Installation of Flues

All the so-called fossil fuels, plus other substances such as wood and wood products, which contain carbon, burn to give off gases, the products of combustion. Our ancestors, and we are led to believe the North American Indian, mastered this elementary fact when they left a hole in the top of the dwelling so that the smoke could escape.

Nowadays we have flues of varying degrees of complexity, which may be broadly classified as conventional and other. The conventional flue is in most cases the traditional chimney, an integral part of the house structure, a tube conveying waste gases from the fireplace to the open air above. It works because the column of warmed gases inside is lighter than an equivalent column of colder air outside, and the lighter column rises. But this is a delicate balance, and easily upset. A down draught caused by some vagary of the wind will make the chimney ‘smoke’.

Some chimneys smoke continually, or always when the wind is in a certain direction. The first may be due to a structural fault or to a permanent adverse condition. The second can sometimes be corrected by neutralising the wind effect. A high building, tree or hill will sometimes cause a wind to lift then to plunge, unfortunately, on to the chimney. There are various types of cowl which are claimed to remedy this or other fault, and would-be purchasers would apply a worthwhile safeguard if they were to insist upon a cure-or-money-back guarantee in writing before buying.

It should be evident that a good chimney will not have a lot of bends. It will, if of any age, be far bigger than is necessary, and the worst offender in this respect is the much praised ingle-nook type of chimney, with ample room in it to accommodate the chimney sweep. Such a chimney is disastrous in modern conditions and almost always smokes. To examine the reason it does so will outline some principles which can be more widely applied.

The ordinary chimney, the nominal 230 mm square one, can evacuate up to ¼ ton of air per hour. Imagine the appetite of the big one! But what is just as bad, it starts at a high level, perhaps 2 m from the floor. That was all right when wood was burned as huge logs. But nowadays it is more often expected to cope with a small basket grate or something on that scale. In the first case the fire is large, its waste gases voluminous and over a large area. They monopolise the entry to the chimney base. In the second case the chimney, as always, taking what is most ready to hand, satisfies its appetite with air from the neighbourhood of the entry. Being satisfied, it turns away the legitimate exhaust, the smoke from the fire. Hence, the chimney smokes.

Part of the solution is to bring the entry nearer to the fire. The only way in which this can be done with such a chimney is by means of a canopy, an artificial extension. But a canopy or extension of the same cross sectional area might fail because it is too large, pulling in diluting air from the sides and so failing in the middle. So there has to be a reduction in area as well. The easy modern way to achieve this is by having a flue liner. This is in effect a new flue which uses the run of the old one for convenience. The old one must be rendered ineffective by sealing off the annulus between old and new. A flue liner can have several beneficial effects. (1) It concentrates the flue upon the job in hand, leaving no margin for adventitious air evacuation. (2) It bypasses any troubles due to deteriorating joints in chimney brickwork. (3) It evacuates flue products at higher velocity, giving more positive results, less condensation in the flue, better carry-off of flue products from the terminal. A higher exit velocity will often overcome a tendency to down draught.

These comments upon the use of canopies etc. were designed to show the difference between a suitable modern chimney and an unsuitable one, and were not strictly relevant to the matter in hand. But it allows us to point out that all modern appliances of the types we are considering require to be connected directly and positively to a chimney or flue, and not left with an open ended pipe discharging in the general direction of a chimney base. That fact underlines the sense and logic of a flue liner, which avoids great changes in cross sectional area as the flue gases go from appliance to terminal.

The appliances themselves have stub flue pipes of the correct diameter, pointing either upward or backward, usually depending upon whether they are intended to stand against but outside of the hearth or chimney wall, or to be at least partly let into the hearth so that an upward flue pipe points in the right direction.

Boilers which incorporate a flue stabiliser have upward pointing stub flues for that reason, and it is expected that to couple into a flue behind, a bend will be used. When flue runs are given a change of direction it is preferable to use slow bends, of the order of 45°, and not right angles.

Gas fires often have a closure plate, a sheet of metal or other material which shields the fireplace opening, and has two holes in it; one for the flue to pass through, and one at or near the bottom which acts as a static draught stabiliser. The closure plate must be firmly secured to the hearth, to avoid unsafe conditions arising. But the gas fire/back boiler, which concerns us, has the gas fire flue transferred backwards to a common flue manifold over the boiler, the lot finishing in a spigot designed to fit straight into a flue liner.

