The information below is available as a concise and informative 20 – 30 minute presentation with lots of pictures, followed by a Q & A session. It can be booked by any interested group and is free of charge (usually 20 mile radius of Cambridge).
Wood as Fuel
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 / wood burner
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 hydrocarbon compounds, VOC’s. These take the form of tars, creosote and resins etc. They constitute about 70 – 80% of the available energy and are an excellent fuel. Most of the rest of the available energy is fixed carbon (charcoal) at around 15 – 18 %. If the stove is not hot enough it’s the VOC’s 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 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 on to the inside of the chimney in the same way as water vapour condenses on to a cool bathroom mirror. In a cool chimney can immediately solidify to form the tarry deposits which your chimney sweep tries to remove. Of course, not all the tars condense inside the chimney. A great deal is emitted from the
top of the stack or flue as un-burnt hydrocarbon or smoke. This is a serious air pollutant. You have also lost much of the available fuel which could have been turned into heat. Incomplete / low temperature burning of wood is bad for your chimney, bad for the environment and bad for your wallet.
How do I know if I am burning wood efficiently?
There are four basic factors which govern efficient combustion of wood. These four factors have a direct effect on the temperature at which the wood burns inside the stove, which in turn governs efficiency.
- The potential efficiency of the stove / appliance
- The efficiency of the chimney
- The moisture content of the wood
- User operation or abuse of the stove
1. Stove efficiency
If the stove is well designed and built, it should be efficient. 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 gives the same problem.
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. Stoves in the UK are often fitted to chimneys which are too big and cold and 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 vaporized before the volatile tars etc. are vaporized and burned. 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. Consider how much energy is required to boil a pan with ½ a pint of water dry on your cooker. You will find a lot of conflicting information about the moisture content of wood for burning but
anything less than 20% is good and less than15% even better. Kiln dried wood or manufactured briquettes should have a very low moisture content. These products cost more but they deliver more available heat energy. 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 to condense and stick. The emission of hydrocarbon air pollutants is greatly increased when burning wood with high a 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 or abuse of the stove
Poor burning habits are the biggest cause of tar / creosote build-up in a chimney. This increases risk of damaging chimney fire.
A great deal is made of the moisture content of wood causing problems in chimneys. Whilst this is a big issue, many users are capable of learning what is dry enough and what is not. 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 leading to tar / creosote problems in the chimney not to mention significant air pollution. Consumers are seldom educated in this area in the showroom, it’s not a top priority for most salespeople. Similarly there can be a lack of informed guidance from the installer. Manufacturers instructions can be too simple and the seldom explain all the issues. If the appliance was there when the householder moved in, there is seldom even a manual, although it may now be available online. Even if there has been good information given it is often forgotten as the user makes up their own mind as to how best run their appliance. Again, it’s all about temperature of the burn. If the air controls are shut down below a given point, the operating 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.
Getting it right:
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. A reasonably vigorous 15 to 20 minute burn will bring the stove properly up to
temperature. 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 is 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. Try to maintain a moderate flaming combustion, where the stove has plenty of flame but it is not being sucked up the chimney. Smoke (un-burnt fuel) should only issue from the top when you first light or are re-fueling the stove. If it is present at other times then one or more of the four factors governing efficient burning are not right. 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 not 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.
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 fuel (VOC’s) 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
Overnight burning / slumbering
Don’t do this. Many people think that it is great to fill up their stove with fuel last thing at night and shut the air controls right down. I am always hearing phrases like “It’s great, it keeps in all night”. This 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 tars / fuel is lost every night. Here is what happens and again, it is all about temperature of the burn. When the air supply to a stove full of burning wood is closed down, the volatile tar vapours (fuel) are still being given off. The temperature inside the stove immediately decreases. The vapourized fuel escapes to the chimney, tarring it up and
causing lots of air pollution. Inside the stove you have begun to manufacture charcoal. Once all the volatile 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 a bit faster 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 little or 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. I have had many customers with good stoves and good chimneys, using dry wood but they have been shutting their stove down overnight. The amount of tar and creosote swept out of some of these chimneys has to be seen to be believed. They have been creating
huge amounts of pollution, wasting fuel and greatly increased the chance of a
damaging chimney fire.
Type of wood
There is a great deal of confusion in the UK regarding the best wood to burn as well as woods to avoid. All wood is good to burn in a stove as long as it is dry. Even seemingly reliable sources of information sometimes get this completely wrong. You may read or be told that only hardwood logs are suitable and that you should not burn pine or conifer as these contain sap? or resins which tar up the chimney. This is rubbish. You may be told that there is more heat in hardwood than softwood. Again, not true. The calorific (heat) value of a kilo of softwood is almost identical to that of a kilo of hardwood, the softwood is just less dense so it takes up more space. You will have to put more pieces of a given softwood
into your stove than you will hardwood, but weight for weight, the heat output will be about the same. With this in mind you should pay at least a third less for a given volume of softwood over hardwood.
Size of Logs
The size of your logs or pieces of wood will make a big 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. A big log has a lower surface area relative to its volume than a small one. If you take two large logs, split one in half and put them in identical burning conditions, the split smaller ones will burn faster and hotter because the increased surface area is able to release fuel more rapidly. This is why we use small kindling and twigs to start a fire – kindling has a large surface area to volume ratio.
Storing / drying your wood
You can easily dry your logs through correct storage. Stack your 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. If you plan to fell trees it’s good to remember that they contain less water in the winter. Keeping the rain off your stacked logs is of secondary importance – Ventilation is the most important consideration. That said, a well ventilated log store with open sides and a roof on it is the best situation. You should easily be able to achieve moisture content of 20% or less in a few months if your logs are the right size and properly stored. 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. If you are a novice, get yourself a good moisture meter. To test the moisture content of any log,
split it first and then test the split surface. If you are buying wood think ahead and buy early.
Open fires are nice to look at but they are very inefficient. Even a poor stove can be three or four times as efficient as an open fire and this does not include the huge heat loss from a building when open fires are not in use. The amount of wood burned on an open
fire in an evening to heat a room can practically heat an average house if burned in an efficient way. If you burn wood on an open fire avoid conifers, willow, sweet chestnut as these tend to spark a lot. Kiln dried wood may burn too fast and hot on an open fire with a strong draw. Try to burn your wood on a bed of ash rather than in a grate or basket. Grates are for burning coal which must have a supply of air from below. Wood burns hotter and more slowly on a bed of ash and the glowing ember stage of burning lasts much longer. Remember that an open
chimney for an open fire acts like a giant vacuum, constantly sucking large volumes of air from the building. If you are running your central heating when the fire is not lit, an awful lot of your nice warm air is being sucked up the chimney. To put it into perspective the heat loss is similar to having a large window open 24 hours a day over the winter. You are however not normally aware of the losses because the chimney sucks warm air out rather than letting cold air in.
Thanks for reading