IT has been said, that the heart of any home is the fire place. Not surprisingly, as through the ages the food was cooked ,water heated and being the only source of heat to ward of the inclement elements became the centre of the household.

Fire places in days gone by were little more than a fire lit on the bare earth, supplying  heat for  the one communal room. The smoke from the fire raising up undirected through the badly constructed thatched roof. The only control, was by the amount of fuel added to the fire. As you can imagine ,with the room being full of smoke and the thatched roof  becoming saturated with tar deposits there could not have been a more unpleasant  not to mention an unhealthy place to live.

When need arouse to add a first floor to these dwellings, there was obviously a need to direct the smoke from the fire burning on the ground floor up and out through the roof ,thus the first chimneys were constructed. These being little more than a construction  of a large ingle nook to catch the smoke, tapering in size as it past through the first floor and thatched roof.

This being a great improvement, as not only was the smoke directed out off the building, but the heat stored in the building fabric of the chimney heated the upper floor. It was also found that as the chimney heated up the velocity of the smoke in the chimney increased allowing the fire to burn more vigorously. 

Many of this type of chimney in some form survive today.

Problems you could experience and how to solve them.

There is nothing more frustrating, having just altered or installed a new fireplace or stove, than to find it SMOKES back into the room. The local builder immediately fits a cowl, which often does nothing to help, or makes things worse!

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

1) Poor flue pull and the emission of fumes or smoke into the room where the fire/appliance is situated.

2) Leakage of smoke and fumes from the flue into adjoining rooms or roof spaces.

3) Ingress of water/dampness due to entry of rain into the flue or condensation from the flue gases.

 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. But once it cools to the temperature of the surrounding air it rapidly slows 

Take note of the following

1) 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.

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

Main course of a smoke being emitted into the room

1. The Ratio Problem
 

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. (See leaflet 1 OPEN FIRES D.1)

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 firebox 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 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:

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 vapour. Lack of oxygen however will produce carbon monoxide, an odourless and highly poisonous gas.

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, smoke or spillage of the fumes, including the carbon monoxide, into the room.

THIS IS A VERY DANGEROUS SITUATION.

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.

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.
 

3. Badly sited chimney terminal

The best place for a chimney to terminate is on or near the roof ridge, and well above it.

Two distinct problems can occur with a badly sited chimney terminal, although the symptoms can be similar.
 

chimney is much too low, a combination of a tall pot plus raising the stack 600-900 mm (2'-3') may be necessary. Note: Building Regulations A 1/2 D gives maximum height for a chimney stack, including pot, as 411, 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 chimney 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 the chimney. 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.

 

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.
 

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 neighbouring 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 squeezed 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 build-up 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.
 

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 

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 chimneystack, 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.
 

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 up draught 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.
 

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.
 

7. Leaky Flue

If the mortar joints are open in an old flue on an outside wall, or in the chimneystack 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 up draught, and causes turbulence in the flue, both of which can lead to smokiness. Re-pointing or rendering can often help. Alternatively it is often best to line the flue.
 

8. Collapsed mid-feathers

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.)

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

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