If an appliance is fitted so that a backward facing flue enters a larger brickwork chimney at right angles, care must be taken to see that the stub flue is correctly set. Failure to attend to this, for instance in the case of a pressure jet oil fired boiler, can result in a very disturbing amount of muffled reverberation, as the jet cuts in and during working.

When flue pipes pass through walls, plates and the like, and have to be sealed in, due allowance must be made for the frequent expansion and contraction which takes place. It is useless to make the joint with cement, even with fire cement or fireclay. A soft heatproof jointing must be used, and the most obvious one is asbestos string or asbestos rope depending upon the size of the annulus. If there is any objection to the use of unshielded asbestos then it may be shielded by a collar which is attached to the flue pipe but not to the back brickwork or metal.

Damp patches on bedroom walls are often the result of condensation inside a flue, itself resulting from slow lazy flue gas travel, and the condensation penetrating bad brick joints. A flue liner cures this.

A flue terminal should always be fitted to a flue liner, and advice upon both is obtainable from the representative of the fuel to be used.

The chimney with an open fire, the culprit which can take away ¼ ton of air every hour, was in some respects better behaved than a more modern combination would be. The effect of all that air was to dilute the flue gases, often to the point at which condensation would not take place. But now, consider a gas or oil fired boiler, or modern solid fuel appliance, with a flue spigot of correctly designed size. In most gas boilers for instance this is 100 mm in diameter, giving a cross sectional area of about 78 cm2. That is the size the flue should be, all the way up. If the spigot discharges through an aperture in a blanking plate into a conventional 230 mm square flue, area 529 cm2, it has more than six times the area it needs and will dawdle. Worse, because of the blanking plate there will be no huge volume of excess air to dilute the flue gases, and condensation will be very much more likely to occur. The evidence all points therefore to the advantages of having a flue liner fitted in an existing flue or chimney to go with a modern appliance.

Because of the way in which wind is affected by high obstructions, it should always be the aim to carry the flue terminal above the roof ridge by about 0.6 m. Any wind striking the ridge will then be rising at that point, which is beneficial.

Having more chimneys than are needed, as in older houses with one open fire per room, is undesirable because of their property of whisking away air, often air which has been expensively warmed. The permanent way to deal with them is to cap them off at the top with a waterproof and windproof cover. Then, just in case the exposed brickwork is porous, do not wholly seal the lower or fireplace end. Instead, perhaps after removing the old grate and surround, fit a sealing plate but leave in it a slot, mesh covered and no more than 150 x 25 mm. This will prevent stagnation. To remove the breast and associated brickwork is builders’ work and should never be undertaken without full professional appraisal of the consequences to the rest of the structure.

At the other extreme is the home without a chimney. It may be a mews conversion, or a new house built with wholly electrical heating in mind. In most cases a conventional flue can be added. One way is to use asbestos flue pipe, double walled to keep condensation to a minimum, with a terminal. Such a flue may be run indoors, in which case there are strict provisions in the Building Regulations about the path it will take. In particular it must be kept away from structural timbers. Alternatively it may be run mainly outdoors, where the asbestos double wall really comes into its own.

The easiest way to run an outdoor flue and still conform with the need to have the terminal above ridge level is to run it up the gable end, other things being suitable. In that way it may be supported off the wall for most of its run.

A more robust, and in most cases better looking job is made by the use of proprietary precast flues such as TrueFlue make. These are in the main consciously styled, have all the necessary insulation built in, and do not need the same degree of support.

The best advice that can be given is: if you have a good working chimney, use it, in preference to any of the other methods of flue gas removal mentioned here. Some measure of sealing in will almost always be required.

There are of course nowadays types of dwelling in which even one chimney per dwelling would be quite out of the question. Multistorey blocks of flats are an example. If these are to be used for other than electrical appliances, then one way is to incorporate a Se-duct or U-duct. We need not give much consideration to these here. They are large centrally placed flues into which appliances from adjacent flats pass their combustion products and from which they draw combustion air. Se-ducts and U-ducts are an integral part of the main structure. Unlike the domestic chimney they cannot be added on at any time, but they are not domestic items except in the collective sense, as in flats.

The balanced flue

This is applicable mainly to gas fired appliances, occasionally to oil fired ones, never to solid fuel. The balanced flue forfeits dependence upon the traction effect of a chimney in return for the assurance that any disturbing influences acting upon the flue outlet will act equally upon the air inlet and so cancel out. This is achieved by ducting the flue outlet and air inlet to a common terminal. Then, if we suppose that a head-on wind is blowing, which would cause an ordinary chimney to reverse, its effect here is counteracted by the same force blowing upon the air inlet and so through the system. It is of course a condition that the combustion chamber must be sealed, hence appliances with a balanced flue are often called room sealed.

