The heat produced from burning firewood is dependant upon moisture content, timber density, and combustion efficiency. We use only hardwood for the best timber density and season it under cover for the best moisture content. Combustion efficiency depends on what appliance you have and how you use it. The forestry commission estimates "a seasoned solid m3 of oak timber has a similar energy value to 280 litres of heating oil."

Moisture content
The moisture content of wood has by far the greatest effect on its heat value. Unseasoned "green" logs contain more water. It takes a lot of heat to boil this water and even more heat to then turn it into steam. Much of the logs heat value is then wasted as this steam goes up the chimney instead of heating the room. This will reduce the net energy released as useful heat.

If you can get them to light at all, logs that aren’t dry will result in a fire that smoulders and creates lots of tars and smoke. These tars can be corrosive, potentially damaging the lining of the flue and increasing the danger of a chimney fire. Wet logs will tend to blacken glass in stoves even if the stove is designed to keep the glass clean. Well seasoned logs can have approximately twice the Calorific Value of green logs.

You should always take care to burn only dried (seasoned) wood, either by buying it dry, or by buying green logs and drying them yourself. Radial cracks and bark that comes off easily suggests well-seasoned wood.

Drying
The calorific value of ‘fresh cut’ or green logs is usually very low and it is important that logs are adequately seasoned before use. Burning insufficiently seasoned logs is not only inefficient, but also leads to excessive smoke and deposits in the flue, including both soot and tars. These tars can be corrosive, potentially leading to damage to the lining of the flue which may not be noticed. There is also the danger of a chimney fire.

The graph below shows the significant difference in calorific value between wet and dry wood.

Green logs will be typically between 50 and 60% moisture and well seasoned logs may end up around 20-25% or less. This can more than double the amount of available energy in the fuel.

Green logs are cheaper to buy than seasoned logs, but will be heavier and will require space to stack and dry before use. For drying it is recommended that roundwood timber is stacked off the ground (on bearers) in a windy, sunny, location under some form of waterproof cover with open sides ideally for at least two years, which can bring the moisture content down to about 20-25%. Radial cracks and bark that comes off easily are signs of well seasoned timber. Alternatively drying can be speeded up by cutting and splitting the logs into firewood sizes and storing in a dry ventilated place. With this method radial cracks are much reduced, the bark may remain attached and the logs retain a more pleasing natural appearance.

Timber Density

Hardwood and Softwood Trees

I have come across the following data from the Forestry Commission which shows the usual moisture content and resultant energy content of some freshly felled forestry timber species before being dried for firewood.

Species	Green density   kg/m3	Oven-dry density   kg/m3	Green Moisture Content (wet basis) %	Density at 30% moisture content kg per m3	Energy value at 30% moisture content kWhs per m3
Hornbeam		570	42%	814	2,850
Oak	1060	560	47%	800	2,800
Beech	1030	550	47%	786	2,750
Ash	780	530	33%	757	2,650
Birch	930	530	43%	757	2,650
Broadleaved baseline (excluding Poplar and SRC Willow)	943	515	45%	736	2,575
Cherry (European)		500	44%	714	2,500
Sycamore	830	490	41%	700	2,450
Lime (European)		440	34%	629	2,200
Elm	1030	430	58%	614	2,150
Alder (Common)		420	48%	600	2,100
Sweet Chestnut		440	55%	629	2,200
Conifer baseline (excluding Grand Fir, Noble Fir)	927	386	59%	551	1,930
Poplar	900	360	60%	514	1,800
Willow (Crack)		350	50%	500	1,750

Plotted as a graph it shows why hardwood is better than softwood as it is dryer when fresh. Ash clearly shows its position as the best firewood and another hardwood, poplar, is at the bottom. Not all hardwoods make good firewood so I avoid using any poplar or willow. But remember this data is not for dried timber. I am hopig to source some similar data for seasoned or dried timber.

The general differences are that hardwoods (deciduous, broadleaved tree species) tend to be denser than softwoods (evergreen, coniferous species) which tend to contain more resins. There is little difference between the Calorific Value per Dry Tonne of different species when tested at the same moisture content. The main difference in CV between species is the moisture content when the timber is green, and the rate of drying.

Wood density
As hardwood species are generally denser than softwood species, a tonne of hardwood logs will occupy a smaller space than a tonne of softwood logs. So a bulk bag of hardwood logs will contain more heat value than the same bag filled with softwood. Dense woods tend to burn for a longer period of time than softwood meaning fewer ‘top ups’ are required to keep a log stove burning for a given length of time. If you measure wood by volume you will receive more kilo-Watt hours (kWh) of heat from a cubic metre (m3) of hardwood than softwood, though this will tend to be offset a little by the slightly higher calorific value of many softwoods.

