The Chimney Sweep Company's guide to chimneys, flues and their problems...
Click on a heading to move to the section...
- How a flue works
- Flue Functions
- The smokey fire
- Primary cause of smokiness
- Secondary causes of smokiness
- Damp in flues
- Diagnosing problems
- Further sources of information
f... Primary cause of smokiness
- The Ratio Problem
- The Air Starvation Problem or lack of room ventilation
- Badly sited chimney terminal
The FIREPLACE OPENING is too large for size of FLUE, i.e. the volume and speed of the smoke passing up the flue is insufficient to clear the large volume of fumes building up in the oversize fireplace opening below. Flues above 6m tall should generally be not less than 1/7th to 1/8th of the area of the fireplace opening, e.g. a 225 mm (9') diameter flue will support a fireplace opening up to about 550 x 550mm (22' x 22') (see Building Regulations J 2.2.) For bungalows, the ratio should be reduced to 1/6th.
As it is generally impractical to consider rebuilding the flue to a larger size, the fireplace opening must be reduced. There are a number of ways this may be done, depending on the size of the original fire opening and the purpose for which the fire is intended. For larger fireplaces and inglenooks, the whole flue should be closed off with a horizontal register plate just above beam or lintel height. A freestanding fire, bricked-in convector open fire, closed stove, or canopy can then be fitted with a flue projecting through the register plate. An access trap must be provided for cleaning above the register plate. For smaller fire openings (up to say 900 mm (3') square, a smoke hood, canopy or tempered plate glass strip, can be fitted across the full width of the fire opening, effectively lowering the height of the lintel. It is best to experiment first, using a strip of sheet metal, fireproof board or even hardboard pinned or wedged in position. (Do not leave the fire unattended if hardboard is used.) This can be lowered until the fire no longer smokes before making a permanent job. Try the experiment for a couple of weeks before finally accepting success. A raised plinth can also be built to reduce the opening size. This can be temporarily built from old bricks built loose and jointed with dry sand. (See diagram 1) Installing a convector fire box is also a good solution in this situation.
All fires must have oxygen to burn. Air contains approximately 20% oxygen. Therefore five times as much air is needed than the oxygen required for proper combustion. Additionally, open fires also take considerable quantities of air to vent the smoke up the chimney.
Modern homes tend to have solid floors, tight fitting doors and draught stripped windows. Lack of ventilation to the room is therefore a common cause of smokiness, more particularly in modern or modernized homes.
Trying to burn either solid fuel or gas with insufficient ventilation will have two effects:
- If insufficient oxygen is available to the fire, incomplete combustion will take place. In simple terms, the products of complete combustion are carbon dioxide and water vapor. Lack of oxygen however will produce carbon monoxide, an odourless and highly poisonous gas.
- Secondly, lack of ventilation will mean insufficient air is available to replace that being drawn up the flue and needed to clear the smoke and fumes from the fire. Result, smokiness or spillage of the fumes, including the carbon monoxide, into the room.
THIS IS A VERY DANGEROUS SITUATION.
If the fire works well when the room door is open, but smokes when the door is shut, the problem is air starvation. This is not a fault in the construction of the fireplace or flue, but a lack of room ventilation. Ventilation requirements are laid down in the Building Regulations part J and the Gas Safety Regulations 1984, (and BS 5440 pt 2 for gas fires).
To cure this problem, additional air must be brought into the room, preferably without introducing unacceptable cold draughts. Either vent directly through an outside wall, or vent into the hall (or a conservatory) and then to outside. If the fire works well with the door to the hall open, this has proved that there is sufficient ventilation from the main house. Also, 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 round the outside of the house, then a simple floor grille cut into the floorboards to one side of the hearth or chimney breast is a good solution. Grilles must have a sufficient free open area. Aim for half the cross section of the flue as a minimum. Do not vent air up immediately in front of the fire opening.
The best place for a chimney to terminate is on or near the roof ridge, and well above it. Two distinct problems can occur with a badly sited chimney terminal, although the symptoms can be similar.
Down draught (See diagram 2)
In this situation, wind blowing over another tall building, tree or hill, descends onto the chimney top, causing a puff of smoke or fumes in the room, usually intermittently.
Pressure Zone (See diagram 3)
Here the chimney is sited in the line of the prevailing wind, with a taller object, house, roof, tree or nearby hill behind the chimney terminal. This can cause puffing or continuous fume emission when the wind is blowing.
For downdraught problems, certain cowls can reduce the problem, or, construct a slab top or dovecote top (diagram 4). Pressure zone problems are more difficult. The best solution is to raise the chimney until it is above the pressure zone. This can be done experimentally by fixing a length of single skin metal pipe over the existing pot. If this works, a tall chimney pot can be installed. (Tall pots are made up to 1500 mm (5') high).
Alternatively, if the chimney is much too low, a combination of a tall pot plus raising the stack 600-900 mm (2'-3') may be necessary. Note: Building Regulations gives maximum height for a chimney stack, including pot, as 4 times the narrowest width, measured from the highest point where it leaves the roof. Cowls rarely do much good in pressure zone situations. The MARCONE chimney pot can be useful to increase chimney height and help to counter down draught. (See diagram 4). It may also help to introduce some room ventilation on the same side as the prevailing wind, helping to equalize the pressure at the top and bottom of the chimney. Try opening a small window on the windward side of the house. If this helps, fit a permanent air vent.
If all else fails, an electric chimney fan may be the only solution.