The theory of the balanced flue is as follows: suppose that the discharge pressure is p and the suction is s, then the differential causing flue operation is p + s. Now suppose a wind of force P blows on the terminal. The discharge pressure becomes p – P and the suction becomes s + P. The differential is + = p + s, I.e. it is unchanged by P.

Balanced flue appliances are usually situated against an outside wall, though some will if required work with extended ducts between appliance and terminal. There are a few rules about the placing of balanced flue terminals, social and technical, which must be observed. A terminal must not be placed near an openable door or window, or at low level near a public right of way. It must be covered by mesh or similar to keep out intrusive objects. For good operation it must be kept away from building corners, and from too near bushes or other buildings. Such features can cause non-homogeneous wind effects.

Traditional flues, Se-ducts, U-ducts and balanced flues have one thing in common. They work for ever for nothing, being dependent upon natural phenomena. Such items as the balanced flue have gone a long way to destroying the monopoly of the hearth as the focal point of the room. But there are still cases in which it may be impossible or undesirable to put an appliance anywhere suitable for connection to a natural flue. It must not be supposed that there is yet a universal answer to that, but in some cases, to be ascertained by enquiry, an appliance may work with an unnatural flue, e.g. a long horizontal run, to which the power is applied artificially. Such flues are ‘assisted’. They include quite complicated controls for safe working, which ensure that the fan starts before the burner lights, and stops after a purging period when it goes out; and if the fan fails to start the burner cannot light.

There is, clearly, a linking of controls between appliance and flue which makes it necessary for the appliance maker to be involved. It scarcely ever happens that a domestic situation cannot be adapted to match a natural flue, but anyone who believes that his does not should consult the gas authorities, or the representative of a major oil company.

A much simpler and cruder device may be used with solid fuel appliances attached to a chimney which is reluctant to work. This is a propellor type fan, let into the chimney, often at an angle, which assists the upward movement of air in the chimney and so creates draught. It carries no special conditions, may be switched on or off at any time and without regard to the state, or even the existence, of a fire. Its greatest benefit is probably in cases of marginal or intermittently bad draught. A fuel supplier, particularly if accredited by the SFAS should be able to advise.

Care of flues and chimneys

With the coming of the Clean Air Acts domestic chimneys no longer carry and discharge such volumes of heavy smoke, loading the walls with soot. One must however expect a certain amount of soot to collect, from solid fuels and from oil burning. In addition solid fuels may give rise to mineral dust which will rise part way up the flue. Another reason for occasionally inspecting a traditional chimney is that solid materials, parging and the like, have a tendency to come adrift and fall to the bottom. In time these could build up and choke the applicance stub flue, leading to a dangerous condition. Therefore, chimneys which used to be cleaned every six months should now be at least inspected once a year. The most that a balanced flue terminal on gas needs is occasionally to make sure that the outside of the terminal is clear. Little boys do try to ‘post’ sticks and things into any such opening, hence the terminal guard. A balanced flue system used with an oil fired appliance should be examined for soot formation during the routine servicing of the appliance.

Assisted flues of the complicated type should be looked at to see that all joints are tight, all controls working as they should, and that flue runs have not sagged and developed collecting points for condensation. This is best done by experts.


It is most appropriate when considering flues, which take away products of combustion, to consider also the rules which deal with the supply of air to the burner – without which there would be no products to take away.

The most thorough investigation into all aspects of ventilation we owe to the gas industry, and them we will quote. Perhaps as a reaction against generations of suffering cruel draughts, people have become very draught conscious. This has led to many cases of over-reacting, and draught stopping to a total extent. Such operations have too often ended in the coroner’s court. The gas industry, whose products of combustion are invisible, unlike the recognisable vapours from bituminous solid fuels, is particularly vulnerable to such happenings and so is most concerned by them. They have therefore specified the minimum requirements for the areas of permanent free openings into any room in which an appliance is at work. A full list is available at the gas showroom, but the main points are given below.

For boilers and similar central heating units with conventional flue the minimum opening at high level into the next room must be 6.5 cm2 for every 0.58 kW of maximum rated output. At low level the minimum is 6.5 cm2 for every 0.29 kW. Half these areas are needed if the openings connect directly to outdoors.

