Woodheaters and woodsmoke
Air quality fact sheet
Department of the Environment and Heritage, 2005
When wood is heated to a high enough temperature it breaks down into a complex mixture of gases. These gases burn in the presence of oxygen to give off heat. If there is not enough oxygen, or not enough heat, the gases will only partially burn and the un-burnt gases will go up the chimney into the air outside.
Once outside, these gases cool down and condense into tiny droplets of oils and tars. These are known as particles. A single particle is far too small to see with the naked eye, but a lot of particles together are seen as woodsmoke. Particles cause many of the environmental and health problems associated with woodsmoke. They are a major source of air pollution in wintertime, especially where many people use woodheaters for home heating.
Woodsmoke contains particles so small that, when inhaled, they can cross the lung lining and end up in the blood stream. (See fact sheet on particles) Woodsmoke also contains other chemicals that can affect our health. The health problems associated with woodsmoke include asthma, chronic lung disease, heart problems and premature births and deaths. Some of the toxic chemicals in woodsmoke are known to cause cancer.
Woodsmoke in Australia is a problem in many towns and cities on very cold, still nights. The colder temperatures mean that more people leave their woodheaters burning overnight. To keep them burning the firebox is often loaded so full of wood the fire is starved of oxygen. This causes the woodheater to smoulder and produce a lot of smoke. On a still night the problem is made worse as the smoke hangs in the air at ground level without a breeze to blow it away. When this happens the air we breathe can become very polluted.
The hot embers of a wood fire are actually the last stage of the burning process before the fire goes out. They are made up of carbon, commonly referred to as 'charcoal', and almost half the heat that comes from a wood heater comes from these hot embers. The embers burn very cleanly and make hardly any smoke.
If you want to keep your woodheater going overnight, you will make much less smoke if you burn the gases off first, before turning the heater down to reduce the air supply. To do this easily you need to take some of the steps listed below.
Many people do not realise that smoke is wasted heat that costs money. If the gases from the fire go up the chimney instead of being burnt, there is less heat available to heat your house. You can minimise the amount of smoke from a woodheater if you:
- burn only dry, seasoned, untreated wood;
- use smaller logs instead of only one large log;
- do not pack the fire box too full as this will starve the fire of oxygen and cause it to smoulder;
- keep the fire burning brightly for the first 20 minutes after lighting and reloading – the faster you can get the fire going the les smoke there will be;
- always have a visible flame if you plan to keep the fire going overnight.
If you are buying a new woodheater, make sure you buy one that conforms to the Australian Standard 'AS 4013'. Woodheaters are also rated for their efficiency so one that is 65% efficient will burn less wood for the same amount of heat than one that is only 60% efficient. Check the label. You will also be better off if you choose one that is the right size for your house. If you buy a heater that can produce more heat than your house needs, you will have to set it to burn slowly. A big heater burning slowly makes more smoke than a smaller heater burning more quickly.
Changes in technology and better woodheater design now mean that many heaters are rated to burn with much lower levels of emissions than required by the Australian Standard. Consider purchasing a woodheater with the lowest level of emissions possible.
Pellet fuel heaters are a new type of heater on the Australian market. They burn pellets of compressed sawdust , the waste product of sawmilling processes, using a hopper to feed pellets into the firebox without the need for human intervention. Pellet heaters have very low emissions and are almost smokeless. Improving the thermal efficiency of your house may reduce the need for heating. Increased insultation in walls and ceilings, thick curtains with pelmets, increased use of thermal mass in floors and walls can all help to regulate the temperature of a house.
More Air Quality fact sheets are available from the Community Information Unit of the Department of the Environment and Heritage, phone 1800 803 772 on the following topics:
- National Standards for Criteria Air Pollutants in Australia;
- nitrogen dioxide, sulfur dioxide, carbon monoxide, ozone, particles, lead, air toxics; and
- smoke from biomass burning
See also our website at Air quality