Cool Fuel Rules

There are many fuel types available to the solid fuel user: coal, peat, wood logs, compressed wood products, smokeless coals. With so much negative information in the press these days … which is the right fuel for you to burn?

Firstly , you must look at your appliance and follow the manufacturer guidelines as burning the wrong fuel type can be dangerous and at the very least invalidate your warranty. Good sweeps are generally a hive of knowledge.

If you care about the environment, then you would want to focus more on renewable fuels such as wood logs.

Selecting and purchasing wood logs

Ash and beech are some of the best woods logs to burn however most logs burn perfectly well (see our burning wood post) if they are dry enough and have been adequately seasoned.

Wood should be purchased from a reputable supplier who should have to comply to regulations and have a vested interest in planting more trees to replace what has been felled.

Wood logs must have less than 20% water content when they are burned. This can ascertained several ways, (see our post on moisture meters) but the easiest and probably most reliable at a price is to purchase logs from woodsure who will have done all checks and tested the moisture content in random batches. You will also be able to complain if the wood you purchased is too high in water content.

If you choose to purchase unseasoned logs then you must season them (typically for 18 months or more) and after seasoning you must test the moisture content with an electrical conductivity moisture tester prior to using them. (again for more on this see our post on moisture meters) Users burning wet wood are one of the biggest problems and challenges our industry faces but it is one of the easiest fixes too.

Why mixing fuels is a problem

Wood and coal burn quite differently. Wood burns through a process of gasification where the volatile oils within become heated, turn to a gas, mix with oxygen above the log and then eventually combust as a mixture of gases.

Coal on the other hand is primarily a carbon, to burn efficiently oxygen must enter and mix from below the fuel. Combustion takes place inside and not above the coal itself.

There are also distinct differences between wood and coal burning types of appliances, such as a grate below or just flat metal base.

Some issues associated with the incorrect use of fuels

  1. Wood on top of coal

If you try to burn wood on a bed of coal you would need to open the top air intake, thus drawing air above the coal and not through it from below. Therefore, the coal would burn poorly and produce high levels of carbon monoxide and carbon dioxide, which acts as a fire suppressant/extinguisher and the effect is compounded by the inadequate heat from the coal.
Wood burns by first emitting volatile hydrocarbons which then become gaseous.
Normally these hot gasses mix with oxygen and burn with the yellow flame we associate with wood. However, in the reduced oxygen environment above a coal bed the volatile gas emitted by the wood log will instead travel into the chimney where it then deposits as a flammable creosote.
Another issue is when combustion temperatures and oxygen levels are too low, then the later hotter stages of gaseous combustion will not effectively take place. During these later phases, carbon soot particles should be burned.
If these particles are not burned they will then exit the chimney as fine dust pollution harming our air quality with all the associated risks.

2. Coal on top of wood

So we understand that coal requires oxygen from below to burn effectively, think bellows in a foundry. If we burn wood below a coal bed we will have a situation where our coal is bathed in carbon dioxide leaving an incomplete burn. The second issue here is that wood produces lots of ash which in turn blocks the ash grate, further lowering the amount of available oxygen to the coal.
If the coal used is smokeless, such as anthracite then high levels of sulphurous compounds will also be emitted. These compounds are highly corrosive and will eat through stainless steel liners and most pot coverings in next to no time. The one exception to the wood underneath coal scenario, is when kindling a fire with wood. Here it is very likely that the air intakes or even the door will be open in order to provide enough oxygen for both fuels for a limited time.


The term slumbering describes the process of the user loading the already burning appliance and then closing down the air supply. This is in an attempt to make the fire burn for long periods, typically overnight.

The big problem here is the fact that fuel requires oxygen for combustion. When oxygen is limited in a combustion reaction, there are several results that occur as are shown below.

  • Much of the volatile content within the fuel is emitted due to the reduced heat but it does not combust. It rises up the flue coating the chimney with volatile fuel, greatly increasing the risk of chimney fire and the requirement for sweeping.
  • Carbon requires lots of oxygen and heat for the particles to combust. When the oxygen supply is limited, dangerous sized carbon particles are released. Many of these particles make it to the outside air as fine dust pollutants , polluting the environment and causing health problems such as respiratory and lung diseases.
  • Heat or the lack of. In order for a fuel to release its stored heat, it must react with oxygen, it being the catalyst for a combustion reaction.
  • Cost. Slumbering will shorten the life of stainless steel liners, increase the need for servicing, sweeping and maintenance. It will reduce the heat value that you should get from the fuel, making it less cost effective too.

In conclusion

Ideally you would only burn dry wood logs in an environment that has adequate air/O2 for moderate flaming combustion to take place. This is achieved by adjusting the air supply so that flames are not sucking up the chimney but not limited too much. Smoke should not be visible in the firebox, only in the fire.
Also by adding only as much fuel as is needed to fill the fire box with flames, because burning too much or too little fuel at a given time is another mistake to be avoided.

Your chimney sweep should give tips on fuels and how to achieve adequate flaming combustion, fuel storage, types, sweeping frequencies and many other important factors surrounding good use practices.