Self supply of wood-fuel

The satisfaction that can be found from growing your own food or building your own house can extend to supplying your own wood fuel. But only if the operation is well planned and realistic. To acheive a ‘payback’ on the often considerable capital investment of a woodfuel system, it will need to be operated efficiently for the life span of the appliance (20 years).

Even little things like the journey to and from log store to boiler are important considerations as they are replicated thousands of times and therefore need to be minimised if possible.

The notes below give a rough overview of some of the considerations to self-supplying woodfuel, but they are only basics and probably no substitute for a site visit from an expert.

It is perhaps worth noting here that a flexible approach to fuel procurement is likely to be more successful long term than a rigid one. Having several different sources of woodfuel beyond your own, takes the pressure off the work load and also allows for exploitation of an oportunity to buy in cheap material if it becomes available. You might start by approaching your local sawmill.

Any wood can potentially be used for wood fuel as long as it is dry. The worst quality timber makes the best sense as wood fuel. Converting trees into wood fuel that have quality and therefore value for furniture/building is not sensible.

How much wood fuel do I need?

A rule of thumb for wood fuel that appears to be useful is that under normal use for each 1 kW of boiler output rate – 0.4 tonnes of dry wood is needed per year.

Therefore:

* 5kW log burner providing space heating for a lounge – 2 dry tonnes

* 25 kW log boiler providing central heating and hot water (c/h & h/w) for a house – 10 dry tonnes

* 80 kW log boiler providing c/h & h/w for a large farmhouse with out-buildings – 35 dry tonnes

You get the idea, of course heating a swimming pool in the summer or maintaining heating for the elderly in the summer could double or triple these calculations.

As mentioned at the beginning, a realistic approach to wood fuel is likely to be more success in the long term. Scale is particularly, important.

Most people could prepare their fuel to supply a woodburner. A small chainsaw, axe and a few spare weekends will suffice. Practice and learning basic techniques will improve the efficiency of the operation and quality of the fuel produced.

Between 16-100 kW the operation is becoming more serious in terms of commitment. The source of wood is an issue that is covered later, but getting it out and preparing is also important.

As with all commodities, they are worthless unless they can be brought to the place where they are needed. For timber this is a real issue as access is often difficult, usually woodlands have been neglected for a reason.

One cost of self supply that is often forgotten is time spent in procurement.
The tasks are:

* Gain permission
* Select trees
* Fell trees
* Cut up trees
* Tidy trash
* Split/cut big logs
* Load trailer
* Deliver to store
* Unload logs
* Put into boiler 2-3 years later

Expert help will also be needed in felling large trees, and even when they are on the ground conversion to logs needs the powerful chainsaws, lifting equipment and experience.

How much wood fuel can my woodland produce?

Woodlands are hugely variable and this applies equally to their ability to yield woodfuel, as to whether there are constraints on the woodland in the form of Tree Preservation Order (TPO), Conservation area, SSSI etc there may be other designated priorities that are above and beyond fuel wood. It is important to check early on with your council and the Forestry Commission, to get the necessary formal permissions, however, the positive side is that thinning or selective felling can be beneficial and a grant may be available to help subsidise the costs of operations.

There is often confusion regarding timber measurement:

1 solid green cubic metre of timber weighs about 1 tonne, when it has been dried it only weighs about 0.6 tonne for hardwood and 0.4 for softwood.

The forestry technique for quantifying a woodland’s wood producing ability is called yield class. It is thoeretical and only an approximation, and as greatly affected by geography and tree age as species or soil type.

Each point of a yield class refers to 1 hectare of woodland producing 1 solid cubic metre (or green tonne) per year. For example:

Oak/Ash wood – could grow 4 cubic metres per annum per hectare – this would yield approximately 2.5 tonnes fuel when dried.

Conifer plantation – could grow 15 cubic metres per annum per hectare – 8 tonnes of dry fuel

A 25 kW log boiler requiring 10 tonnes of dry wood would need 4 hectares of broadleaf woodland or a little over 1 hectare of conifer, derived from thinning and potential sustainable in perpetuity. As, explained this is affected by many factors and a woodland on the north coast of Cornwall would yield very little.

In reality, many woodlands are neglected and have not been thinning much at all, therefore there is often an additional amount of timber that can be thinned from the stand without detriment, intially, before normal management is resumed.

Woodland type has a significant impact on how easy or difficult operations are going to be. Historically, when woodland work was conducted by men with axes and horse, smaller tree sizes were more manageable than massive specimen. This is perhaps true now, for the aspirant self-supply wood fueller that a coppice of ash is far more useful than a collection of mature trees.

Drying and storage

All woodfuel needs to be dry (15-20% moisture content – moisture meters are supplied with Boiler/Accumulator pack). Logs are best cut green and then allowed to dry.

In our maritime climate air drying logs/timber lengths is a challenging problem taking 1-2 summers minimum for cut and split logs to be ready and longer for entire logs. This immediately, raises the requirement of 3 shed/barns for storage, one for each year and some forward planning to ensure there are the right quantities of logs for each year.

Based on a domestic log system of 25 kW output the rough requirement for normal use is 10 tonnes of dry logs per year. In a pile this will roughly occupy a volume of 40 cubic metres, or a store 5 metres by 5 metres and 2 metres high. As 2-3 of these are required to create a drying cycle.

The log store should be close to the boiler/stove so that ‘time and motion’ fuelling journeys are minimised. However, some non-combustable barrier between boiler and fuel store is sensible.