This section discusses the way chimneys function and should be of particular help if you are trouble-shooting an existing problem or if you are designing a new chimney and want to avoid some of the pitfalls.
The business of connecting to an existing chimney, installing a flexible liner, or erecting a prefabricated sectional chimney is discussed in the Installation section.
1. Chimneys at their simplest
At simplest, a chimney is just a vertical tube containing warm gases (A.) All things being equal, increasing the volume of warm gas by enlarging the diameter (B) or making the chimney taller (C) should cause it to develop a greater upward thrust.
Much in the same way that a big hot air ballon will lift a heavier payload than a small one.
The trouble is, all things seldom are equal with chimneys – and therein lies their complexity.
2. Chimney diameter (a)
Every chimney has an optimum diameter for the appliance it serves. If the diameter is too large, the flue gas temperature will drop owing to heat lost through the walls (A.) The symptom is a sluggish draught, weak resistance to ‘blowback’ in turbulent wind conditions – and a tendency for the flue to foul up and need almost constant maintenance.
Confusingly, a chimney that’s below optimum diameter (B.) can exhibit a similar tendency – but it will also act as if it’s being permanently constricted. The tell-tale sign is a regular tendency for smoke to dribble – or worse still belch into the room, coupled with an almost permanent smell of soot.
In C. the chimney diameter is correct; heat is reasonably evenly distributed (except at the extreme top where the weather is encountered,) and draught is stable enough to extract all smoke and fumes to the atmosphere.
3. Chimney diameter (b) - The rule of 1:8
How do we determine correct chimney diameter in any given situation?
A rule of thumb that has been used for years is to use the 1:8 formula. A square 10″ x 10″ chimney has a cross-section of 100 square inches and will serve an aperture 800 square inches at the bottom.
For those whose school days are in the dim & distant past you calculate the cross-section of a round chimney using the formula 3.14 x r². In the example at right, the radius of an 11″ chimney is 5.5″. So 3.14 x (5.5″ x 5.5″) = 95″² or near enough 100″²
The 1:8 rule of thumb is not foolproof but it’s often used as a basis for designing new chimneys – and it’s a useful starting point for diagnosis when things go wrong.
A chimney that is very short, or which is runs cold owing to extreme exposure, may need to be closer to 1:7
A warm dry chimney with plenty of height may allow you to get away with 1:9
One trap to avoid is to think your chimney should be the same diameter as the flue collar on your stove. This is not always the case. Always consult your appliance manufacturer’s recommendation first (if it is available) before falling back on the 1:8 formula.
Table showing the size of apertures that can be served by different sizes of square and round chimney.
|ROUND CHIMNEY (Diameter)||SQUARE CHIMNEY||SIZE OF OPENING SERVED|
|inches||mm||inches||cm||sq inches||sq cm|
4. The 1:8 rule applied
In the diagram at right, A. is an open fire prone to smoke problems and ‘blow-back’ because at a ratio of 1:10 the aperture is too large for the chimney.
In B. a convector fire of the Jetmaster or Rutland type has been installed. bringing the ratio down to 1:8 The smoke problem has disappeared and the fire now lights easily, draws well and suffers little or no blow-down.
In C. a small stove has been installed. With the door of the stove wide open during refuelling the ratio is only 1:6 with the result that air rushes into the stove and up the chimney with considerable force.
In all three examples, the same chimney is being used but its performance alters dramatically according to the size of opening it serves.
5. Open fire chimneys & closed stove chimneys.
The virtuous thing about open fires (A.) is that they draw a large volume of air upwards into the chimney. This helps to distribute the heat evenly as well as drying up any damp patches.
A chimney serving a closed stove (B.) behaves in a completely different way. Only a tiny volume of air is drawn in – not enough to distribute the heat upwards, and not enough to dilute the tars, creostes and water vapour contained in the smoke.
So the stove chimney often runs at a very high temperature indeed just above the stove, but a little further up it may cool rapidly to the point where the ‘nasties’ have all the time in the world to condense out.
The result can be both a fire hazard and actual degradation of surrounding masonry caused by the acidic, black goo that may penetrate outwards wherever the mortar is porous. Even a brand-new tiled liner may suffer if there are flaws in the joints, or pin-holes – or, as has happened on more than one occasion, the tiled liners have been installed upside down!
|Chimney in cross-section serving an open fire||Chimney in cross-section serving a closed stove|
|Soot particles travelling up the chimney from an open fire. Note that they are spaced well apart because they have been diluted by the large volume of air sucked into the chimney. The hot flue gases remain hot to the top of the chimney and also heat up the chimney itself.||Soot particles travelling up a chimney from a closed stove. Because the smoke is not diluted much, the particles are packed together at high density. Also the low volume of slow-moving flue gases get chilled by the heavy mass of masonry in the chimney, causing creosotes to precipitate on the way up and create a major fire hazard.Note the cracks in the liner caused by chimney fire allow discoloured condensate to penetrate masonry and plasterwork.|
6. Lining a chimney
If you are planning to use an existing masonry chimney, the best insurance against problems when you burn wood or peat in a closed stove is to install a flexible stainless steel liner. The diameter of the liner is critical – if in doubt, consult the stove manufacturer or your stove supplier. It is also essential that you use a heavy-duty ‘multifuel’ liner and install it in strict accordance with the manufacturer’s instructions. Typically the liner will have a corrugated outer skin to resist abrasion when it is being installed and a smooth inner skin to make cleaning as easy as possible.
