How a chimney and flue works
Terminology used in connection with chimneys.
- 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 chimney stack
- Chimney stack – A chimney stack 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:
- To carry the products of the burning fuel from the fireplace, and to release them through the chimney pot.
- 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. 2. The smoke will rise while it is hotter than the surrounding air. The greater the temperature difference, the faster the smoke will rise. 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. Chimney Information
1)The problem of air starvation or the lack of room ventilation is often over looked.
The problem of air starvation or the lack of room ventilation is often over looked. All fires must have oxygen to burn. Since air contains about 20% oxygen. Five times as much air is needed to give the needed quantity of oxygen for proper combustion. Also, open fires require substantial quantities of air to clear the smoke up the chimney.
In modern homes solid floors are the norm, as are draught free doors and windows. It follows then that the lack of ventilation into the room is consequently a very common cause of the smoking fireplace, in both the modern house, and those that have been modernized.
Attempting to burn solid fuel with inadequate ventilation will have two results:
- If there is inadequate oxygen available for the fire, it will result in incomplete combustion. When complete combustion occurs, the products produced are Carbon Dioxide and water vapour. However a lack of oxygen will produce Carbon Monoxide, which is an odourless and highly poisonous gas.
- A lack of ventilation will mean there is not enough air available to replace that trying to be drawn up the flue in order to clear the smoke and fumes from the fire. Resulting in smoke and fumes spilling into the room, including the highly poisonous Carbon Monoxide.
THIS IS A VERY DANGEROUS STATE OF AFFAIRS
2)How to increase air ventilation
How to increase air ventilation If the fire works properly when the door to the room is open, but it smokes when the door is closed, then the problem is air starvation. This is not a defect in the construction of the fireplace or flue, but a lack of ventilation to the room. To cure this problem, additional air must be allowed into the room, you might vent directly through an outside wall, but this can introduce cold draughts. Better, is to vent into the hall using a “Draught Master” above the door, and then to outside. If the fire works well with the door to the hall open, then there is sufficient ventilation from the main house. A vent from the hall to outside is usually more acceptable than a vent from the living room direct to outside. If the room has a suspended wood floor with air bricks below to the outside of the house, then one or more grilles cut into the floorboards to one or both side’s of the hearth can be a good solution. The grilles must have enough free open area. Try to get half the cross section of the flue as a minimum.
3)Badly positioned chimey terminals
Badly positioned chimey terminals The best place for a chimney to terminate is well
above the ridge of the roof. Even so, tall objects
nearby can still affect it.
Two different problems can occur with a badly
placed chimney terminal, although the symptoms
can be alike.
In this position, the wind is blowing over a tall tree,
or it could be a hill, or say, a block of flats, it descends
onto the chimney top, causing smoke or fumes to puff
into the room from time to time. For downdraught problems,
certain types of pot can help reduce the problem or the
construction of a slab top or dovecote top can help.
b. Pressure Area
In this situation the chimney is sited in the line of the prevailing wind, with the house roof, or it could be a tall building, behind the chimney terminal. This can cause puffing or even a continuous smoke spillage when the wind is blowing.
Pressure area problems can be more difficult to solve. The simplest way would be to raise the chimney until it is above the pressure zone. This can be done with a tall chimney pot. (Chimney pots are made up to (5') tall). On the other hand, if the chimney is much too low, you could raise the stack by 2 to 3 Feet and add a tall pot.
You should note that Building Regulations Govern the maximum height for a chimneystack, including pot, the narrowest width, measured from the highest point where it leaves the roof, would limit the maximum height of the stack.
You could try opening a small window on the windward side of the house, if this helps then fit a permanent air vent. Doing this helps to equalize the pressure at the top and bottom of the chimney.
Different Fuels Available
- Smokeless Fuels - With familiar names such as Coalite, Homefire, and Cosycoke, These smokeless fuels are easy to use and, contrary to popular belief, are also economic to use, producing up to 1/3rd more heat than ordinary bituminous house coal, so it is not really any more expensive to use than coal.
- Bituminous Coal - Coal has been around for generations and is a very familiar fuel. As we don't live in a smoke controlled area in Cornwall it can be used on all open fires, producing that warm yellow flame so loved by all.
- Wood - For many people there is no better fire than a wood fire; however it is essential that the wood used is well seasoned and dried properly. Emissions and residues from burning wood that is "green" e.g. new cut logs, cause deposits such as tar and creosote to build up in the flue and cause no end of problems, and the possibility of chimney fires and structural damage.
- Seasoned wood is that which has been stored in the open yet under cover for several years, and then it should be brought inside to dry further. The object of all this is to reduce the moisture content of the wood to around 25%. Green wood will contain as much as 60 to 65% moisture, and all that moisture has to go up your chimney as vapour, where it will condense on to the sides of the flue and form tar/creosote deposits. It will also give you a very dead fire as all water is boiled out of the wood before it can give you any heat. If you must burn wood, and particularly in a wood stove, then you must also have a flue that is insulated so that the vapour and other gases from the fire stay above dew point until it vacates the chimney pot. If you are not able to do any of these things, then my advice it to burn smokeless fuel or house coal, you will save yourself a lot of aggravation and expense in the long run.
- Manufactured Fire logs - Relatively new on the market these are made from a compound of wood and wax. They are said to be odourless, clean to use, environmentally friendly, and easy to store. They claim to burn for two hours. As to the cost of using them I have no information.
A Smoking Fireplace
