Wood Fuel

The use of seasoned wood wood is essential with Vigas boilers. Maximum moisture content should be as near 20% as possible. Optimum drying conditions for wood are plenty of air movement coupled with a rudimentary roof to throw off the rainwater. Remember, closed sheds do not dry wood as effectively as open, airy sheds. Any species of wood may be used in Vigas boilers as long as it is seasoned.
Wood fuel will season most effectively once split and cut to length. Many types of wood will season in 12 months but heavy dense species such as Oak will take considerably longer. Vigas boilers require the wood fuel burnt in them to have a reasonable surface area to facilitate maximum gasification.

Vigas boilers are designed to burn dry wood material from sawdust to logs. Differing moisture content and size of fuels will affect efficiency heat output and the length of burning time. During gasification the quantity of released gas depends on the size and surface area of the fuel. The greater the surface area, the bigger the volume of gas produced. Hardwoods, (for example, Ash, Oak, Birch and Beech) are gasified more slowly than softwoods (for example, Fir, Pine, Spruce and Larch) and generally burn for a greaterr length of time. It is possible to gasify all kinds of wood in Vigas boilers, however the optimum moisture content should not exceed 15 to 20%.

A useful rule of thumb for splitting would be, over 150mm diameter- split in half. 250mm diameter, split into three. 300mm diameter, split into four.

	Split logs cut to half metre lengths ready to charge a Vigas 25 kW or 40kW log burning boiler.		Ideal moisture content of split logs 20% or less.

Where do I buy wood?

Wood fuel can be found under the 'Firewood' or 'Coal and Solid Fuel' section in telephone listings. Local tree surgeons will also be a good source of fresh cut timber. Local farmers often sell firewood and good quality dry waste wood can be had from joinery machine shops and timber merchants.

When asking for wood prices, it is useful to be clear from the outset about the following:

• The moisture content (dry basis) or how long it has been seasoned for.
• The species of wood.
• The price per volume or weight.
• The average length and diameter.
• Delivery costs and whether this includes a tail lift or crane.
• Packaging whether loose, wire cage, pallet and /or big bags.

What is moisture content?

When first cut, half the weight of 'green' wood from the forest is water. This does not appear on the surface of the bark but is locked into the cellular structure of the fibres of the wood as sap. If you try to burn such wood you will get the following problems.

• It is heavy to move.
• It does not easily ignite.
• It gives off little heat.
• It chokes up the chimney with tar.
• It increases the acid content of flue gases.
• It increases the ash.
• It smokes a great deal.

Burning 'green' wood is like taking a perfectly good fire and throwing a bucket of water over it!

The simple way to burn wood well is to allow it to dry naturally. Two years is the preferred minimum drying time for newly cut wood, more for some hardwoods, but it should never be dried for less than a year.

Moisture content at any stage of the drying process can be measured in several ways.

• Using an electronic moisture meter which shows the figures as a percentage on a screen.
• Artificially drying a sample and comparing the weight before and after drying.
• Years of experience can allow one to assess moisture content by handling samples of wood.

Above all, do not forget:		burn wood not water!

How much wood do I need?

The heat energy required for full central heating for most houses lies somewhere between 14,000 and 30,000 kWh per year, irrespective of fuel source. Wood can be used to occasionally heat one room or alternatively it is possible to heat the whole house through the entire burning season (October to March).

It is important not to forget that, in estimating how much heat is likely to come from wood, volume for volume, stacked or loose wood contains less useful wood than a single solid piece. This is because logs or split logs are uneven and have air spaces around them. It is good to remember this when measuring a pile with a tape measure, for exmaple if the pile is 1 metre high and the logs are 0.3 metres long, then the length of the pile will be 133 metres long just before the winter period!

The UK average wood fuel consumption for an efficient boiler is approximately half a tonne per kW per year. In other words, a 25kW boiler would burn approximately 12.5 tonnes of seasoned wood per year subject to heat load.

Our associate company can supply all your wood fuel requirements.

Email: mineheadsawmills@supanet.com

Tel. No. 01643 704381