Innovation

Firewood

 

A ROUGH GUIDE TO WORKING OUT THE COST OF HEAT WHEN BUYING SPLIT FIREWOOD LOGS

 

Many people use logs to provide at least a proportion of their home heating. Whilst some might be sourced for free, it is often necessary to purchase it from a local firewood merchant. This may come in a relatively informal way, by the trailer load or dumpy bag and sometimes by the tonne. Sadly, some claims that your load is ‘a tonne’ may be grossly exaggerated- a cubic meter of dry softwood may only weigh 1/5 of a tonne.


The Purpose of this chart is to provide a means of working out the cost of the heat that you are getting from your firewood. This information may be useful as a way for the cost conscious customer to understand what they are getting from different suppliers and to be able compare different heating fuels. The supplier, too, can use it to work out how much fuel he is delivering.



Unfortunately the calculations are complicated by the moisture in wood - between 20 and 50% by weight. This adds to the weight and reduces the useable energy because up to 20% is used to evaporate the water. It will also burn badly and foul the chimney - it is essential to properly dry wood first. Moreover, softwoods are significantly lighter than hardwoods, and also the amount of heat given out by different heating appliances varies enormously (as low as 35% for a poor open fire and up to 85% for a good boiler).



How to use the chart (it's not as hard as it looks!)

 

firewood3.jpg

If you are buying as a loose load of split logs:

Measure/estimate the volume of the load and work out what you are paying for a cubic metre.

 

 Ask the supplier what species of timber are in the load, hardwoods or softwoods, heavy species such as oak, beech or ash, light species such as spruce or poplar.

 

If you wish to calculate the price per tonne you will need to know the moisture content - 20%, 35% or 50%: see below, otherwise the price per kWh is based on 35% moisture.

 

On the chart, above 'Firewood price, £ per cubic metre' select the appropriate timber species

 

 Locate the price you paid per cubic metre and move horizontally to the right to the red column and read off the cost of energy using a log stove at 70% efficiency (or 35% for an old open fire - in brackets). Add 5% for timber at 50% moisture, subtract 5% for 20% moisture.

 

 Continue to the right blue columns to find how much you paid per tonne (if you wish to know this), according to the moisture content.


Example: From the chart, hardwood logs at 35% moisture content costing £40 per cubic metre are equivalent to £95 per tonne and 4.3 pence/kilowatt hour in an efficient stove, over 8 pence in an open fire.



If you are buying logs by the tonne (you should be certain that they have actually been weighed):

 

Locate the price in the blue £/tonne columns depending on the moisture content.

 

Move to the left horizontally and read off the cost of energy using a log stove at 70% efficiency (or 35% for an old open fire - in brackets). Add 5% for 50% moisture,  subtract 5% for 20% moisture.



In conclusion, firewood can be a sustainable, low-carbon fuel and you can't beat a cosy log fire. However, unless you are getting your firewood for free, the heat may not be particularly cheap. For comparison, domestic electricity costs 12p per kW hour and heating oil costing 40p/litre in an 85% efficient boiler will cost 4.6p/kWhour (see www.coedcymru.org.uk/calculator.htm ). Perhaps you should source hardwood instead of softwood logs, or upgrade your heating appliance - most old stoves are a lot less than 70% efficient. In any case, you should purchase the logs one to two years in advance and dry them thoroughly.

 

Notes. 2. The solid mass content of loose split logs compared to solid wood was estimated as 0.45 (from 'Purchase and Sale of Wood for Energy Production' in 'Technology-Environ-Economy, Centre for Biomass Technology' (Denmark) 2002. ISBN 87-90074-28-9). This is roughly similar to the figures from www.woodfuelwales.co.uk and www.biomassenergycentre.org.uk For neatly stacked logs, divide the price per m3 by 1.7. The solid wood densities were taken from the TRADA Wood Information Sheet (1999). The 'Softwood' mean density was taken as 423kg/m3, the 'Hardwood' mean 700kg/m3 and the intermediate group mean 534kg/m3 all at 15% mc. Sycamore and cherry densities are 630kg/m3 so you need to extrapolate between the hardwood and intermediate columns on the chart. The wood will never be supplied oven dry and is unlikely to be air dry (20%). The presence of moisture does not significantly alter the volume of a piece of wood. The moisture is taken on a wet basis, i.e. % of total weight, not on a dry basis as in the timber industry. In reality, most is not likely to be supplied less than 30% moisture. 3. Freshly felled timber is around 50% by weight of water. Timber well dried outdoors (up to two years drying) may reach 20% moisture but not many suppliers provide this. Without a moisture meter it is hard to know the moisture content although the supplier may indicate how long ago the timber was felled. The figure of 35% has been used, with +/- 5% pence/kWh for 20/50% moisture. Even though the chart shows the pence per kWh for wood at 50% moisture you should not try to use this as it will burn badly and make smoke. It is important to stack all firewood well, under cover in an airy location and dry for at least a year. (For further explanation of calculations used here see www.coedcymru.org.uk/calculator.htm ) 6. The ability of appliances to burn the fuel and deliver heat ('the efficiency') is highly variable. We have chosen 70%, as a number of modern stoves according to Hetas should be around this figure. A good wood fuelled boiler may be 85% efficient but an open fire may only be 35% efficient, which will double the energy cost as shown in brackets. For different efficiencies, multiply the p/kWh by 70 and divide by the new efficiency. Also, it is important to operate the stove optimally, according to the instructions, with dry wood. In reality, it may be hard to know how much heat a stove is actually delivering. Whilst every effort is made to ensure that the information provided in this leaflet is correct, Coed Cymru advises that you should obtain independent confirmation before making any financial commitment. Coed Cymru 01686 650777 12th November 2008. This leaflet was produced with the aid of funding from Calu, Bangor.


Whilst every effort is made to ensure that the information provided in this leaflet is correct, Coed Cymru advises that you should obtain independent confirmation before making any financial commitment. For further information please contact Coed Cymru, 01686 650777
(OR see
Staff Page for email addresses and officers' telephone numbers).