The heat locked up in a fuel is expressed as its ‘calorific value.’ To measure the calorific value of wood in a laboratory, you take a small sample and oven dry it until all moisture has been driven off. Then you weigh it carefully and place the sample in a device called a ‘bomb calorimeter’ which enables you to burn the sample completely while measuring the amount of heat produced. With wood, the number arrived at is likely to be around 8,600 BThUs.

This is all very academic and bears little relation to what goes on when you throw a log on the fire! But the results are of interest nevertheless in that they show show us that there is close correspondence between the calorific values of all types of wood fibre. In other words you can take sap wood or heart wood from oak, spruce, beech, rhododendron, balsa, teak, pear tree – pretty well any wood you can lay your hands on – and they all turn out to match each other quite closely in terms of calorific value.

NOTE – wood and peat are both famous for giving you “two heats” – the first when you harvest and render them ready for use and the second when you burn them! Although different species of tree may deliver equal calorific values, some will always be much more cost-effective to process than others because they are much easier to saw and split. 

Different species of tree retain different levels of sap during growth, and of course many conifers produce resins which can be quite flammable. For this reason, green or unseasoned fuelwoods vary greatly in their burning properties though ash has always been admired for the heat it produces in almost any state – hence the old rhyme that ends “But ash green, or ash old is fit for a queen with a crown of gold.” In an earlier era it was normal practice to burn wood while it was still green, since dry logs burned too rapidly in an open hearth. Thus people became keen observers of how different types of unseasoned wood burned.

The interesting thing is that these observed differences all but disappear when logs are fully seasoned. And proper seasoning is what we must achieve when we run a modern closed appliance, since moisture degrades heat output severely and also plays havoc with the chimney. (See the chimney section for more information on this.)

The chart below is rather academic in that the column on the left represents a 100% dry sample (ie one that has been oven dried) and the ones to the right include theoretical deductions for the energy lost when varying proportions of moisture are boiled off. In practice, burning wet wood in a stove always produces much worse results than the figures suggest for two reasons;-

• Combustion efficiency is degraded. A fire must reach a certain minimum temperature in order to burn cleanly and efficiently. When excessive moisture is present the fire is ‘quenched’ and produces a dense pall of smoke containing unburned tars and creosotes.

• Stove efficiency is degraded. The normal human response to an under-performing fuel is to over-run the stove. This significantly increases heat-loss to the chimney.

You will incur several penalties if you persist in burning wet wood over a lengthy period of time. First, you burn far more fuel. Second, you foul up your chimney and probably lay up a fire-hazard. Third, over-running a stove shortens its life by subjecting it to excessive heat along the flue path. Fourth, you lose control because it tends to sulk or go out unless you run it continuously at ‘full throttle.’

One useful check on the moisture content of your fuel is to run the stove for a short period on some demolition timber, or wood reclaimed from a dry discarded pallet. If this transforms its performance you will know that you have more work to do to get your main wood pile properly seasoned!

Another big variation to be aware of is the varying bulk-density that occurs between different species of timber. As we have seen, spruce and other softwoods can deliver the same amount of heat weight for weight as oak when they are both fully seasoned. But the chart below shows that if you are burning mainly spruce, you will need around 49% more fuel by volume than if you are burning oak or beech.

This extra volume needs to be taken into account not just when you are storing your wood, but also when you are burning it. So if your principal fuel is likely to be softwood and you are still at the point of choosing a stove, give some thought to going up to the next biggest model above the one indicate by your heat requirement. That way you’ll get the output you want without suffering unduly short stoking intervals.

In countries with a woodburning tradition, wood is typically sold by volume. The standard unit used is the ‘cord’ which is 8’0″ x 4’0″ x 4’0″

There is little point in buying wood by weight since you would then pay more for wet wood than for dry wood!

All the same, buying by volume has its drawbacks since sections that are twisted and mis-shapen obviously don’t compact into a neat pile as readily as long, even sections of the type shown in the diagram at right. This of course offers plenty of scope for haggling, but experienced buyers can generally “guesstimate” what is on offer with an acceptable degree of accuracy. This approach corresponds roughly to the Forestry Commission’s practice of buying and selling timber by cubic measurement.

As was noted earlier, hardwoods and softwoods have different densities and can be expected to fetch different prices when bought by the cord.

Once you have your wood, the most important thing is to get it fully dried out or ‘seasoned.’ The procedure is to saw and split it to increase the surface area, and then store it off the ground under shelter but exposed to as much wind as possible.

A polythene tunnel, open at both ends works particularly well since it picks up a useful amount of heat during the day time while allowing plenty of ventilation to pass through. The tunnel can be floored with pallets and/or paving slabs placed over a sheet of black polythene to act as a barrier against moisture and weeds.

In the picture below, the fuel is stored within a simple wooden frame. The bars to the front are removable so that they can be added or removed to suit the height of the woodpile.

In the table below, wood has been allocated an arbitrary value of 1 so that we can compare other fuels with it directly. Note that when wood is converted to charcoal it effectively becomes pure carbon and has a calorific value equal to the finest quality coals.

The calorific value of cattle dung is virtually identical to wood, making it an important energy source in the Third World. This is unfortunate since when fuel is scarce (as is often the case,) it gets burned instead of being returned to the land to improve soil fertility.

Comparing fuel energy values (for the purpose of comparison, wood in its dry state is taken as having a value of 1.)