A rough guide to calculating the cost of heat when buying 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 supplies from a local firewood merchant. This tends to come in a relatively informal way, by the trailer load or dumpy bag and sometimes by the tonne, the moisture content may vary between 20 and 50% of the total weight, and hardwoods can be much denser than softwoods so it is hard to know how much energy is being bought with each load.

The purpose of this leaflet is to provide a means of:

estimating the cost of the heat that is being delivered by the stove or boiler.

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 wood he is delivering.

Unfortunately the calculations are not straight forward because of the large amounts of moisture that can be present in wood. Not only does this add to the weight but significantly reduces the useable energy because up to 20% is used to evaporate the water.

The weight of a cubic meter of randomly arranged split logs (the usual method of supply) will depend on the moisture content and whether they are hardwoods or softwoods. (See Appendix 1)

Moisture content(% Wet basis)	Hardwood Weight of 1m3 split logs (tonnes)	Hardwood Energy MWh per m3	Softwood Weight of 1m3 split logs (tonnes)	Softwood Energy MWh per m3
0 (oven dry) 20 (air dry) 30 40 50 (newly felled)	0.28 0.35 0.40 0.47 0.56	1.41 1.35 1.32 1.27 1.20	0.16 0.20 0.23 0.27 0.32	0.80 0.77 0.76 0.73 0.68

The moisture content, by weight, may vary between 20% (not very common) up to over 50%. Unfortunately, it is hard to know what the actual moisture content is without a moisture meter or drying and weighing some, so using 30-40% as a rough guide will do, but the supplier may be able to indicate how long since the timber was felled. It is always advisable to stack and air dry the logs for as long as possible (a year or more) after delivery so as to improve the burning quality.

If firewood is supplied by the trailer load then it will be necessary to know the volume of the trailer to calculate the weight of each load. So if you receive a trailer load of 1.5m3 of logs at 30% moisture costing £51 then the cost of 1 tonne of logs is:

Cost of the load of logs = £85/tonne (£51)

Volume of load (1.5m3) x 0.4 )from table, column 2, for 30% moisture)

[The solid mass of dry timber in 1m3 of loose logs may seem relatively low, especially if the customer was under the impression that they were getting a half or even one tonne.]






The total heat available from 1m3 of loose logs is also shown in the table, assuming that 1kg dry wood equals 5.36kWh and subtracting the heat that will be used for free water and combustion water evaporation.

Estimating the cost of heat that is actually obtained from the firewood will depend to a large extent on the efficiency of heating appliance in terms of how much heat is being delivered into a room or into the hot water of a boiler. This is a difficult area for log burning stoves and it is not easy to know how much heat is going up the chimney and how much into the room. With open fires the efficiency may be as low as 35% but with some well designed stoves it could be over 70% under optimal conditions. The manufacturer might be able to help, but beware that their figure does not simply refer to combustion efficiency. Purpose built log boilers can be very sophisticated and the manufacturer may be able to provide reliable information. Remember that high moisture content will significantly reduce efficiency.

The following formula has been developed from the spreadsheet that was used for the Coed Cymru Heat Cost Comparator and gives the heat cost in pence per kWh delivered heat from the price of logs supplied (£/tonne) at different moisture contents and different appliance efficiencies. The absolute calorific value of oven dry wood is taken as 5.36kWh/kg

First work out the price per tonne from the first table. Then use this formula:

Pence/kWh = C(from table) x £/tonne

% efficiency of appliance

.

Moisture Content of Wood %	C
10 20 30 40 50	2.25 2.58 3.01 3.66 4.67

For example: From the first table, 1 cubic meter of hardwood at 30% moisture costing £34 is equivalent to 0.4tonnes or £85/tonne. If the stove is delivering heat at 60% efficiency then the cost of heat is:

			3.01 x £85 = 4.3 pence/kWh
			60%

For comparison, electrical heat currently costs over 10p/kWhour and heat from an 85% efficient oil boiler costs around 3.5p/kWh. For the supplier, producing firewood logs and selling in small loads is labour intensive and relatively costly. For the customer, logs in reality may not be a cheap source of heat but a log stove can be an attractive household feature, there is often the opportunity to source free firewood and wood fuel should be environmentally sustainable and will displace carbon emissions from fossil fuels. A modern log boiler should deliver much better efficiencies and run for longer unattended but the capital outlay will be higher.

Appendix 1. Methodology for determining weight per cubic meter of split logs. The solid mass content of loose 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.woodfuelswales.co.uk and www.biomassenergycentre.org.uk The wood density for hardwoods was taken as 0.63 tonnes/m3 dry weight from 'Handbook of British Hardwoods' R.H. Farmer 1972, HMSO and assumes locally grown timber (Wales), predominantly oak, beech and ash. Hence the weight of 1m3 loose oven dry split logs is 0.45 x 0.63 = 0.25 tonne. Softwoods are considerably lighter, and using Sitka spruce (the main softwood grown in Wales) with a dry density of about 0.36 tonnes/m3 ('A Handbook of Softwoods', Building Research Establishment, 1977 HMSO) the weight of 1m3 of loose oven dry split logs is 0.45 x 0.36 = 0.16 tonnes. The wood will never be supplied oven dry and is unlikely to be air dry (20%) so the weights have been adjusted accordingly in the table. 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 timber is not likely to be supplied at less than 30% moisture. If the logs are neatly stacked then weights of logs per cubic meter in the table should be multiplied by 1.7.

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 Andy Stewart at Coed Cymru, 01686 650777
(OR see Staff Page for email addresses and officers' telephone numbers).

Coed Cymru, The Old Sawmill,
Tregynon, Newtown,
Powys SY16 3PL
Tel 01686 650 777

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