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The guide to chimneys, flues and their problems…

Click on a heading to move to the chimney section…

A … Introduction
B … Definitions
C … How a flue works
D … Flue Functions
E … The smokey fire
F … Primary causes of smokiness
G … Secondary causes of smokiness
H … Damp in flues
I … Diagnosing problems
J … Further sources of information”>Further sources of information

a… Introduction

There is nothing more frustrating, having just altered or installed a new fireplace or stove, than to find it the chimnney SMOKES back into the room. The local builder immediately fits a chimney cowl, which often does nothing to help, or makes things worse! The next door neighbour, the milkman, and old Bert down at the pub all nod sagely and come up with their patent cures. Several months and a few hundred pounds poorer, professional advice from the local National Fireplace Association member showroom is finally sought.

Chimneys work on simple physical principles, not by legend or magic and old wives’ tales. This paper will briefly look at these principles and help to identify the causes of smokiness.

Problems associated with chimneys can be broadly divided into 3 main types:

  • Poor flue pull and the emission of fumes or smoke into the room where the fire/appliance is situated.
  • Leakage of smoke and fumes from the flue into adjoining rooms or roof spaces.
  • Ingress of water/dampness due to entry of rain into the flue or condensation from the flue gases.

This article will mainly deal with the first above, ‘THE SMOKY FIRE’. Fume leakage from the flue will be dealt with more fully in a future article about lining old flues.
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b… Definitions

It is important to understand the terms used in connection with chimneys. These are principally as follows:

  • FLUE – The flue is the void or passageway through which the products of combustion are removed from the fire to the outside.
  • CHIMNEY – A chimney is the structure surrounding one or more separate flues.
  • FLUE PIPE – A flue pipe is a single skin metal pipe used to connect a fire or appliance to a chimney.
  • FLUE LINER – The flue liner is the material used to form the flue within the chimney. Flue liners can be of fire clay, refractory quality concrete or metal (usually high grade stainless steel).

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c… How a flue works

Before looking at the cause of  problems in chimneys, it is important to understand how a flue works. A flue is basically a column of hot air and gases, which is lighter than an equivalent column of cold air outside. Observe a bonfire on a still day. The smoke naturally gathers in a column rising vertically above the fire, forming an invisible flue. But once it cools to the temperature of the surrounding air it rapidly disperses. This observation of a bonfire provides two important rules for the best performance of chimneys:

  • Smoke wants to rise vertically, therefore any bends or sloping sections in a flue are going to slow down the flow and hence the clearance of smoke from the fireplace.

RULE – Flues should ideally be vertical. If an offset is necessary, it should be as near vertical in angle as possible. 30 degrees from the vertical is the recommended maximum. 45 degrees bends are really too steep, although permitted in some circumstances.

  • The smoke only rises as long as it is warmer than the surrounding air. The greater the temperature difference, the faster it will rise.

RULE – Chimneys and flues should be insulated to keep the gases warmer therefore improving the ‘draw’ and the clearance of smoke from the fireplace or appliance.
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d… Flue Functions

It is also very important to understand the functions the chimneys are intended to perform.

For most wood and coal fires the flue has two jobs to do:

  • To clear the products of combustion from the fire and to discharge them outside the building.
  • To create a flow of air through the burning fire bed to provide sufficient oxygen for the efficient combustion of the fuel (this second purpose is often overlooked.)

For most gas fires however only the first above is essential. It is generally not desirable for the flue to pull air through the burner. Hence most open-flued gas fires are fitted with some form of draught breaker or diverter. (This reduces flue pull and ensures any down draught does not enter the combustion chamber and blow the flame out).
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e… The smokey fire

The main purpose of this leaflet is to look at the causes and remedies for the emission of smoke and fumes from open fires and appliances into-the room. (With gas fires, this is termed ‘spillage’). It is a criminal offense under the GAS SAFETY REGULATIONS 1984 to allow spillage from a gas fire to take place. Gas installation work must only be carried out by a CORGI registered installer. (See future NFA leaflet on Fuel Effect Gas Fires.)

The faults leading to a smoky fire can broadly be analyzed to three and ten secondary causes.
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f… Primary cause of smokiness

1. The Ratio Problem
2. The Air Starvation Problem or lack of room ventilation
3. Badly sited chimney terminal

1. The Ratio Problem

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 and chimneys 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.

2. The Air Starvation Problem or lack of room ventilation

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:

  1. 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.
  2. 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.


