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Other causes of a smoking fireplace

1. A badly formed throat or gather


The throat or back of the lintel above the fire should ideally slope upwards at 45 degrees into the flue. But more often than not a standard concrete lintel is used across the fire opening presenting a flat surface above the fire. Add to this a stone fireplace surround, level with this lintel and as the smoke hits this flat area some will spill out into the room. And if the gather over the fire is of rough brick or stone creating an uneven surface, it will also slow down the smoke trying to enter the flue. The lintel will need to be replaced and the gather smoothed, also you could try using a sloping metal smoke hood, it might be just enough to help.

2. Partly blocked flue


This can be due to the chimney not having been swept for a long time; it should be swept at least once a year. But also in older chimneys, pieces of the mortar lining can fall across the flue, or a brick or piece of stone from the mid-feathers that divide it from a neighbouring flue might fall and partly block the flue. It might be possible to move this blockage with sweep's rods. If not it will be necessary to open up the flue to clear it. It may also be necessary to reline the flue. Modern flues are constructed with clay liners, if mortar is squeezed from the joints and not properly cleaned off as they are built, there will be restrictions in the size of the flue at each joint. Also if to form an offset, 45-degree bends are used, and the mortar is allowed to fall onto the bend, this will partly block the flue. This mortar can be hard to remove and it might be necessary open up the flue to clear it. (See figure 6.) It is also a favourite place for soot to build up.


Tar from wood burning can also partly or even completely block a flue. This is a Particular problem in flues, which are not of an insulated type. If wood is burnt, and a smoke problem has gradually got worse, this is likely the cause. The blockage usually starts near the top of the chimney where the gases cool rapidly. A sweep's rods and a scraper or steel wire brush may dislodge some tar. However, since it is normally as hard as rock it is difficult to remove. In extreme cases the chimney will need to be opened to clear these deposits. Chemical chimney cleaners may help to loosen the tar deposits if they are used often enough, over a period of many weeks, with frequent sweepings to remove the loosened tar. There are specialist contractors who can ream out obstructed flues. But if you chimney is in any way unsound reaming it could be disastrous, as it is very forceful in its action.


However if wood is to be burnt, it must be DRY and well SEASONED. That means several years old. Stored in the DRY, not out in the rain. And only in chimneys with well insulated flues.


New logs contain up to 60% moisture

Air dried logs have 40% moisture

Room dried logs as little as 20% moisture


In any event there is a lot of moisture contained in wood and it all has to travel up the flue as steam, and as it cools it will condense out and leave a tarry deposit. Not only slowly blocking the flue but also presenting an ever increasing fire risk. And attacking the flue surface.


3. Birds Nests


These can be a real nuisance, especially if the chimney has been out of use for some time, your chimney is seen as an ideal place for the Jackdaw to build his nest, it is he who favour’s the chimney in which to make his home, and if it is unused, then so much the better for him, however the Jackdaw is not adverse to building his nest in your chimney even when you are using your fire, it would seem that the old adage “prevention is better that cure” really does apply in this case, why? Because removing a bird’s nest from a chimney can be a very difficult and time consuming task, also an expensive one, and some times impossible without major disruption to the customer if it is necessary to remove a stove from its flue in order to gain sufficient access to work. I have taken as many as a dozen large dustbin bags full of twigs from a chimney blocked by bird’s nests, in that instance the birds had been building one new nest each year for up to ten years, this is an extreme example but it does illustrate how difficult the problem can be, in an open fireplace it is fairly easy to work, but when it is a closed stove such as a Parkray or Charnwood stove then the restricted access can necessitate the removal of the appliance in order to clear the nest. So what will keep those pesky Jackdaws at bay? See the Picture Here it shows an example of a very sturdy Bird Guard that will last many years in the very corrosive environment on top of your chimney pot. Much better than the frail chicken wire netting so often used but which rusts away in less than a year, only to allow the bird’s back in again.

4. Unsuitable chimney pots


Although many are decorative in the extreme, some chimney pots can be quite unsuitable for their purpose, and add-on cowls are often far too restrictive when compared to the size of the flue. The most suitable is the plain, straight-sided pot of the same size as the flue. (See Figure 7.) Another thing, very often round pots are fitted onto older square flues by placing pieces of tile across the corners of the flue. If this is done without care, an obstruction can be formed inside the base of the pot, on which soot and tar can build up. Better is to use the type of pot shown here it will fit more readily to your square chimney top without the need of bits of tile.


Never be tempted to use one of the cowls shown in the two pictures below.


They are not intended to be use on chimneys that are being use for solid fuel fires. They are ventilator caps, to keep unused chimneys dry, e.g. keep out the rain while allowing some air movement




5. 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 and grander houses, flues were all built over to a large central chimneystack, Those large and imposing chimneystacks might look grand but, when they involve long near horizontal runs of flue, they will be liable to give trouble and get blocked with soot and debris. It is frequently necessary to install additional tight-fitting soot doors to provide access points for cleaning. They make for a really complicated sweeping session requiring much labour and expense.


In modern home construction using clay liners, builders create sharp bends using two 45-degree clay bends, mostly to take the flue up into the centre of the house so that it can vacate at the highest point on the roof. But some times they do it in the misguided belief that a “dogleg bend” as it is often called, will make a flue draw. However this sharp offset can easily be blocked with, soot and tar build up (See Figure 6.) That is why regular chimney sweeping is so vital. It is best to keep offsets as gentle as possible such as 30-degrees. Horizontal sections of flue pipe are sometimes used to connect a stove to a flue. If a horizontal connection has to be made then keep it short, not longer than 150mm. If a thick wall has to be penetrated, this is best done at an angle of 45-degrees. Make sure there is suitable access for sweeping. (See Figure 8.)


