We took a new woodburner that complies with the national environmental standard, and ran it with eight different fuel loads, including the standard test fuel as well as split radiata logs, macrocarpa, blue gum, and manuka. We tried seasoned and unseasoned radiata, and large logs of radiata and blue gum.
What we found
Nothing that you'd normally regard as firewood gave anywhere near the low emissions that are obtained from burning the test timber. Wood type, log size, and dryness can make a huge difference to performance and exhaust emissions.
Our real firewood gave less heat than the test fuel, and it burned for longer with greater emission levels. Seasoned blue gum was the cleanest burning, but even it produced more than three times the emissions of the test fuel.
The emissions were much worse with damp wood, because heat from the fire is used to evaporate the water in the wood. They were also much worse with large logs, which have a lower surface area in relation to their volume (compared with the test fuel). High firebox temperatures are the key to keeping a fire burning cleanly, and modern woodburners are much better at this than models from a few years back.
Amongst the real firewood, the overall efficiency figures didn't vary much. Less heat for a longer time gives an efficiency figure that's similar to more heat for a shorter time. Again, the worst result was for wet wood.
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