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Wood Burning Solid & Multi-fuel Stoves

Environmental Impact

Although coal, gas, and electricity have been gradually replacing wood as the main source of heat in the homes of Britain over the last 300 years, it is still an important source of energy. It has another advantage though; when did you last sit round and enjoy the smell, flames and ambience of your gas boiler? But is it green? Well it is certainly greener than the other three could ever be (with the exception of hydroelectricity and electricity produced by other renewable sources, which is less than 5% of all electricity production). It is renewable, trees are always being planted. Then there is dead and fallen wood, not all trunks are suitable for sawn timber.

There are vast acreages of coppice in Europe that were used to provide poles, fences and other farm essentials. If not regularly sawn down these will wither and die destroying whole eco-systems. The amount of CO2 produced by burning wood almost exactly equals that used by the tree growing. If left to rot on the ground wood produces methane, a gas 20 times more damaging to the atmosphere than CO2. When burnt properly, wood produces virtually no smoke and no acid, so let us turn down the central heating and leave our children and grandchildren some oil, gas and coal, burn wood in a nice stove with a glowing conscience and glowing cheeks from the all round warmth of real flames.

Wood is considered by the government to be CARBON NEUTRAL and wood boilers over 7Kw with installation are subsidised by having only 5% VAT levied on them. There is approximately 20 million tonnes of renewable wood available annually in the UK and about 50million tonnes in France.

WOOD BOILERS.

From the above notes on combustion of wood you can see that if you just put a water jacket in the combustion chamber, you will cool the combustion process below the required 3-500C, incomplete combustion will occur and you will have a tarry shiny inside to the stove and a tarry chimney to boot. A recipe for chimney fires, a waste of money and effort as you have failed to burn a high percentage of the wood you have so carefully stored and loaded into your stove.

Proper stand alone wood boilers use a forced draft system to aid the primary then secondary combustion. Then, and only then, are the hot flue gases allowed to pass through the boiler/heat exchanger to give up their heat to the water. These are large and not very pretty though.

Charnwood now make a 'flue saver boiler', which takes the waste heat from the chimney to heat water so it does not effect the combustion. A further 2-3 Kw of heat can be extracted this way. see www.charnwood.com.  When connected to a thermal store type cylinder these are a superb method of reducing you carbon foot print by about 40%.

Other complete wood boilers to consider though are the;-

New Aarrrow Eco range, Esse 700b,Charnwood 16b, Charnwood Island IIIb.

WOOD PURCHASE / CALORIFIC VALUES.

The figures used in this section are taken with kind permission of the Forestry Commission from their leaflet entitled ‘Wood as a Fuel’. 

A solid cubic metre of fresh felled timber weighs about 1 tonne, or 1000 KG, contains up to 600 kg. of water. Selling or purchasing by weight is therefore a bit of a misnomer as the weight will fall by up to 225kg with air drying alone.

Merchants should sell logs or lengths by solid measure. A lorry with a capacity of 1.8Mtr.Cu. holds about 1 solid cubic meter (Cu.Mtr.) of wood.. A wire net cage 2m x 1m x 0.9m high will hold 1 solid Cu.Mtr. of wood. Cleanly trimmed straight conifers occupy about 15% less space.

1.8 Cu.Mtr. of broad leafed (beech) cut logs (1mtr solid wood) air dried to about 20% moisture contains approx. 1764 kWh. of energy which if burned in a stove at 70% efficiency will give up to the house 1,235kWhrs. or a one bar electric fire on for 1,235 hours (seven weeks non stop).

To calculate the cost per kWh. of a load of wood take;-

Length x width x height of the truck in metres & multiply this figure by 980 and you have the approx. number of kWh of the load. Divide your load cost by this figure to get the cost per kWh. This can then be compared with your electricity or gas bill. For oil, take your price per liter cost (do not forget to include the VAT) and multiply it by 0.154 to get the cost per kWh. Also remember that oil and gas central heating boilers are only about 85% efficient overall so you should divide this answer by 0.85 to get a good comparison.

In English, 1 lb. of good dry hard wood produces 1kWh/3,340 Btu’s. approx.

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