Why you should sweep your chimney
It is estimated that there are 30,000 chimney fires each year in the UK. Of those, around 9,000 cause serious damage to property and occupants. Wood is being used more and more to heat homes due to the rising cost of oil, gas and electricity. Sales of wood burning appliances have risen considerably over the last few years but many people are not aware of the risks associated with burning wood.
Causes of chimney fires
Burning fuel creates emissions which cool as they rise up the chimney and turn into creosote. The creosote sticks to the chimney lining and being highly flammable, catches fire when the sparks from the burning flames below rise up the chimney. This causes the chimney to burn, sending flames out of the top of the flue but also down in to the room.
The most common causes of chimney fires are:
- Burning green wood that has not been seasoned or dried properly;
- Blockages in the chimney, such as bird nests, that cause restriction in air flow;
- Chimneys that have not been swept regularly;
- Not using a wood burning appliance to its full capacity;
- Letting embers smoulder for a long time.
Most people think of carbon monoxide poisoning being caused by leaking gas but the potentially fatal condition can be just as easily caused by badly managed solid fuel appliances. Carbon Monoxide(CO) can be emitted from any appliance burning carbon based fuels eg. gas (mains or bottled), solid fuel (coal, wood), petrol, oil or paraffin and less than 2% of CO in the air can kill in two minutes. Annual figures show that 200 people needlessly die or become seriously ill due to carbon monoxide poisoning and because symptoms are similar to those of flu, it can often go undetected.
Chimney fires and carbon monoxide poisoning are easily avoided.
- Inspect and sweep your chimney regularly, at least once a year, more if you use your appliance for long periods of time;
- Use a registered chimney sweep from National Association of Chimney Sweeps who work to a code of practice and are fully insured(findone here)
- Only use properly seasoned, dry wood on your fire and don’t burn any wood that has a high sap content such as pine;
- Never burn rubbish or card in your fire as it will cause sparks which can alight in your chimney;
- Use a fireguard in front of your fire to avoid any sparks catching light in your home;
- Ensure that your appliance is fitted by a HETAS engineer;
- Fit a chimney birdguardto your chimney pot to deter nesting birds
- Ensure you have smoke and carbon monoxide detectors in your home
Remember, chimney fires will not occur in a clean, intact and properly installed chimney.
Get your chimney swept, it’s really not worth the risk.
Chimney Problems and How to Cure 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! 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 is finally sought.
There are many problems that can occur in a badly fitted or badly situated flue. Here we focus on the smokey fire and some suggested fixes. It is not complete and every situation is different.
How a flue works:
The flue is the void or passageway through which the products of combustion are removed from the fire to the outside. It is a column of hot air and gases which is lighter that an equivalent column of cold air outside.
2 things to remember:
Smoke wants to rise vertically so any bends or sloping sections in a flue are going to slow down the flow and the clearance of smoke from the fireplace.
Smoke only rises as long as it is warmer than the surrounding air. The greater the temperature, the faster it will rise.
For most wood and coal fires, flues have two jobs – to clear the products of combustion from the fire and to discharge them outside the building and to create a flow of air through the burning fire bed to create sufficient oxygen for the efficient combustion of the fuel. For gas fires, the first part is essential but it is generally not desirable for the flue to pull air through the burner.
The smoky fire
The faults leading to a smoky fire can broadly be analysed to three primary causes.
1. THE RATIO PROBLEM:
The fireplace opening is too large for size of flue so 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.
As a rule, flues above 6m tall should generally be not less than1/7 to 1/8 of the area of the fireplace opening(a225mm diameter flue will support a fireplace opening up to about 550 x 550mm). For bungalows the ratio should be reduced to 1/6.
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 can 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 and replaced with a freestanding fire, bricked in convector open fire, closed stove or canopy.
For smaller fire openings(upto900mm 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.
A raised plinth can also be built to reduce the opening size or installing a convector fire box are also good solutions to this problem.
2. THE AIR STARVATION PROBLEM:
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. Lack of ventilation is a common cause of smokiness and spillage of fumes including Carbon Monoxide into the room.
To cure the 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 conservatory and then to outside. If the room has a suspended wood floor with air bricks round the outside, then a simple floor grill cut into the floorboards to one side of the hearth is a good solution.
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 problems can occur from a badly sited terminal.
Downdraught– 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.
Pressure Zone– The chimney is situated 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 types of chimney cowl can reduce the problem or you can construct a slab top or dovecote top on to the chimney pot. It may also help to introduce some room ventilation on the same side as the prevailing wind, helping equalise the pressure at the top and bottom of the chimney. Try opening a window on the windward side of the house and if this helps, fit a permanent air vent. If all else fails, an electric chimney fan may be the only solution.
In all these situations, it would be advisable to seek advise from a chimney and fireplace specialist.