6. Flue is too large


If a flue is very large, then the flue gases will cool quickly as they enter it, and the flue never really gets warm consequently reducing the up draught and spilling back into the room. This particularly occurs with inglenooks and other large fireplaces. Some try to solve the problem by extending the neck of the canopy or flue pipe from the appliance well up inside the chimney above a register plate. Adding as much as 2 metres. This may well stop the smoking, but it can cause another problem. How to thoroughly cleaning the flue? An access door must be provided of sufficient size for cleaning the large void above a register plate. A better solution would be to line the whole chimney with a liner sized to suit the type of fire being used. (See Figure 1.) And fit a freestanding fireplace or closed stove.


7. Cold and exposed flues


A chimney that is built on the outside of the house rather than within the house, because it is exposed to the elements on three sides is going to be cold and will cool the flue gases rapidly causing smokiness, you might need to use gas blow torch and point it up the flue in order to warm it and get it to “pull” be for you attempt to light the fire, once lit, keep it going well so as to keep the flue warm. It will never be as good as a protected chimney e.g. one in the centre of a building. (See Figure 9.)


8. The leaking Flue.


When the mortar-joints between the bricks are open in an old flue on an outside wall or in the chimneystack above the roof, and even in the attic, cold air can be drawn or blown into the flue. This results in two things, first it cools the hot rising smoke or fumes, as a consequence reducing the up draught, and second it causes turbulence in the flue, both of which can lead to the smoking fireplace. Re-pointing the brickwork, or rendering/plastering the outer surface of the stack, not forgetting in the loft, can often help. But it is often best to reline the entire flue with an insulated liner.


9. Broken Mid-feathers.


In older chimneys, in order to divide the flues from each other in the stack they built mid-feathers using bricks or slates or even thin pieces of stone between the flues but didn’t bond them into the outer walls of the stack. In time and with corrosion from the soot, a portion of these "mid-feathers" can deteriorate and disintegrate. This can easily block a flue that is in use (see No 2 above) or fall down a disused bedroom flue for example. The effect of these missing mid feathers can be similar to No 8 above, causing draughts and Turbulences in the rising flue gases. If smoke is seen rising from two or more of the chimney pots. Then you have most likely, identified the problem, the solution is either, to rebuild the stack, or have the flue in use relined. (See Figure 10.)


10. Siphonage. (See Fig 10)


This nuisance occurs when two chimney pots are close together; the wind will blow the smoke from one flue directly across the top of the other pot, so that the smoke is then siphoned down the unused flue. It can also take place when the mid-feathers have disintegrated inside the chimneystack or there are open joints linking two flues together. (See No 9 above and Figure 9.). There are three possible solutions: Fit a taller pot to the flue that is being used. Re-line the flue if the leak is inside the stack. Improve the ventilation to the room where the fire is situated. This nuisance can also arise where two rooms have been made into one, if an Aga, multi-fuel stove, or a gas fire is in one chimney, and an open fire in the other chimney of these formerly separate rooms, the open fire will frequently pull fumes into the room from the other appliance. The open fire when in use will always have the stronger “pull” than a closed appliance. Greater ventilation must be provided for the open fire, to stop it drawing the air it needs down the other flue.


11. Poorly installed liners.


Sadly, a good number of chimneys are poorly built and it would seem with no knowledge or regard for the principles or regulations concerned. The first liner is some times simply propped onto concrete blocks, so that a suitable flue gather is not formed. Sometimes the liners are fitted with the rebated and socketed joints upside down. If no mortar is used to joint the liners and the void around the liner is not filled, the flue will have leaking joints, which can cause the problems discussed in No 8 above. Furthermore any moisture or tar from burning over wet wood running down the flue will leak out through the joints, and cause stains on the wall. The only answer to this ruinous state of affairs is to re-install the liners correctly; it is unlikely that you can put a new lining within the defective one, since this will result in a considerably reduced flue size, perhaps not even suitable for a closed stove or room-heater.


And finally...


12. Damp in flues


The burning of any solid fuel will produce moisture in the form of steam. New logs as we have seen can contain 60% moisture. Now if the flue is too cool, these vapours will condense, and when they combine with the by-products of combustion, they will produce tar and acids. The result will be either brown stains coming through the walls, or runny tars leaking out around register plates and flue pipe joints, predominantly where wood burning stoves are concerned. These problems usually occur in unlined flues, usually those built before 1965, or where clay pipe liners have been installed, but, upside down. The problem normally involves re-lining the chimney. For closed woodstoves, an insulated lining is crucial.

Rain can also result in comparable problems, (see No 2 in the introduction) generally getting in via the pot, but leaking mortar joints, defective flashings etc are also points to look at. Get your preferred local builder to investigate and put right. Both the Slab Top, (See Figure 11.) H Pot and Marcone chimney pot mentioned earlier are fairly effective at keeping direct rain out. But try to avoid using chimney pot cowls, as some can severely restrict the flow of smoke from the flue, and may actually cause that dreaded problem of the smoking fireplace!!!! The MARCONE chimney pot can be useful as it can be quite tall and will increase chimney height and its design might help to both keep out rain and counter down draught. As will the H Pot (See Figure 12.)


Identifying problems


I have here attempted to give a broad understanding of the problems, and possible solutions to smoking fireplaces. But it must be accepted that smoky fire problems, are often caused by a combination of a number of faults. And trying to isolate and deal with each one in turn will require a little experimenting, and much careful scrutiny. It would be both sensible and wise to seek expert and experienced advice.



For more information, or advice, contact Bullen's Chimney Cleaning Service's on 01726 69690 or email us using the Contact Us page.



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