At simplest a chimney is just a vertical tube enclosing a volume of gas. However for the tube to function as a chimney and generate a safe reliable draught six things need to be addressed.
(The business of connecting to an existing chimney, installing a flexible liner, or erecting a prefabricated sectional chimney is discussed in the Installation section.)
The flue gases fed to a chimney should be hot enough to promote an adequate draught and must be kept as hot as possible until they exit at the top. This is best achieved by constructing the chimney in lightweight insulated materials that heat up rapidly. The chimney should also be contained within the warmth and shelter of the building for as much of its length as possible. Twin wall sectional chimneys use just 25mm of insulation so it is particularly important to minimise their exposure to the elements. Clearly the situation shown at right is undesirable and should be avoided.
Any masonry chimney is by definition ‘stone cold’ and will quickly chill hot flue gases fed to it. The best way to counter this is to fit a low-mass flexible liner which heats up rapidly and helps to keep the flue gases hot throughout its length.
A tall chimney applies more air-pressure to its base than a short one. All things being equal the chimney in the tall building at right will generate a stronger, more stable draught than the short chimney in the lower building.
A short chimney of the type often installed into chalets, mobile homes and top floor sitting rooms may struggle to generate adequate draught particularly when flue gas temperatures are low, as is the case when a stove is run at reduced output.
Note that whenever height is added to a chimney the likely impact on temperature must always be taken into account. In the building at right, adding height has caused the chimney to stick up like a flagpole in a way that will cause its upper section to run cold. In such cases the benefits of extra height may well be cancelled out by excessive heat-loss from the exposed section. In addition, any chimney that runs cold at its upper extremity is likely to foul up rapidly owing to the large volumes of condensate produced.
A chimney that is excessively large in diameter will encourage the flue gases to ‘loiter’ and cool down whereas the ideal is for them to move up smartly and exit the chimney while they are still in a hot, gaseous state. Thus an oversized chimney will likely suffer from a sluggish draught and condensation problems and may tend to foul up rapidly.
A chimney that is too small in diameter may generate a perfectly adequate draught under normal running conditions, but as soon as the door of a stove is opened it will get over-loaded by the greatly increased volume of air that is drawn in and will tend to spill smoke to the room.
In an effort to avoid these problems appliance manufacturer specify the diameter of chimney that should be used for each of their various products. Needless to say their stipulations should always be strictly observed.
When no specification is available a useful rule of thumb is to use the 1:8 ratio. In the diagram at right, the chimney diameter (shown as the blue circle A) needs to be approximately one-eighth the cross-section of the stove door-frame B outlined in blue.
Note that the 1:8 rule of thumb is just that. The ratio can and does vary significantly from one installation to another according to the thermal properties of the chimney, its height and its normal running temperature.
All chimneys are influenced by weather conditions. The diagram below illustrates what happens when wind encounters a building. A “wind envelope” develops in which the air pressure builds on the windward side and reduces on the leeward side. This creates a great deal of confused air-movement in the midst of which the wind velocity across the top of the roof may be greatly increased.
A chimney terminating above the ridge in an area of ‘clear air’ will be less afflicted by these conditions than one further down the slope which may suffer from quite violent surging and blow-back problems. The angle or pitch of a roof plays a part in all this – steep roofs with dormer windows or other projections create more turbulence than shallow-pitched roofs with clean, uncluttered profiles.
Other factors that disrupt chimney draught include adjacent buildings, trees and hills the influences of which vary greatly according to wind speed and direction.
The ideal chimney is dead straight and has smooth, even, internal surfaces at a constant diameter. Bends and cowls create friction that can cause back-pressures to develop that may impact on combustion efficiencies and even cause flue gases to ‘spill’ to the room. Where bends are unavoidable they should be kept as gradual as possible.
The chimney at left uses a pair of 15° bends spaced some distance apart which is preferable to the more pronounced ‘kink’ in the chimney at right.
Where it is necessary to offset a chimney the amount of offset should not exceed 20% of its height, so a chimney 5 metres high should not be offset by more than one metre.
All chimneys need to be accessed from time to time so that they can be inspected and cleaned.
Stove manufactures routinely specify the amount of draught a chimney should generate under normal running conditions. A manometer is the instrument used to measure draught and at simplest consists of a ‘U’ tube containing a small quantity of coloured liquid. Connecting one leg of the tube directly to the chimney causes the liquid to rise on that side while the liquid on the opposite side falls by an equal amount. The difference is read as “millimetres in water” against a graduated scale.
