The majority of thatch fires are chimney related, and most often associated with the installation and use of modern wood burning stoves. The potential for a fire is entirely dependent on understanding and managing the risks during the design and installation process and the subsequent selection of the correct fuel.
Choice of wood as a fuel:
Unseasoned logs cost you money and increase the risk of flammable tar build up in a flue.
33 logs at 60% moisture content are required to produce an equivalent heat output of 10 logs with a moisture content of 25%.
Never burn recovered chemically treated building off-cuts.
The commonly available and most suitable hard wood types are Ash, Beech, Birch, Blackthorn, Elm, Hawthorn, Hazel, Hornbeam, Maple, Oak, Rowan, Sycamore, Wild Cherry, Willow, Alder, Apple, Pear, Holly.
Wood is divided into two main categories, hard and soft. A rule of thumb is that around twice as many softwood logs as hardwood logs may be required to achieve the same heat output. Softwoods, (conifers) include spruce and pine, burn much faster than hardwoods and have a tendency to spit and crackle Freshly harvested wood contains a naturally high amount of water, between 65-90% depending on the species. Removing the water is known as seasoning. This term suggests a period of time, and for natural air drying up to two years is recommended. The more efficient a fire burns, the less fuel is required and less tar is deposited.
It is important that logs should be dried and stored under cover for a minimum of a year to get the moisture content down;
this releases more heat energy per log into the room and reduces the amount of creosote and tar deposits in the chimney.
The correct fuel uses only well-seasoned wood with a moisture content <25%.
Black smoke is a sure sign of burning the wrong fuel. The pub on the left has experienced 3 chimney fires in the past 12 years.
A correctly set stove, burning kiln dried wood will burn cleanly. Note the temperature gauge on the flue, which manage optimum operating temperatures
Problems caused by using the wrong fuel:
- Wet logs cause the chimney to cool, condensation occurs and a residue is formed. This residue is brown or black and can be flaky, sticky, runny, tar-like or hardened and will sometimes be all of these in the same flue. This tar is flammable.
- The chimney may become completely blocked or the volatile residue can ignite causing a dangerous chimney fire.
- Corrosion, excessive condensation from wet wood which normally forms in the upper part of the chimney is acidic in nature and will corrode the inner surface of a metal liner, eventually leading to perforation and failure of the liner.
The Classic ‘at risk’ thatch fire scenario:
The safe use of a wood burning stove in a thatched property is totally dependent on the responsibility of the user in understanding the risks associated with a modern wood burning stove installed in a traditional single brick chimney. New buildings with thatch are safer as the known problems have been designed out.