Tar (or Creosote) can build up in your chimney or flue liner for several reasons and is a major cause of chimney and house fires. Firstly, it is important to know what it is. Tar is the residue left behind due to incomplete combustion of the gases and impurities which are released when you burn your fuel in your solid fuel appliance, whether you have a stove or an open fire. This can happen for a number of reasons which we will examine below.
This (to me) seems to be the most common reason for tar build up for people with Wood Burning or Multi-Fuel Stoves. To preserve the life of your stove and chimney liner it is essential to burn quality wood. This means using only well seasoned or kiln dried logs and resisting the temptation to burn wood treated with paint or preservatives. The best way to ensure you are using dry wood is to get yourself a Moisture Meter. These are available almost everywhere and are quite inexpensive, although as with most things in life it is advisable to obtain the best quality you can afford. This meter has (usually) two prongs which you insert into your log and this will give you a digital read out of the woods moisture content. The figure you should be looking for is a reading of less than 20% (although 15% is considered optimal by most Chimney Sweeps). If you have access to kiln dried wood then this usually reads much lower than this so kiln dried is the safest way to go if you don`t have the means to season or store large quantities of logs.
Your stove will have one or more air/ventilation controls fitted to it. This usually takes the form of a slide or a handle that you use to control how much air gets to your fuel to assist in the combustion process. When your stove or appliance is fitted it is vital to learn how to use these controls to get the most efficient burn possible. Your installer will supply you with instructions from the stove manufacturer but he should also verbally explain these controls to you in real time in order for you to fully understand how your stove functions.
If you close down the supply of air too much (it is amazing how many people do this habitually) then the fuel does not get enough air to burn off all the impurities/pollutants. This in turn means the gases are not hot enough to ensure the smoke emitted exits the chimney cleanly and in turn forms a build up of tar in your chimney/liner which is extremely combustable and can lead to house fires. This deposit can also lead to a complete blockage in the flue so no gases can escape at all. Gases such as Carbon Monoxide (CO) can leak back into your house and this is a potential danger to life. This is why it is also vital to have a correctly fitted CO detector in the same room as your stove.
It is worth knowing that the Government plans to bring in new controls in 2022 to control the type and quality of fuels that people are burning and to ensure that all appliances meet DEFRA requirements in order to reduce air pollution and to cut the number of chimney/house fires. It is important to note that they are NOT going to ban wood burning stoves as certain misleading articles in the press have claimed!
If you are burning the correct type of fuel at the optimal temperature (you can ensure this by using a stove pipe thermometer) and you still have a problem with the build up of tar then it is important to look at the efficiency of the flue/chimney and how the waste gases of combustion are kept hot enough to ensure they exit cleanly and efficiently.
Firstly, it may be that your chimney is not high enough and efficiency can be affected by surrounding structures such as other buildings or trees. Climatic conditions can cause down drafts that can affect how you chimney performs. Prevailing winds can also have an effect. Another issue can be that the chimney/flue is not insulated enough to ensure its efficiency. It could be that it is situated on an external wall or gable end and could require extra insulation to maintain efficiency. Much of these problems can be avoided with good forward planning before installation of your stove. An experienced Chimney Sweep can help with the recognition/diagnosis of these issues and it it better to seek advice sooner rather than later as solutions may become more expensive later on.
So, after examining the causes of tar build up in stoves, chimneys and liners we must consider the costs involved, both to the customer and to the environment. Burning the correct type of fuel in a modern efficient stove can be one of the cleanest ways of heating your home providing it is done correctly with fuel from a sustainable source. All wood will release the same amount of carbon dioxide (CO2) into the environment once the tree is dead, that it took in during its lifetime. Burning your logs in an efficient manner will ensure that other pollutants are burned off and all that is released into the environment is this same amount of CO2 meaning that burning seasoned wood efficiently can be considered a carbon neutral form of heating! Of course other factors such as transportation and energy needed to kiln dry will come into the debate.
If you find that you get a build up of tar then you should now be able to understand the possible reasons why and the remedies available to you. Of course to remove the tar you will need the services of a qualified, registered Chimney Sweep as it is important to do it in such a way as to avoid damage to your chimney/flue. Sometimes this can even require expensive chemical treatments so it is now clear why you should do your part to avoid this problem to begin with. So, remember to get your chimney swept regularly and always seek advice if in doubt!
Andy Smith is a Lancashire based Chimney Sweep registered with the National Association of Chimney Sweeps (NACS). Smiths Chimney Sweeping serves most areas of Lancashire and the North West including; Chorley, Bolton, Wigan and Blackburn.