The Right Fuel

Any device that requires a fuel can only perform correctly if it has the correct fuel. A car for instance operates best when you use the right fuel, wood boilers are no different. Freshly cut wood can contain up to 80% moisture , useful energy is therefore boiling off this water. Therefore it makes sense, before you burn wood to remove as much moisture as possible.

The process in which moisture is removed from wood is referred to as seasoning. This is not a complicated procedure. First the tree needs to be felled, preferably in the winter when the sap content is low, then cut into manageably sized logs and split. The split logs can then be stacked in a dry area with plenty of air flow bark side down, the bark on wood is there to keep moisture in. Over a number of months (typically 6- 12 months) dependent on wood type, the moisture content will gradually decrease as the wood dries out. The rate of drying will depend on climatic conditions, if you obey the simple rules of keeping the wood raised and dry, the wood will season.

The alternative to seasoning naturally is to force dry the wood by Kiln drying. This involves forcing the moisture from the wood by baking the wood at high temperatures in a climatically controlled environment (normally a large oven). The advantage of this process is time and a consistent moisture level , a consistent fuel delivers consistent performance with excellent calorific values. The disadvantage is of course cost and an increase in the carbon foot-print of the fuel versus naturally seasoned wood.

The ATTACK boiler range requires a moisture range of 12-20% for optimum performance, our partners and installers will provide a moisture meter with each installation allowing you to check that your fuel is in the required range. Having seasoned wood will allow your boiler to run as the manufacturer intended and to produce the efficiencies quoted. By using seasoned wood you are actually harnessing the energy in the wood for heating your home instead of burning the wood to boil of moisture.

The graph below illustrates the effect moisture content has on the calorific output from the fuel.

 

The Attack range of boilers can accept most forms of clean wood, seasoning being the most important factor. Pallets or any scrap wood from building work (timber for building materials has already been seasoned) can be used. However man made timbers cannot be used , no MDF, Plywood or Chipboard as these all contain adhesives which are detrimental to the boiler and will release possibly explosive or toxic gases during the combustion process. Any timber that has been painted should also be avoided.

Problems associated with unseasoned wood include a build of tar in the boiler and flue, this tar will cause the boiler to run inefficiently even when seasoned wood is used at a later date, the tar build up acting as an insulator. When high levels of tar are present the air flow will become restricted in the boiler, adversely effecting the combustion process. When unseasoned wood is burned, a large percentage of the energy is used to boil off the moisture present, with smoke and vapour billowing from the flue, this will no doubt upset any neighbours. Smoke will come out of the flue until the boiler gets up to temperature and the gasification process starts, if smoke continues into the burn it is a sign that the wood is not seasoned and has a high moisture content. A fuel with a high moisture content will feel heavier, burn quickly and produce more ash.

Most of the problems with having a Wood Gasification boiler are down to the lack of seasoned fuel. Hopefully if you have read through the above, then you have gone some way to understand that dry wood is good wood and good wood equals good efficiency.

Where to buy the wood?

There are many places selling wood fuel; fuel specialists, tree surgeons, garages, farmers – the list goes on. Some will be seasoned, some will be freshly cut, you can purchase either but make sure you are getting what you pay for. One of the many benefits of wood as fuel is that there many outlets for supply, giving you the option to buy locally and bargain for the best price. The moisture meter supplied by Attack UK will allow you to test the wood before you buy. You will pay more for seasoned wood, use the meter to ensure the wood is as described. It is more cost effective to purchase freshly cut wood and season yourself, space permitting. Should space be at a premium however, purchasing pre-seasoned is probably the better option. A consistent supplier should mean that you know what to expect each time, and you can plan your fuel schedule, this is better than a supplier who will give you dry logs one month and wet the next.

Main considerations when purchasing:

  • How is it priced? Don’t get confused when being quoted for m3 and kg or tonnes. The most consistent way to get a price is to have quotes for volume (m3), as the volume is unaffected by the moisture content. The type of wood will affect the price, with hardwoods ( oak, beech ) being more expensive than softwoods ( birch, pine, Douglas fir )

  • Is it seasoned? You can expect to pay more for seasoned wood, if they say it’s seasoned ensure you check it on delivery. You will soon become accustomed with what is seasoned by the look, weight and feel.

  • Are the logs suitably sized? You need to check that the supplied logs will fit into the boiler. You don’t want to be spending time having to saw sections of logs to the right length to ensure they fit inside the boiler but at the same time you need to ensure maximum fill of the hopper at each loading. Typically for a maximum possible length of log of 550mm, we would recommend a length of 500mm to allow for ease of fill.

  • Hard or Softwood? There are benefits to both, softwoods tend to burn quicker and give a rapid rise in temperature but need filling more often ,hardwoods burn slower but are more expensive . A mixture of both is ideal as the softer wood will burn quickly to give immediate heat them the hardwood will provide a continuous longer burn and keep the fire alight.

  • How will it be delivered? If you are purchasing a large quantity of timber, then handling and access needs to be considered. Most suppliers will deliver with a hydraulic lift leaving you to move, stack and store. Delivering on pallets or jumbo bags is another alternative, but these will need to be stored in a dry location and may need to be man handled.