Wood is an excellent fuel. It is plentiful, and renewable. It can often be sourced very close to the point of use and is nearly carbon neutral. At a basic level it requires little or no processing – cutting and splitting. It can be used to manufacture other products e.g. pellets, briquettes or chips. The use of wood as a fuel however is not well understood. Consumers are often unaware of how to get the best from their wood fires / stoves. Most people think they know how to burn a few logs but there are several very important factors which are frequently overlooked. Burning wood in a stove / woodburner
The basic problem is that the wood is often not burned in an efficient way. That is to say it is not burned at a high enough temperature. Most of what is burning when you use logs are “volatile hydrocarbons”. These take the form of tars, creosote and resins etc. They constitute about 70 – 80% of the available heat energy and are an excellent fuel. When we see flames coming from wood, it’s the volatile gasses that are burning.
Most of the rest of the available energy is fixed carbon (charcoal) at around 15 – 18 %. When we see wood and embers glowing, it is the fixed carbon that is burning. If the stove is not hot enough it’s the volatile tars etc. that can cause a problem. These volatile hydrocarbon compounds must first be turned in to a gas (vapourised) before they can burn. If the fire is not hot enough these volatile vapours are still given off but are not completely burned in the appliance and they escape to the chimney. If the chimney is cool enough, some of these tars will condense and solidify on to the inside of the chimney in the same way that water vapour condenses on to a cool bathroom mirror. In a cool chimney they can immediately solidify to form the tarry deposits or creosote glaze which is a problem for all of us.
Of course, not all the tarry vapours condense inside the chimney. A great deal is emitted from the top as un-burnt hydrocarbon or smoke. Smoke is simply unburned fuel. This is a serious air pollutant. Much of the available fuel which could have been turned into heat has also been lost. Incomplete or low temperature burning of wood is bad for your chimney, bad for air quality and bad for your wallet.
There are four basic factors which govern efficient combustion of wood. These factors have a direct effect on the temperature of the wood burning inside the stove, which in turn governs efficiency and the degree of air pollutants.
The four factors are:
1. The efficiency of the stove
2. The efficiency / construction of the chimney
3. The moisture content of the wood
4. User operation and control of the stove – this is usually the most important but most misunderstood factor.
The sizing of the stove is also a significant factor. If the kW output (size) of the stove is too large for the room then this will be an additional problem. Low temperature burning is likely as the room will quickly get too hot and the operator is likely to close the air controls off too much.
1. Stove efficiency
If the stove is well designed and built, it should be capable of burning efficiently. Many are not but basically, if it is capable of properly pre-heating the combustion air and introducing it to the fuel in a turbulent manner, it should be quite efficient. If it is not capable of a good level of pre-heating then the operating temperature will be lower and vaporized fuel (tars etc.) is lost to the chimney. If the stove is too large for the room the operator is likely to burn small, cooler fires which give the same problem. Many stoves already installed in the UK have a very low potential minimum efficiency as they are capable of having the air closed off too much.
2. Chimney efficiency
Most people find it difficult to think of a chimney as being efficient or otherwise but it is a key part of the system. It must exert an appropriate and consistent draw / pull throughout its length. To achieve this it must be high enough. It must be the right size (cross section) and be well insulated. The top of the chimney must terminate in the right place – free from turbulent and disturbed air. Even a very efficient and clean burning stove will not reach its potential without the right chimney above it. Lastly, the pot / cowl / terminal should not impede the exit of combustion gasses.
3. Moisture content of wood
The moisture content of wood is a key factor for efficient burning. Any water in the wood must be “boiled off”. This requires an input of heat energy which is then lost to the chimney as steam. An average sized log may contain well over ½ a pint of water and still seem reasonably dry. You may find a lot of conflicting information about the moisture content of wood for burning but anythingless than 20% is good and less than15% even better.
Kiln dried wood or manufactured briquettes should have a guaranteed maximum moisture content. If your wood is too wet, the temperature in your stove can’t get high enough to achieve good efficient combustion and lots of un-burnt tars enter the chimney. To make things worse there is much more water vapour in the smoke. All this has the potential to cool down enough inside the chimney and then to condense and solidify. The emission of unburned tars, creosote etc. is greatly increased when burning wood with high moisture content, as is the chance of a chimney fire. Some people use a bed of coal or smokeless fuel to get their wet logs to burn. This is very polluting and a waste of money. What’s more, the increase in water vapour helps form acids which will etch the stove glass and attack the metal parts of the stove and chimney. Not good.
4. User operation and control of the stove
Poor burning habits are the greatest cause of tar / creosote build-up in a chimney. This increases risk of damaging chimney fire. It wastes fuel and causes unnecessary air pollution.
A great deal is said about wet wood causing problems in chimneys. This can be an issue but most users quickly learn what is dry and what is not, particularly if they have a good chimney sweep to help them. Less well understood is the huge influence that the user has when controlling the rate of burn of the stove.