You get best results if you backfill between the liner and the old chimney with loose, dry insulating material – vermiculite (trade name ‘Micafil’) is particularly well suited to this job.
With a liner, heat is displaced right up the chimney and the surface temperature rises rapidly as soon as the stove is lit, helping to keep condensation to a minimum. At the same time the chimney is protected throughout its entire height against moisture penetration. Properly installed and maintained, a liner should virtually eliminate chimney-fires and will generally improve the draught applied to the stove so that its performance stays lively, responsive and efficient.
To clarify; a liner is indicated whenever wood/peat are burned in a closed appliance. There is seldom any need to line a chimney when a closed stove is being run exclusively on solid fuel – or when wood/peat are being burned in an open fire.
Cross section of a chimney equipped with a low-mass liner when serving a closed stove.
Here the soot particles remain tightly packed as before but now the flue gases stay hot to the top of the chimney because they are contained within a low-mass flexible liner back-filler with insulating material.
The higher temperatures eliminate or drastically reduce precipitation of flammable tars and creosotes.
For information on the installation and maintenance of chimney liners visit the Installation section.
7. Beware the long, slow burning cycle.
However good your chimney might be, you can do a great deal of damage to it simply by running in incorrectly.
A common fault is the ‘slow burn’ syndrome. Loading a stove with wood or peat and turning it down to its lowest setting for hours at a stretch (particularly overnight) is never good practice. Why? Because in the first place you need a minimum temperature in the firebox to burn the fuel cleanly. Go below that temperature and you get a continuous pall of dense, humid smoke loaded with unburned tars and creosotes.
Secondly, since the chimney is no longer being heated adequately by the stove, it cools down. Draught then becomes sluggish and at the same time high levels of precipitation develop.
An identical problem tends to arise when a stove is too big for the job it’s doing. A natural response is to under-run it to keep the room from over-heating – but again with the risk of storing up serious chimney problems.
The solution to all this is to aim for an installation that needs to be run brightly for reasonable periods of time. And if you fall into the trap of under-running your stove, at least get into the habit of burning it up brightly at regular intervals. This will help to dry up moist patches and blow out the ‘cobwebs.’
Click here to see what an unlined chimney can look like in cross-section when when it is serving a closed stove.
8. Chimney constructional materials – the case for high insulation & low mass.
Temperature plays such a major part in a chimney’s performance that it’s worth giving special consideration to the thermal properties of the materials used in its construction.
Old-fashioned brick chimneys date back to an era when architects & builders were more or less forced to work with the raw materials already on-site. And a well-built masonry chimney had a lot going for it when open fires were the norm, because as we have seen, open fires are ‘kind’ to chimneys. By causing a large volume of warm, dry air to to percolate upwards in a continuous stream, they keep everything in good order.
By contrast, some modern, high efficiency stoves leak so little heat to the chimney that it runs almost permanently cold – with consequences that can be dire. Draught performance plummets while precipitation levels soar. It doesn’t do to try and make a stove installation too efficient. As well as the heat needed to create adequate draught we need a minimum surface temperature of at least 50°C. throughout an installation just to stop it from precipitating undesirable quantities of flammable goo.
So modern woodburners are best served by well-insulated low-mass flues. This was demonstrated earlier when we noted all the good things that happen after a bricks & mortar chimney is fitted with a low-mass liner properly back-filled with insulating material.
However, anyone about to build a new chimney for a wood stove would be well advised to look at the very real advantages to be had by using a modern stainless-steel sectional chimney. These have all the things we look for ‘designed in’ including low-mass high insulation properties and ease of maintenance.
9. Keep the chimney warm by keeping it inside.
The other thing that has a big impact on temperature is of course exposure to the elements. You may have noticed that in all the illustrations in this series, the top few centimetres of chimney have always been given a bluish tinge. This is to illustrate the fact that we can do nothing to stop the head of a chimney being cooled by the weather.
We can however do something about keeping the rest of the chimney as warm as possible. The best way is to locate it within the main body of the house (A). Not only will this raise the operating temperature several vital degrees – it will also ensure that heat lost from the chimney percolates into the building where it will do some good instead of being lost to the cold night air (B)
If a chimney is badly located the end result will always be the same; degradation in performance and the prospect of long-term maintenance head-aches.
