Fire is the production of light and heat by combustion. Combustion is the combination of a substance with oxygen accompanied by light and heat. The fire warms the air, which expands and rises. Smoke consists of the volatile products of combustion, such as unburnt coal, and other visible products, which float on the air. The first requirements are (a) that a fire shall have sufficient air to cause it to burn, (b) that the air carrying the smoke shall be warm enough to rise, (c) that it shall also be kept warm until it emerges from the chimney pot, and (d) that it shall not be obstructed in the flue by downdraughts or debris. If these simple requirements are met there should be no trouble from smoky chimneys.
Chimneys smoke when there is insufficient oxygen to cause combustion, such as:
(a) When the room is unventilated;
(b) When the doors and windows have been sealed against draught;
(c) When two intercommunicating rooms, each having a fireplace, are not sufficiently ventilated, and the fireplace with the stronger draught consumes all the available air and starves the other;
(d) On the windward side of the house the wind exerts pressure and blows into the ventilators on that side. On the lee side there is suction and air is drawn out of the ventilators. It is not correct to assume that a fire is receiving sufficient oxygen even when the windows of the room are open. A typical domestic fire requires as much fresh air as can be provided by 125cm2 of unobstructed opening. If this ventilation comes from under doors, windows or between floorboards, draughts will occur, as the fire will draw air across the room from the window or door. It is better to install ventilators near or under the hearth, as the warmed air expands and rises upwards; it is essential to keep it warm, otherwise it will grow cold and cease to rise; as it cools it will contract, reach dew-point and condense, forming weak sulphurous acid which attacks the parging, expands it and causes it to fall down the flue, sometimes in the form of dry sand. The sulphurous acid also attacks the jointing of the brickwork and, by combining with carbon smoke, causes dirty marks to appear on internal walls (usually over the first-floor fireplaces in small houses).
Rapid cooling of smoke occurs:
(a) When the flue is cold or damp, for example in an uninsulated outside stack.
(b) When an appliance is used that retains the hot gases in order that ovens etc may be quickly heated. The ordinary modern range is designed in this way, and in the very efficient types the outlet for the flue gases is about the size of a 10p coin. Consequently the volume of gas that staggers out into the flue is very small, and the flue remains cold, so that it is sometimes found that existing flues which have worked well with old types of fires become difficult when modern efficient grates are installed. In this case a grating can be inserted into the flue at about ceiling level so that the small amount of hot air from the appliance is augmented by the dilutent hot air from the kitchen, and condensation does not take place so easily.
In one way or another most modern appliances are designed to stop the hot air going up the flue, so there is danger of condensation attacking the parging. It is therefore advisable to line new flues with suitable 8in diameter circular liners, and to provide a drain to the flue pipe for condensation. This is usually done by continuing down with a straight length of flue after the elbow joint to the cooker, and installing a tray to catch the condensation, and a small access door.
Obstructions in the flue
Obstructions in the flue may be due to soot, loose parging or brickwork, birds’ nests or other foreign bodies; but usually they are caused by downdraught.
When the wind is blowing horizontally across the top of the pot, some up-draught is provided to help the hot air in the flue, otherwise the wind gives little help. But wind on the windward side of a building forms an area of pressure and tends to rise in a wave on contact with a steep roof (over 30 deg pitch) and dip to form an area of suction just before the ridge. In the lee of the building all the roof slope will be in an area of suction. Any chimney within the area of pressure is apt to be in a downdraught, so that the tops of all chimney stacks should be above the ridge line and out of the areas of pressure, from whichever direction the wind blows. Trees and other buildings will also affect the downdraught of chimneys.
With roofs of less than 30 deg pitch the wind produces suction and not pressure, the suction becoming stronger towards the ridge.
It should be noticed that where the ventilation is on the lee side of the building, and therefore in the suction area, not only can the fire be deprived of oxygen but the air can be sucked out of the flue through the fireplace opening, causing a downdraught.
Design of flues
Flues should be without bends. The only disadvantage is that rain may come down the chimney, but this can be stopped by means of a “Chinese hat” placed on the top of the pot.
Where bends are necessary, they should be kept to an angle of at least 60 deg, definitely not below 45 deg, and the same width of flue should be maintained for the entire length of the stack.
Flues should be restricted at the throat and at the chimney pot. A throat of about 250 cm2 is quite sufficient (a slot 10cm x 25cm), and the opening should be funnelled into the throat as smoothly as possible. This funnel is usually about 20 cm long, and if a drawing is made on these lines, using the plan of an actual top of a standard clay fire back, it will be found that there is a smoke shelf on the main wall which is horizontal and level with the top of the throat and 20cm above the top of the fireplace opening. The theory was that downdraught hit the smoke shelf and rebounded. Modern theory is that the smoke shelf can be sloped off in line with the throat.
In old buildings with wide, high, open fireplaces the construction of the throat, even by means of an iron plate with a slot in it, gives very worthwhile results. In ordinary sized flues some otherwise untraceable troubles can be cured by restricting the throat of the flue.
The flue should be constricted at the top by a slightly tapering chimney pot. Flues of less than 5m long generally require lengthening by extra brickwork or chimney pots.
Flue linings
In older properties the flue may be brick finished or finished with pargeting. In either case the condition may now be such that flue gases can escape into floor and roof voids or through to internal plaster surfaces causing staining to appear. In other cases owners install wood-burning or multi-fuel stoves or use old flues as outlets for oil or gas-fired boilers, without due attention to the flue. The controlled burning of all modern appliances is such that most older flues are inadequate to draw the gases away quickly to the outside. If not attended to moisture and cold will collect in the top of the flue and accelerate sulphurous discoloration of walls initially at first-floor level. When this occurs not only must the cause be corrected but it will also involve replastering and redecoration. The reduction in draught associated with modern appliances is such that lingering gases can permeate through gaps in mortar joints into bedrooms, creating a health hazard.
There are a number of proprietary flue linings that can be installed relatively easily, but at a cost. Popular among such systems are those which involve the use of an inflatable rubber tube around which is pumped a special insulating aggregate. If the flue is absolutely straight, installation can be undertaken with the minimum of interference to the property, but if there are bends in the flue it is necessary to remove a number of bricks to make certain that the tube remains central in the flue. Obviously it needs to be finished correctly at top and bottom. One advantage of this system is that all the old cracks in the flue are filled and CIKO Chimney Linings Ltd maintain that benefits include the strengthening of the stack, curing of damp patches on chimney breasts and improvement of updraught and reduction in downdraught.
An alternative is the flexible steel tube such as Multiflex which is now suitable for use for solid fuel, wood, oil and gas burning appliances and comes in different diameters.
Most proprietary systems are guaranteed for 10 years subject to the usual provisos about correct installation, usage etc.
Unused flues
Where a flue is to be left unused for a short time it must be left ventilated. If not, rain runs down the flue and will cause seepage and staining to chimney breasts. If a flue is to be left unused for a long time then a suitable flue vent insert should be placed in the chimney pot, to reduce rain penetration, and the flue left ventilated.
The full cost of relining a flue including the making good of decorations can easily run into the upper hundreds or low thousands of pounds. It is not always easy for a surveyor to check the condition of a flue but if the client — enchanted by the glow of the wood-burning stove — is not to be disenchanted by gases, stains and high relining costs then they should at least be advised of the possibility by the competent surveyor.