Open Fires
An open fire is the traditional way to burn logs, it can also be attractive and cosy. It is however, a very inefficient method of heating, as the uncontrolled air flow takes not only the hot air from the fire up the chimney, but also draws centrally heated air in from the rest of the house as well. This is replaced with cold, outdoor air drawn into the house through drafts and vents. Often an open fire will run at very
low efficiencies (.25%) resulting in large amounts of smoke and ash for very little heat output. Remember that when an open fire is not in use then the chimney will allow large amounts of cold air into the room, so an open fi re may well be increasing other fuel costs.

Log Stoves
Traditional log stoves provide radiant heat to a single room. They offer an attractive, renewable, low carbon heat source that can be cheap to buy and to run. They achieve significantly higher efficiencies than open fires (.70%), lowering fuel requirements and less ash. Log stoves need a flue if there is no lined chimney present. Many stoves are also available with a back boiler option, allowing the generation of hot water to heat radiators or provide domestic hot water.

Efficient stoves
Traditional log stoves provide radiant heat to a single room. They offer an attractive, renewable, low carbon heat source that can be relatively cheap to buy and to run. They achieve significantly higher efficiencies than open fires (.70%), and this results in a lower fuel requirement (and fewer trips to remove ash). Choose the smallest fire box you can for your heat requirement (the installer should be able to help you with this), with controlled hot secondary air, and ash retained in the base of the fire. Operating a stove with doors open considerably reduces efficiency. An insulated chimney is essential. Whenever water vapour is in the chimney, the temperature at its outlet must be above 100'C to avoid water condensing. Visible smoke emission from the chimney is a sign of inefficient combustion. Log stoves are also available with a back boiler to provide hot water, but this usually reduces the efficiency.

After adding fuel, set to fast burn ensuring all of the gasses are fully burnt. Only set to slow burn when all the wood has been reduced to charcoal and ash. Newly added wood set to burn slowly will create smoke and tarry deposits in the chimney. Users of older stoves are advised to consult their stove centre about current recommendations on how to achieve the best results stoves should not be banked up with logs overnight. A bright fire which has turned wood into charcoal should be left with the day’s ash, no secondary air and minimal primary air.

Nornal Operation burning wood plenty of air fast efficient burn no smoke

Over Night burning charcoal restricted air slow efficient burn no smoke

Over Night burning wood restricted air slow inefficient burn lots of smoke

Smoke Control Areas
Logs can be used as fuel in smoke control areas except if burnt in officially approved “exempt appliances”. The website www.uksmokecontrolareas.co.uk gives lists of Smoke Control Areas, and the appliances approved for use in them. The problem is that the high cost of becoming "officially approved" means that many stoves that would comply are not submitted for official approval. It is not permitted to burn wood on an open fire in these areas.

Net carbon costs of woodfuel:
All traditional fuel (i.e. excluding nuclear) releases carbon dioxide (CO2) when it is burnt.
However, the net CO2 released by burning sustainably produced wood is considerably less than the CO2 released when fossil fuels are burnt:
Net CO2 emissions by fuel type

Fuel type:	Life cycle CO2 emission:
Wood	7 kg/MWh
Natural Gas	270 kg/MWh
Oil	350 kg/MWh
Coal	480 kg/MWh
Electricity	530 kg/MWh

Sustainably produced wood grows at least as much as is harvested each year.This way CO2 is taken out of the atmosphere and stored in the wood at least as much as is produced when the wood is burnt. Some CO2 is produced in harvesting, processing and transporting the logs. By burning sustainably produced wood instead of using fossil fuels CO2 emissions can be greatly reduced.

USEFUL FACTS AND FIGURES:
1m3 of wood (standing or recently felled) comprises about 50% water (by total weight)
= approximately 1 tonne of unseasoned/fresh/wet wood
= approx. 0.72 tonnes of seasoned wood comprising about 30% water
= about 3m3 of loose woodchips (by volume)
= about 2,500kWhrs of usable heat energy for broadleaf wood
or about 1,800kWhrs of usable heat energy for conifer wood
= about 1.4m3 of stacked firewood logs or 2.5m3 of loose logs (In reverse
1m3 of stacked logs = about 0.7m3 of solid wood and 1m3 of loose logs =
about 0.4m3 of solid wood).

* Data and information sourced from Forestry Commission leaflets.

Links

www.biomassenergycentre.org.uk

www.uksmokecontrolareas.co.uk

www.forestry.gov.uk/woodfuel

www.logpile.co.uk