There has been a lot of debate about wood stoves and their environmental impact.
Muswell Hill Sustainability Group runs regular evenings on how to use your wood stove efficiently, reducing emissions to the minimum. Contact info@mhsgroup.org to find out when our next Woodstove Wisdom evening is.
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Why the correct use of your wood burner’s secondary air supply is essential
So you have now got your wood burner installed and can happily burn as much wood as you like safe in the knowledge you have done your bit for the environment… Well, actually that might not be the case!
For a wood burner to convert wood fuel to heat efficiently and effectively requires that the wood burner be operated correctly. Correct operation of a wood burner means delivery of the additional wood burner benefits that are not provided by an open fire. So what does this mean?
Fundamentally for a wood burner to operate correctly you need to burn both the wood itself and the exhaust gases that are expelled during combustion. An open fire cannot achieve this and the exhaust gases are expelled from the chimney in a non-combusted state. This is why a wood burner is said to be 80% efficient (if operated correctly) and an open fire 20% efficient. 80% of the potential heat of an open fire goes up the chimney. Hence, if you don’t operate your wood burner correctly you are losing a lot of heat energy up the chimney and causing much more air pollution compared to a wood burner used efficiently.
However, help is at hand. To burn those exhaust gases you just need to correctly use the two air supply controls on your wood burner. These control the Primary and Secondary air flows. The primary air supply arrives below your fire whilst the secondary air supply arrives in the form of air jets above your fire. The most important of these air flows is the secondary one as it is the secondary air flow because that promotes combustion of the exhaust gsses.
Use the primary air supply to get the fire going and burning hot. Then shut it down. From this point on ‘manage the fire’ by adjusting the secondary air flow. If this is done correctly you will see the flames dancing around (see photo) and actually see little jets of gas burning as they exit the wood. Another way to see you are getting a high level of secondary combustion is that your logs ‘maintain their shape’ until completely burnt (when you see this you will understand what I mean) and your smoke coming out of the chimney will not smell.
Another no-no with respect to your stove is burning wood slowly (cool) with minimal air supply. In addition to poor secondary burn this approach will also lead to nasty black tar deposits in your flue, which gives a greater likeliness of chimney fire, and deposits on the glass.
So remember the simple rules are:
1) Burn hot
2) Secondary air supply to dominate.
Follow both these rules and compared to not doing it you will get much more heat (x2), burn less wood (half) and radically reduce emissions. Then you can enjoy burning your wood in the comfort of knowing you really are doing the right thing.
Muswell Hill Sustainability Group 2017
Further reading:
Norwegian Wood – Chopping, Stacking & Drying Wood the Scandinavian Way Lars Mytting, 2015 ISBN-139781419717987 Publisher Abrams Image