The Chimney Sweep Company's guide to chimneys, flues and their problems...
Click on a heading to move to the section...
- Introduction
- Definitions
- How a flue works
- Flue Functions
- The smokey fire
- Primary cause of smokiness
- Secondary causes of smokiness
- Damp in flues
- Diagnosing problems
- Further sources of information
g... Secondary causes of smokiness
- Badly formed throat or gather
- Partially blocked flue
- Incorrect chimney terminal
- Sharp bends and long offsets
- Flue is too large
- Flue is too tall
- Leaky Flue
- Collapsed mid-feathers (or withes)
- Siphonage
- Badly installed liners
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
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.
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)
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.
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).
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.
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.
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.
^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.)
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:
- Fit a taller pot to the working flue.
- Line the chimney if the leakage is inside the stack.
- 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.
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 roomheater. It is sometimes possible for a specialist contractor to ream out the defective liners and re-line, but this is expensive.