Modern digital manometers do the same thing with greater precision and deliver a reading in pascals. A value of 1 – 2 mm in water or 10 – 20 pascals is typical for most stoves.
Inadequate draught impacts on combustion efficiencies resulting in excessive fuel consumption and a tendency for the chimney to foul up rapidly. Excessive draught may cause a stove to over-fire unless it is truly air-tight and fitted with precise air-valves.
Sound chimney design aims at keeping flue gases safely above their “dew point” – i.e., the temperature at which water vapour condenses to liquid. Dew-point varies according to humidity, pressure and sulphur content. Most text-books quote a temperature of around 50°C. for domestic installations but even minute traces of gaseous sulphur trioxide (SO3) can push it as high as 130.5°C.
Unfortunately stove flue-gases never condense to pure water – the water invariably contains contaminants. When wood is burned the contaminants include tar and creosote – often in sufficient quantities to thicken the water to a treacly consistency that adheres stubbornly to the flue-path and may build up to create a serious fire hazard. When coal products are burned they release sulphur and sometimes chlorine gases both of which convert readily to aggressive acids (sulphuric acid and hydrochloric acid). Either can cause structural damage to an unprotected masonry chimney and may destroy or shorten the life of a stainless steel liner.
Note – imported coal products appear to be completely unregulated and may include batches with a high sulphur/chlorine content. Burning these fuels will invalidate warranties both for a stove and for the flue components serving it. The only coal products that are safe to burn are the ‘HETAS Approved’ grades listed in the stove handbook.
No chimney, however well designed, can cope with a mismanaged stove. A common mistake is to smoulder fuel instead of burning it at the temperatures needed for clean combustion. Smouldering produces a pall of dense smoke that will rapidly block up the flues with soot. If the fuel being smouldered is damp wood, the problem will be many times worse since the creosote and tar that is produced can create a permanent blockage. In such cases the only remedy is to remove and replace the flexible liner – a procedure that is invariably messy, disruptive and expensive.
Clearly long, overnight burns are to be avoided, and if it is necessary to turn the fire down low for short periods – for instance while out shopping for a couple of hours – it is good practice to burn the fire up brightly for a few minutes upon returning. This will help dry up and drive out any moist residues that may have accumulated in the flues.
Over-driving a stove is less common since it results in soaring fuel consumption which most households want to avoid. Nevertheless if a stove is under-sized there will be a temptation to run it hard on cold evenings – a practice that can subject the base of the chimney to ‘blow-torch’ temperatures high enough to cause serious damage.
If wood is burned regularly it will often create some degree of ‘stickiness’ in the flues which the sweep’s brush may simply skate over. The best way to tackle this problem is to pre-treat the chimney by burning a quantity of the driest fuel available. Anthracite or smokeless fuel will deliver best results and during their use every effort should be made to ventilate the chimney as much as possible so that the volume of air that passes up is hot enough and dry enough to drive out any moisture that may be present. Under ideal conditions, tar and creosote deposits will shrink as they dry and will often detach from the flues and ‘tinkle’ down to the bottom of the chimney quite audibly. By the time the sweep arrives he should have no difficulty scouring out the more stubborn deposits and getting the chimney thoroughly clean.
It should be noted that chemical flue-cleaners contain aggressive substances that attack and damage any stainless steel components within a chimney – including flexible liners. Therefore they must not be used, though they may be a useful way to pre-treat a masonry chimney before a flexible liner is fitted.
When all of the above factors are taken into account it is obvious that the design of a new chimney demands care and (ideally) the eye of a ‘practiced hand’. It is equally obvious that an existing chimney should always be carefully appraised before being committed to use with a new stove.
Finally, chimney maintenance is not simply a matter of calling in the sweep once in a while. On the contrary it is a continuous process requiring the correct fuels to be burned in the correct manner so that combustion remains ‘clean’ and any oversights are dealt with swiftly and effectively. Modern clean-burn stoves make this easy since a blackened glass is an early sign that all is not well. What has been deposited on the glass is also being deposited in the flues and therefore requires corrective action. Is an approved grade of fuel being burned? Is it as dry as it should be? Is it being burned at a high enough temperature? The question are simple enough – as are the answers. All that is needed is to apply them!