire will ramp up slowly and surely. And this gradual nested burn is exactly what we want. The video link below offers a clear demonstration of top down burning, as well as info on how best to reload for optimal efficiency:

One final note. Top down burning requires a steady supply of smaller diameter wood (3 inch, 2 inch, 1 inch). Chopping wood into these smaller pieces may seem a daunting chore, but it can be done simply and efficiently by finding an old car or truck tire and setting it up at a good chopping height. Stack a bunch of bigger splits or rounds into the hole (with grain up and down) til it is fairly tightly packed. Then whack away at it with a maul. You’ll find the wood splits easy and stays in place, which eliminates bending down over and over to pick up stuff that otherwise goes flying. This is a BIG back-saver. And the cushion of the rubber perimeter makes it easy on the wrists too. An internet search for “splitting wood in a tire” will show all kids of examples. Eg:

OK, hope that helps. Please kindly pass this info along to your wood-burning neighbours.

And happy heating as we clear the air.

Patrick Amos



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