As a fire burns in a fireplace, the air from the room goes through the grate and fuels the fire. As the fire burns, it releases numerous gases which rise, as they are lighter than the surrounding air. These gases then escape up the chimney and are released into the atmosphere. As these substances pass through the chimney, a substance commonly referred to as soot begins to accumulate on the walls. This includes a flammable substance called creosote. Over time, these deposits can begin to obstruct - and eventually completely block – the chimney.
Such an obstruction can lead to an inefficient, and potentially unsafe, fireplace. This can be explained by the fact that a well-functioning chimney is necessary not only to allow potentially harmful gases to escape from the room, but also to ensure that the fire burns efficiently. As hot air ascends in the chimney, the fire sucks in more air from the room through the grate, in order to fill the space that has been left. Without this continuous supply of air, the fire would burn itself out. Deposits of flammable substances such as creosote also have the potential to cause chimney fires. Thus, a clean chimney is also a safer, more efficient chimney.
How a chimney and flue works
First it will help if we understand some of the terms used in connection with chimneys. These are basically as follows:
- Flue - The flue is the passageway in a chimney through which all the combustion gases (smoke/fumes) are removed from the fire to the outside of the building. The minimum height for a flue serving solid fuel appliances is 4.5 metres or 15 feet, to create enough up draught.
- Flue Pipe - A flue pipe is a metal pipe, used to connect a fire or appliance to a chimney flue.
- Flue Liner - The flue liner is the material used to “line” the flue within the chimney. Flue liners can be of fire clay, refractory quality concrete or metal (usually high grade stainless steel).
- Chimney - A chimney is the structure surrounding one or more individual flues.
- Mid-Feathers – Mid-feathers are the dividing structure separating the flues in a chimneystack
- Chimney stack – A chimneystack is the structure in which a number of flues are contained (part of it is visible on top of the building surmounted by chimney pots
The Role of the Flue.
For most solid fuel fires (wood, coal, and even smokeless fuels) the flue has two tasks to perform:
- 1st To carry the products of the burning fuel from the fireplace, and to release them through the chimney pot.
- 2nd To generate a flow of air through or over the burning fire to provide sufficient oxygen for the efficient combustion of the fuel.
How the flue works.
The up draught in a flue is due to a combination of the height of the flue and the difference in temperature between the flue and its gases and the outside air. The warmer and taller the flue the more efficient it will be A flue is essentially, a column of hot air or gases, which being lighter than a corresponding column of cold air outside of a building wishes to rise vertically. To illustrate, picture a bonfire on a still day. The smoke naturally forms into a vertical rising column above the fire, thus it forms an invisible flue. But as it cools to the temperature of the surrounding air it begins to dissolve. This illustration of a bonfire and its column of smoke provide two important lessons for the best operation of any chimney:
1. Smoke wants to rise vertically; therefore any bends or sloping sections in a flue are going to slow down the flow of smoke from the fireplace or appliance.
LESSON ONE –A flue is best if it goes straight up. If any bend is needed, it should be as near shallow in angle as possible. 30 degrees from the vertical is the ideal. Bends of 45 degrees are in the main too steep, although they are allowed.
2. The smoke will rise while it is hotter than the surrounding air. The greater the temperature difference, the faster the smoke will rise.
LESSON TWO - Flues should ideally be lined with an insulating material to keep the gases hot this will increase the pull of the flue and the removal of smoke from the fireplace or appliance.