A lack of consumer awareness regarding the correct operation of their appliance is the single most important factor when it comes to inefficient burning. This usually leads to tar / creosote problems in the chimney not to mention significant air pollution. Consumers are often unaware of the problems. Information given when the stove was installed may be missing of forgotten.
Manufacturer’s instructions regarding control can be too simple and they seldom explain all the issues. It’s no wonder consumers are unaware.
Often the user just makes up their own mind as to how best run their appliance. But, it’s all about the temperature of the burn. If the air controls are shut down too far, the burning temperature drops and vapourised fuel escapes to the chimney again. Following manufacturer’s instructions on burning and operation is good but a basic understanding of what is going on helps get it right.
Newer super efficient stoves are designed to capture and burn all the fuel. The operator can’t shut down the air to the point that combustion is restricted. This type of stove will simply not work with wet logs. They are the future but there is still a large stock of less efficient stoves, so how to get it right is very important.
A reasonable load of appropriate fuel should be placed in the stove along with any firelighters you may wish to use. Light the fuel, close the door and make sure all air controls are fully open. Follow the manufacturer’s instructions but as a basic guide a 15 to 20 minute burn will bring the stove up to correct operating temperature. Try to maintain a moderate flaming combustion, where the stove has plenty of flame but it is not being sucked up the chimney. If your stove is designed to pre-heat the combustion air, it is even more important to get it up to operating temperature. If it does not reach this temperature before you begin to close the air down, the combustion will not be so complete and you will be losing fuel (tars etc.) to the chimney.
Once the optimum operating temperature is reached you will probably need to re-fuel. Let the fuel start to burn before beginning to close any air controls. If your stove has more than one control it is probably able to pre-heat the combustion air. Now is the time to completely shut the control which allows the room temperature air in. You can then begin to set the rate of burn with the pre-heated air control. Don’t shut it too much! You have a box full of hot burning fuel and if the air is reduced too much, combustion is incomplete and you’ll be losing fuel to the chimney again and yet more unnecessary pollution.
Smoke (un-burnt fuel) should only issue from the top when you first light or are re-fueling the stove.
If smoke is present at other times, it’s usually because the fuel is not burning hot enough. This is usually because the air controls are closed off too much. It’s a good idea to go outside and have a look at the top of your chimney when the stove is up to temperature and the wood is burning nicely.
You should see no, or very little smoke. Return to the stove and shut the controls, then go back outside and have another look at the top. You should now see lots of smoke. There will be a point beyond which you should never close the air controls. A little experimentation and you should be able to find this quite easily. Super efficient stoves have been designed to prevent the user from closing them off beyond a certain point. They also introduce high temperature pre-heated combustion air in a turbulent manner and hold the fuel / air mix in the combustion box for longer.
Your local professional chimney sweep should be able to take you through the process for your stove. If they can’t do this or don’t think it’s important then perhaps change your chimney sweep!
The design features of very efficient stoves ensure that the burn occurs at optimum temperature most of the time, so there is very little escape of un-burnt volatile fuel to the chimney. If there is no un-burnt fuel inside the chimney, there can never be a problem with tar / creosote build-up. Choose your stove carefully.
Don’t do this. Some people still think that it is great to fill up their stove with fuel last thing at night and shut the air controls right down. I’m glad to say it’s becoming less common as professional sweeps help educate their customers but it still happens.
“Slumbering” the wood is very bad practice and more than half the available fuel load may be lost to the chimney. In an average size stove this could easily mean that over a kilo of vaporised fuel gasses are lost every night! Here is what happens and again, it is all about the temperature of the burn:
When the air supply to a stove full of burning wood is closed down, the volatile vapours (fuel) are still being given off. The temperature inside the stove immediately drops. The vapourized fuel escapes to the chimney, tarring it up and causing un-necessary air pollution. Inside the stove you have begun to manufacture charcoal. Once all these volatile fuel compounds have been driven out of the wood you are left with quite a pure carbon – charcoal. This will then burn at whatever rate the available oxygen allows and the process can take all night. Burning an identical load of wood faster and at a reasonable temperature will significantly reduce or eliminate the loss of fuel to the chimney. If you are in the habit of burning at night, then set your air controls so that no smoke is coming from the chimney top. You are now burning much more fuel in the stove rather than letting it escape to the chimney. The stove will deliver much more heat to the building but over a shorter period of time.
The size of your logs will make a difference to how they burn. The greater the surface area of the wood relative to its volume, the faster the tars etc. can be vaporised and burn. 5 to 6 inches wide is best.
You can easily dry your logs down to 20% moisture with correct storage. Stack logs so the air can get at them. If you cut and split them yourself try to do this when the wood is fresh cut as it is much easier on you and your tools. Once split, you have greatly increased the surface area of each piece and it will dry much faster. A well ventilated log store with open sides and a roof on it is the best situation. You should be able to achieve moisture content of 20% or less in around six months if your logs are the right size and properly stored, a year would be better. A moisture meter is a very useful tool. To test the moisture content of any log, split it first and then test the split surface.
Beware of the word seasoned, it means nothing in reality. The only important consideration is the moisture content, not how long it has been cut or stored.
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