Stop your money going up in smoke
How to get more out of your woodheater
Department of the Environment and Heritage, 2002
Australian's have been burning wood for centuries to keep themselves warm in winter. Many still enjoy the warmth and comfort of a woodheater.
Recently, scientists have shown that smoke from a woodheater means:
- unburnt fuel emitted into the atmosphere;
- pollution causing respiratory problems; and
- potentially hundreds of dollars a year wasted.
Minimising smoke haze contributes to significant health benefits for everyone by improving our air quality, reducing the maintenance and running costs of your woodheater, and saving you money.
Reduce woodsmoke in 5 steps
Follow the five simple Breathe the Benefits steps to reduce woodsmoke:
- burn dry, seasoned, untreated wood;
- stack wood under cover in a dry, ventilated area;
- use small logs;
- burn the fire brightly; and
- have a flame if burning overnight.
Burn dry, seasoned, untreated wood
Wet and green (or unseasoned) wood will not burn properly and produces a lot of smoke.
Freshly cut wood contains about half its weight in water. A reasonable log of firewood, if still green, can contain around two litres of water. Energy and money are wasted in boiling the water in each log, the fire does not get as hot as it should and excessive smoke is produced. More wet wood is needed to get a reasonable amount of heat. Using dry wood therefore saves money.
Two pieces of dry, seasoned wood banged together will make a loud hollow crack, whereas unseasoned wood makes more of a thud.
Stack wood under cover in a dry, ventilated area
Wood is ready to burn when air-dried for at least 8 months to a moisture content less than 20%.
Store wood under cover in a dry, ventilated area, i.e a well-vented woodshed, in a fashion to allow the wood to continue to dry out. If freshly cut (green) wood is stacked in the open it will dry out and eventually reach about 15% moisture content.
Stack wood away from the house walls because the wood can act as a pathway for insects, eg ants or termites in some regions of Australia.
Use small logs
Small logs burn brightly and do not crowd the heater as larger logs can do. Smaller logs will make a fire easier to light, and help in establishing a vigorous fire quickly after refuelling. This will reduce smoke emissions.
Use at least two or three logs in the heater, never one large log. However, if a woodheater is stacked full of small logs there might be such rapid combustion that there is not enough oxygen present to achieve complete combustion and smoke would increase. Leave enough space over the top of the fuel load to allow flames to develop. For slower burning, two or three larger logs will usually prove most satisfactory.
Burn the fire brightly
A robust fire produces less smoke than a slow or smouldering fire.
A smouldering fire at any time, whether caused by wet wood, not enough air, not enough kindling or poor loading of logs (i.e blocking off combustion air), will cause excessive smoke. Run the heater on high burn rate (air controls fully open) for 5 minutes before and 15 to 20 minutes after adding more logs. Check the heater chimney regularly for smoke. If the heater is producing lots of visible smoke for more than 15 minutes after lighting or refuelling, adjust the full load and increase the air settings. Just a little bit of attention to the fire will make a big difference to how much smoke is produced. Testing has shown that a poor fire can produce up to 10 times more smoke than a brightly burning fire.
When lighting a cold heater, always use sufficient dry kindling to establish a good fire quickly.
Have a flame if burning overnight
A smouldering fire or one damped down produces excessive smoke and should be avoided.
Run the heater at full burn rate for 15–20 minutes before turning the heater down for the night. If there is a poor fire during the night, emissions of smoke will be very high. If the fire is burnt well before it is turned down to a slow or medium burn, even if turned a bit too far down so the flame disappears for a while, the emissions will not be as bad as having a poor fire throughout the whole burn cycle.
Keep the air controls high enough to prevent the flame from disappearing, especially for woodheaters that do not carry Australian Standard 'AS4013' certification.
Selecting & installing woodheaters
Selecting a woodheater
Choose a heater with a low emission factor: The 'emission factor' is measured in grams of smoke per kilogram of wood burnt (g/kg). The current Australian Standard is 5.5g/ kg, but some heater models on the market are well below this level. Check to see if your heater conforms to the Australian Standard 'AS4013'.
Choose a heater that best suits your circumstances: The heater output is measured in kilowatts (kW). A big heater burning slowly usually causes more smoke than a smaller heater burning fast. The Australian Home Heating Association (national office 03 9592 2522) can provide you with information on which woodheater is best for your situation.
Installation
All heaters must be installed in accordance with the Australian safety standard AS2918. However, having a flue taller than the minimum height required by law can help disperse the smoke causing less smoke in your immediate neighbourhood.
Ensure your woodheater installer shows you how to get the best out of your heater. Read and follow the instructions that come with your heater.
Insulation
A well insulated house will stay warmer longer and so may not need a heater to be left burning overnight.
This will save fuel and reduce smoke emissions. In spring and autumn, a well insulated house may not require any extra heating at all.
Read the Australian Greenhouse Office's 'Global Warming Cool It' booklet for information on how to insulate your home (http://www.greenhouse.gov.au/pubs/gwci/ ).
Woodsmoke: causes and cures
When wood is heated, energy is required to drive off any moisture before higher temperatures can be reached. As the wood continues to increase in temperature, complex chemical reactions begin to take place. The wood begins to break down, releasing a mixture of gases, such as methane and hydrogen, and many complex tars and oils. If a flame or glowing charcoal are present to ignite these gases they will burn vigorously, giving off heat and providing the visible flame we all associate with wood fires. This combustion requires oxygen from air. If there is not enough oxygen these gases will not burn, or only partially burn.
Any gases escaping the flame unburnt will escape through the flue. As they cool, many of the gases will condense into tiny droplets of oils and tars. These particles are too small to be visible to the naked eye, but in large numbers become visible as white or faint blue smoke. The smoke is wasted fuel, so the more a heater smokes, the more fuel is wasted.
The final phase of wood combustion accounts for almost half the heat released from wood – through glowing charcoal. During this phase, very little smoke is emitted, as charcoal is a very clean burning fuel. However, all the gases must have been burnt off while there was a lot of flame and before the air access is restricted, i. e. before the fire is turned down.
Although wood is a natural product, its combustion generates more than 100 pollutants. Some chemicals in woodsmoke are known to cause cancer, and others cause respiratory and other health problems. We can reduce particle pollution of our air by burning our woodheaters more cleanly and efficiently.
Which wood to burn?
The net benefit of buying dry, seasoned wood
Buying wet wood means paying a lot of money for water! Freshly cut wood contains about half its weight in water. Order or collect your wood in the summer months. Deciduous trees should be harvested in winter, when there is little sap, then dried until the following winter. Buy wood that has a low moisture content. You could save up to 40% when buying dry, seasoned wood.
Reducing smoke through proper combustion has great environmental benefits. A heater run poorly overnight can produce 5 times the amount of pollution than a heater burnt efficiently.
Lower maintenance costs may result through less creosote accumulation in the flue. This means financial benefits through reduced flue cleaning. Remember to check your flue regularly for creosote build-up.
Need more information?
For further information, contact the Department of the Environment and Heritage on (freecall) 1800 065 823 or e-mail airquality@environment.gov.au, or contact your local State/Territory environment protection agency.
Breathe the Benefits was a Natural Heritage Trust funded Clear the Air project .
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