Room sealed models require the same areas at high level, but at low level only half the allowance for conventional flue models. Room sealed appliances are those having a balanced flue or Se-duct.

The minimum areas given above are permanent free areas, I.e. in a typical grille they are the openings, not the total face area. Although the proportion of permanent opening varies from grille to grille, it would be unwise to assume that the area of opening is more than 30% of the total, and it would be wise to choose generously sized grilles.

The principles, and to a large extent the details, given by the gas industry may be held to apply to any fossil fuel which is burned under controlled conditions. If in doubt be generous, for it is most necessary that air shall be freely available to the combustion process. If the flame is starved of oxygen there is trouble ahead. Most of the complaints about open fires smoking arise through air starvation which is caused by the phenomenal appetite of that appliance for excess air. It is an unbeatable spiral, showing how unsuitable uncontrolled combustion processes are in the modern home.

With the accent so much on supplying air for combustion, it might be asked why room sealed or balanced flue appliances require air openings. The answer is that they are what they claim to be, namely for ventilation. Combustion appliances emit a certain amount of warmth, and unless this is removed by deliberate ventilation it can build up a local temperature.

This could become uncomfortable, in some circumstances even dangerous.

Ventilation control

The operative word is control. A lot of houses can be shown to leak like baskets, with air movement taking place between floor boards, through window and door jambs, up unused chimneys, from room to room and away in the attic, and so on. A serious effort should be made to stop all of these, because they carry away a lot of the air so expensively warmed by the heating system. Only when stray and unofficial draughts have been stopped can we realistically set about controlling air movement. Control has two meanings. There is the air allowance for the room containing the fuel burning appliance, while for other rooms it is generally accepted that there shall be I ½ air changes an hour, not least to avoid building up a concentration of carbon dioxide in occupied rooms.

The stopping of unregulated draughts is usually thought of as a part of an insulation programme, and we will treat it as such and discuss it in a preceding post0.

The deliberate provision of air movement we will look at in two ways. First there is the air for combustion: if it is admitted, for example via a door, and the appliance is against the opposite wall, there is bound to be a draught cutting across the room. But if the air opening is on the same wall, even adjacent, then the body of the room will experience no direct draught. But it could, by travelling under the floor or on the floor, discharge near to the fire, just as readily. An opening made in a wall for such a purpose rarely needs more protection than that which excludes wild life. Although a flap may be fitted, the fire when out will not exert any pull on the duct since it is making no demand for air.

If this simple scheme is not possible, or not welcomed, a ground floor room with suspended floor might even benefit by having a mesh covered air opening cut into the floor. So long as the floor is well covered, for instance using good quality under-felt, it will not noticeably increase heat loss. But it will ensure the ventilation under the floor which is necessary to combat dry rot.

If in the end we come back to the door as the source of fresh air, do not rely upon air entering under the door. It not only gives the worst draught, it is also possible for someone else to stop it off, for example by fitting a thicker carpet. The proper way to use the door is to have a deliberate opening at the top, perhaps by cutting an inch off the top.

Or one might borrow a trick from warm air practice, and let a ventilator into a wall which communicates with another part of the house. This is the kind of detail which ought to be decided on personal grounds, for example is it preferable to encourage air movement to or from a room other than that to which the door leads, or should air movement take place in some other than the door-to-fireplace line?

A special example, open to be borrowed for special circumstances, comes from warm air heating of the ‘natural convection’ type. It involves fitting an extractor fan at high level in a wall, in order to move warm air from one room to another or to a hall for onward travel.

It is difficult to think of any case in which the last example would be required. The general rule is that combustion processes will grab all the air they need, so long as the air is free to travel their way. This means leaving ample area of grille etc., but in one connection there must be a warning. It is by no means unusual to have an extractor fan fitted in the kitchen, with the sole purpose of removing moisture laden air from the kitchen. Very often the boiler is in the kitchen too. In that case the air inlet allowance of 6.5 cm2 per 0.29 kW is inadequate, since it has to supply air for both boiler and extractor, and the latter is more powerful. There is then no rule about the size of opening, but it must be judged according to the results of a ‘spillage test’ which an expert can carry out. This involves putting everything to work, including the fan, and then finding out whether air travels continuously forward, or whether there is any tendency for flue gases to ‘spill’ back into the room.

Finally, we cannot put too much stress upon the need for air openings to be permanent. They must not be closeable at anyone’s whim or for any reason.

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