Sometimes when a chimney is suspected of running cold it is worth examining the amount of fouling up that occurs at the extreme top. A warm chimney that’s running well will tend to precipitate a crust of soot and tar in the top half metre only – where the flue gasses are chilled suddenly. A cold running chimney may appear to be relatively clean at the extreme top, for the simple reason that most of the really nasty stuff has been precipitated further down.
10. An exposed, chilled chimney can reverse its draught.
The residual heat in a house will continue to produce a weak, convective airflow even when the heat is low or has been turned off. As warm air rises in the building and seeps out through the upper levels, cold air is drawn into the building at ankle level to replace it.
Under these conditions, the air in an exposed chimney may be chilled to such a low level that it sinks down and dribbles in a ‘backflow’ out through the stove and into the house, carrying with it the unpleasant smells of tars and creosotes.
Attempts to light the fire when the chimney is in this condition can be frustrating. Smoke and flame will continue to dribble out into the room until there is sufficient heat in the vertical column to generate a strong, positive updraught. In the mean time, you can spend much time on your knees blowing into the embers while your eyes smart.
All of which is a further strong case for designing every chimney in such a way that it is kept well within the main body of the house and remains warm and dry at all times!
11. The Importance of height.
Height plays an important part in the performance of a chimney. A minimum volume of warm, buoyant flue-gas needs to be present inside any chimney to generate adequate draught. In some case things are so finely balanced that increasing height by a metre or less can make a big difference to operating comfort. This won’t necessarily work though if the extra metre you add runs cold owing to extreme exposure to the weather.
Chimney height is particularly critical when a large, central-heating cooker with ovens is being installed (A.) For much of the time these appliances are in ‘tick-over’ mode and lose only a tiny dribble of heat to the chimney. Yet they demand a vigorous draught to pull on the fire so that it burns efficiently and so that the heat is pulled around the lengthy channels serving the ovens and hobs. To get the optimum performance from this type of appliance, a warm-running chimney of adequate height and diameter is essential. As an example, the Rayburn Nouvelle central-heating cooker demands a minimum chimney diameter of 7″ (175mm) and a minimum height of 18′ (5.5m) If there are any bends in the chimney, the internal diameter must be increased to 8″ (200mm.)
When a woodstove is installed into a boat (B) or caravan, there is no option but to keep the chimney short. The safest set-up in this situation is to choose a stove with a short fluepath and a chimney that is fully insulated throughout. Generally this implies a twin-wall stainless-steel sectional flue. The result is a higher flue gas temperature than in A. This helps to compensate for the smaller volume of gases present and in most cases the draught generate will be sufficient for the more modest needs of the smaller stove.
12. Don't bend the rules when you bend the chimney.
There are compelling reasons for keeping a chimney as straight as possible. Even slight bends introduce friction and in marginal situations this may be enough to upset performance. Where bends are unavoidable, the standard way to relieve unwanted friction is to size up the entire chimney diameter by an inch or two. But as we have seen earlier, increased chimney diameter means increased conductive heat loss through the chimney wall. And the whole emphasis in chimney design is to minimise heat loss so that the flue gases remain hot right to the point where they exit the stack.
The second reason for keeping a chimney straight is maintenance. Every bend can trap and hold soot, and if there is a major fall of soot followed by blockage, you can be quite certain the blockage will occur on a bend. It’s also a lot easier to maintain a chimney if you can see all the way down with a good torch. That way, stubborn patches of creosote and tar can be readily identified as can any damage to the chimney lining.
The third reason to keep a chimney straight is cost. Whether you offset in bricks and mortar or in prefabricated stainless-steel sections, every bend costs money. Cutting a hole in the roof and dropping a dead straight prefabricated chimney onto the top of a stove can cost half as much as building up from an insulated “Tee” on the back of the appliance and introducing sundry offsets higher up in an attempt to avoid minor obstacles.
There used to be a theory that putting a bend in the chimney directly above the hearth shielded the fire from rain and somehow reduced ‘blow back.’ It turned out to be only a theory and has no place in modern design!
Unavoidable offsets should always be kept as gradual as possible. 45º degree bends are strictly taboo except when a short length of flue pipe is being used to connect a stove into the base of the chimney. Higher up, 30º degree bends are acceptable but should be avoided if possible. By far the best way to offset is to use a pair of 15º bends placed a reasonable distance apart.
13. High up the roof is best.
The exit point for the head of a chimney is another critical factor in its performance. Every effort should be made to locate the terminal high up on the roof in an area where the wind blows across it horizontally.
The ridge of a roof creates turbulence and down-draughts as do dormer windows and other projections. The safest bet is to bring the chimney head out within the top third of the slope.
Locating the chimney far down the slope towards the eaves invites surging and blow-back which will vary according to wind speed and direction. To compensate for this it is often necessary to increase the height of the chimney, but due to increased exposure it will then run colder with all the attendant problems noted earlier.
In addition, a tall, exposed chimney sprouting from down around the eaves is an eyesore, and access for cleaning purposes can become a real problem.
A shallow-pitch roof creates less turbulence that a steep-pitch roof, making location of the chimney head less critical.