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.
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3. Badly sited chimney

The best place for chimneys to terminate is on or near the roof ridge, and well above it. Two distinct problems can occur with  badly sited chimneys 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.
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Downdraught caused by near obstacles

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).
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 Chimney downdraught caused by high pressure on roof slopes and low chimneys

Alternatively, if chimneys are 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 chimneys, 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 a chimneys 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 chimneys. 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.
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Chimney top pots and slab tops

g… Secondary causes of smokiness

1. Badly formed throat or gather
2. Partially blocked flue
3. Incorrect chimney terminal
4. Sharp bends and long offsets
5. Flue is too large
6. Flue is too tall
7. Leaky Flue
8. Collapsed mid-feathers (or withes)
9. Siphonage
10. Badly installed liners

1. Badly formed throat or gather

The throat or back of the lintel should slope upwards at 45 degrees into the flue. Often a standard concrete lintel is used across the fire opening forming a flat soffit above the fire. Add to this a stone fireplace surround with a course of stone in front of this lintel and the smoke will strike this flat area and trickle out into the room. Also the gather above the fireplace is sometimes of rough brick or stone and will severely restrict the smoke reaching the flue. Either this lintel must be replaced or, possibly, a sloping metal smoke hood will help
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2. Partially blocked flue

With older chimneys, a large piece of the mortar lining may fall across, or a brick or piece of stone from the upper dividing masonry to a neighboring flue may collapse, partially blocking the flue. This obstruction may sometimes be dislodged with sweep’s rods. Otherwise it will be necessary to open up the flue to clear the blockage. It may be necessary to line the flue.

In modern flues constructed with clay liners, mortar squeezes from joints are often not properly cleaned off during construction. Particularly if 45 degree bends are used to form an offset, mortar droppings can be left on the bend, partially obstructing the flue. These can be difficult to remove and may also require opening the flue to clear.

Tar buildup can also partially, or even wholly, block a flue. If wood is burned, and a smoke problem has gradually got worse, this is a likely cause. The blockage is usually near the top of the chimney where the gases are cooling. If tar is forming in a tall pot, replace this with a shorter one with, say only 150 mm (6″) projection. Sweep’s rods and a scraper or steel wire brush may dislodge some tar. However, it is often baked on like hard pitch and may be difficult to remove. In extreme cases the chimney will need to be opened to clear these deposits. Chemical chimney cleaners may help to loosen tar deposits if used over a period of weeks with frequent sweepings. Professional advice should be sought concerning the suitability of chemical cleaners, as some can be harmful to certain flue materials. There are specialist contractors who can ream out obstructed flues. If wood is being burned, IT MUST BE DRY AND SEASONED.
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3. Incorrect chimney terminal

Many chimney pot designs and add-on cowls are far too restrictive when compared to the size of the flue. The best terminal for most chimneys is a plain, straight sided pot of the same size as the flue. Also, many pots are fitted onto older flues by placing a piece of slate across each corner of the flue. If this is done carelessly, an obstruction can be formed inside the base of the pot, on which soot and tar can build up. Remove and refit the pot. There are strict rules for terminals allowed for gas log/coal effect fires. (Refer to BS5871 pt. 3. 8.3.4)
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4. Sharp bends and long offsets

In older flues, offsets were usually formed at small angles from the vertical by corbelling brickwork across. However in some larger homes, flues were all carried over to a large central chimney stack, often involving long near horizontal runs of flue. These will tend to give trouble and block with soot and debris. It is often necessary to install additional tight-fitting soot door access points in such flues.

Soot door example in sealed stove
In modern house construction using clay liners, sharp offsets are often created using two 45 degree clay bends in the mistaken belief that this makes a flue draw. This sharp offset then easily blocks with a few sticks from an enthusiastic jackdaw, or soot and tar build up. In extreme cases it may be necessary to open a section of wall and rebuild a gentler offset. Horizontal sections of flue pipe are sometimes used to connect a stove to a chimney. Any horizontal connection should be no longer than 150mm (6″). (Building Regulations J 1.19).If a thick wall has to be penetrated, this must be done at no less than 45 degrees (See diagram 5).
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5. Flue is too large

If a flue is very large, then the flue gases will cool quickly as they enter it, thus reducing the updraught and spilling back into the room. This particularly occurs with inglenooks. The best solution is to extend the neck of the canopy or flue pipe from the appliance well up inside the chimney above the register plate. Adding 2m (6′) will often cure the problem. It is, however, better to have a flue a little oversize than undersize. An access door must be provided for cleaning above a register plate. Alternatively, line the whole chimney with a liner sized to suit the type of fire being used.
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6. Flue is too tall

With a very tall chimney, the flue gases will often cool so much before they reach the terminal that a cold plug of air is left at the top of the flue and acts like a cork. This can occur when first lighting the fire and later in the evening when the fire has died down. Try keeping a hotter fire burning, or fit an appliance with doors which can be warmed up more quickly than an open fire. It may be necessary to install an insulated liner in some cases. Seek professional advice first.
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7. Leaky Flue

If the mortarjoints are open in an old flue on an outside wall, or in the chimney stack above the roof, cold air can be drawn or blown into the flue. This has two effects– it cools the hot rising gases, reducing the updraught, and causes turbulence in the flue, both of which can lead to smokiness. Repointing or rendering can often help. Alternatively it is often best to line the flue.
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Dangers of reverse pressure in adjacent chimneys8. Collapsed mid-feathers (or withes)

