Updated: 25 Feb 2010

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Introduction

Modern woodburners are clean burning, but only under ideal conditions, with a specific type of wood. We tested with 8 other kinds of firewood.

All woodburners sold for non-rural use must pass a stringent national environmental standard for efficiency and for emissions. That should be great for the environment, but there's a catch.

The very test that woodburners must pass to ensure they meet the standard actually results in them burning dirtier than they should when they're used in everyday conditions.

What's the problem?

For woodburner-compliance testing, the woodburner sits on accurate scales in a special test room where the heat output can be measured. Each test run is started with an existing hot ember bed. Cut lengths of clear (no knots) 100mm round, very dry radiata are used.

The scales allow a measured amount of fuel to be burnt down to a set residual weight. The flue is connected to several measuring instruments, and the average heat output, emissions, and efficiency are calculated from the measurements taken during the test run.

In order to achieve good results in this test, manufacturers design and "tune" their woodburners to burn as cleanly and efficiently using the same kind of fuel.

The trouble is, this test fuel is nothing like real firewood. Real firewood has more resin and other volatile compounds, and these require more air than the test fuel to burn. That means a woodburner tuned for the test fuel won't burn real firewood as effectively, with the result that more of the wood goes up the chimney in the form of smoke and soot.

And smoke and soot aren't good for you. Some of the microscopically-small particles they contain can lodge in your lungs, causing problems such as asthma and other respiratory diseases. There is no known lower "safe" limit for the pollution that woodburners produce.

About our test

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.

Test results

Guide to the table

1 Wood type

• Seasoned radiata was clear, with "normal" resin content. It was not "old man pine" but more representative of plantation firewood.
• Macrocarpa and blue gum were normal firewood, with no distinguishing abnormalities.
• Manuka was in log form, up to 85mm in diameter, and had been seasoned for a long period in the open air.
• Unseasoned radiata was from a plantation tree, felled two weeks before the test and cut and split one week after that. Because the wood was wet, the woodburner was left on high-burn setting for approximately 25 minutes (compared with the normal test of approximately 10 minutes), or until the wood was burning brightly before the air control was turned down to a medium setting. Turndown any earlier resulted in the fire smouldering and going out.

The testing of large logs of dry radiata and blue gum was performed with the fuel as supplied by the firewood merchant. The logs were not split further.

2 Burn time
The time taken for the fuel to be consumed, from when it was first placed on the hot embers.

3 Average output
The average of a series of heat-output readings taken over the burn time.

4 Emissions
The exhaust solid content measured as grams per kilogram of fuel burnt. The national environmental standard requires emissions of no more than 1.5g/kg using the test fuel (kiln-dried radiata).

5 Efficiency
The heat that ends up in your lounge relative to the energy content of the fuel burnt.

Our advice

Winter airborne pollution is a serious problem in a number of New Zealand cities and provinces. Minimising pollution means keeping your woodburner working well. And that requires careful selection of the fuel you burn, and the way you burn it.

You can do your bit to reduce emissions, even if you have an older burner:

• Make sure the wood you burn is well seasoned. You'll need at least one year of well-ventilated drying, with the wood protected from wet weather - longer for harder woods. Wet wood cools the fire and reduces heat output, which greatly increases emissions.
• Don't burn big logs (over 110mm diameter).
• Don't burn treated wood, painted wood, or driftwood. They all cause dioxins and other toxic substances to be produced.
• Keep the fire hot - smouldering fires are very dirty. This means refuelling relatively often: perhaps every 20 to 30 minutes for radiata, less often for the harder woods.
• Don't load the firebox up and turn the heater down to burn overnight. This is the most polluting and least efficient way of running a woodburner. It doesn't burn as much because it cooks the firewood, produces little usable heat, gums the flue with soot, and pours out large quantities of pollution into the still night air.

Our view

There is a need in many parts of New Zealand for woodburners to produce as little pollution as possible. Because of the nature of the national environmental standard compliance test for woodburners, that is not happening right now.

It would take only small design/manufacturing adjustments for woodburners to produce fewer pollutants than they do now, when burning typical firewood, if the compliance test was updated.

Two things are needed:

• Manufacturers need to develop woodburners that can cleanly burn a wider range of woods.
• Regulators need to develop a test fuel and testing method that more closely matches typical firewood, and the way householders normally operate their burners.

"Low-emission" woodburners would then be worthy of their name. And then we could all breathe easier.