In old chimneys, the flues in the stack were often divided from each other by building bricks ‘on edge’ or slates or thin pieces of stone between the flues and not bonding them into the outer walls of the stack. With time and corrosion from the smoke, a section of these “mid-feathers” or “withes” collapse. This may cause blockage of a flue that is in use (see 2 above) or fall down a disused bedroom flue. The effects of these missing mid feathers can be similar to 7 above, causing cross draughts and eddies in the rising flue gases. This problem can often be identified if smoke is observed rising from 2 or more of the chimney pots. The solution is either to rebuild the stack or have the flue in use lined. (See diagram 6.)
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9. Siphonage

This problem is best understood by looking at diagram 6. It can occur when two flue outlets are close together, the wind blowing the smoke from one flue directly across the outlet of another. It can also occur when the mid feathers have collapsed inside the chimney or there are open joints linking two flues together. (See 7 above). There are three possible solutions:

  1. Fit a taller pot to the working flue.
  2. Line the chimney if the leakage is inside the stack.
  3. In the room where the fire is, improve the ventilation.

This problem can also occur when 2 rooms have been knocked into one, with, a cooker or gas fire in one chimney and an open fire in the other. The open fire will often pull fumes into the room from the other appliance. An open fire will always win a “tug of war” with a closed appliance. Solution: increase room ventilation.
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10. Badly installed liners

Regrettably, a number of chimneys are carelessly constructed with no knowledge or regard to the principles or regulations involved. No proper flue gather is formed. The first circular liner is simply propped onto concrete blocks, leaving triangular openings at the corners. Often the liners are fitted with the rebated and socketed joints the wrong way up. No mortar is used to joint the liners and the space outside the liner is left open. The resulting chimney is therefore full of leaks and open joints causing the problems described in 7. Any moisture or tar running down the flue will leak out through the un-mortared joints, causing staining on the wall. The only solutions to this disastrous situation are to either cut out one face of the chimney and re-install the liners properly, or to insert a new lining within the defective one. This will result in a smaller flue and therefore usually means installing a closed stove or room heater. It is sometimes possible for a specialist contractor to ream out the defective liners and re-line, but this is expensive.
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 h… Damp in flues

The burning of any fuel produces water vapour. If the flue is too cool, this vapour will condense, and mixing with other by-products, will produce tar and acids. This can result in either brown stains coming through the walls, both inside and outside the house, or runny tar leaking out around register plates and flue pipe joints, particularly with woodburning stoves. These problems can occur in unlined flues built prior to 1965 or where clay liners have been installed the wrong way up. The solution generally requires lining the chimney. For closed woodstoves, insulating round the liner is advisable.

Rain can also cause similar problems, either entering the flue directly at the top, or through leaking mortar joints, defective flashings or back gutters. These should be checked and repaired by a local builder. Both the slab top and Marcone chimney pot (see diagram 4) are effective at keeping direct rain out. Generally avoid using chimney pot additions, i.e. hood inserts, as these are too restrictive to the outflow of smoke and may lead to other problems.
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 i… Diagnosing problems

This document has given a general understanding of the problems and possible cures. The table below of typical symptoms will give a quick reference analysis back to the relevant section. Smoky fire problems are often caused by a combination of several defects. Isolating and dealing with each one in turn may need some experiment and careful analysis. Experienced advice should always be sought from your nearest National Fireplace Association member showroom before attempting a cure. Many member showrooms will be able to offer a home advisory visit and survey.

Possible Causes
Paragraph Ref.
1. Fireplace with large opening smokes all the time. Opening too large for flue size
Incorrect terminal
Partially blocked flue
Flue too large
2. Fireplace with standard (small) fire opening smokes all the time. Flue too large
Flue too small
Badly formed throat
Incorrect terminal
3. Fire only smokes with doors and windows closed. Lack of room ventilation
4. Fire smokes continuously in certain wind directions. Chimney in pressure zone
5. Fire puffs occasionally in certain wind directions. Chimney in downdraught situation
6. Fire (woodstove) worked well for a time, smoking gradually got worse. Flue partially blocked with soot/tar build up. Sweep the chimney.
7. Fire suddenly started smoking (after chimney was swept) Flue partially blocked by mortar/mid-feather collapse.
Sweep chimney again


(The word Fire is used to mean both open fires and closed appliances)
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 j… Further sources of information

  • The Building Regulations 1990 Part J : Heatproducing appliances. (Revised 1992)
  • The Building Regulations 1985 Part A. 1/2D. Structure.
  • British Standard BS 8303 1986 Installation of domestic heating and cooking appliances burning solid mineral fuels. (Revision due 1993).
  • British Standard BS 6461 1984 Installation of chimneys and flues for domestic appliances burning solidfuels.
  • Gas Safety (Installation and Use) Regulations 1984.
  • British Standard 5440 Pt I and Pt II Installation of Flues and Ventilation for Gas Appliances.
  • British Standard 5871 Pt I, II and III Installation of Gas Fires (including Decorative Fuel Effect).
  • British Coal: Chimney Guide (1991). British Gas: District Technical Offices.
  • British Flue and Chimney Manufacturers’ Association: Technical Leaflets

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This information is designed to give a general guidance to analysing and curing smoky fires and fume emission.

This is a difficult subject where experienced advice is invaluable.
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