When you chose to browse the Linnorie Firewood Services website, you were probably looking for solid fuel (logs) to keep your home cosy during the dull and cold winter days.
Wood is a natural and with 400,000 years of usage the oldest fuel. Compared with other fuels produced from wood (chip, pellets, briquettes) logs are the most natural one and the best value for money (£ per energy unit [kWh]).
But it is a non-homogeneous material. Dependent on type of wood (species), where it grew or weather conditions the characteristics are never exactly the same. But in some of the rules you can trust at any time.
Below are some facts so you get the right logs and the best value for your money:
Pro's of Softwood = con's of Hardwood
SW from a producer (owner of woods / forest) costs around
half per tonne compared with HW
HW grows more slowly
HW is less regularly shaped and has more branches which reduces
automation and makes harvesting and processing more labourious
SW-forests can be harvested in a more mechanised way
SW has per weight the higher caloric value (see caloric value below)
which means more engergy per kilogram
SW seasons more quickly - typically within 1 year under a well-ventilated shelter
SW is easy to light
SW burns at higher temperature which means
more complete combustion: breakdown of phenols (wood tar) into combustible components
more complete combustion means also less ash to be removed. Our wood gasification boiler reaches 1100° Celcius in the burning chamber and heats 4 houses. It only needs to be de-ashed approximately 12 times per year with one tray of ash
Far less build-up of waste substances in the fire, flue and chimney and hence fewer of the problems associated with this.
SW is the best value for money for all systems with controlled airflow, such as stoves and log boilers or when you just want to make a room cosy for the evening. If you are not sure please ask.
Pro's of Hardwood = con's of Softwood
HW has the higher density for a given moisture content
which means more kilogram per volume
HW has a longer burning period
HW does not spit or send hot cinders out into the room.
HW is the right fuel for systems with uncontrolled airflow, such as open fireplaces, or poor heat storage capacities of either the heating system or the structure of the room / house
SW is the right choice in all the other cases for money-saving heating.
Before some of the wood type specific values are listed, LFS is explaining some of the key variables:
Conversion Factor (CF)
Moisture Content (MC)
Density (@ x% MC)
Hardwood (HW) or Softwood (SW)?
1. Conversion Factor (CF)
The conversion factor expresses the change in volume between
the three standard forms of wood as determined by the air gaps:
The CF is a variable of
the length of the logs. Long ones have bigger air gaps
the way a person stacks the logs or levels loose logs in a bag
the shape of the logs. HW has bigger air gaps.
A block of 1m3 of solid wood expands as a rule of thumb to 1.5 to 1.6m3 when it is stacked and 2.5m3 when it is loose (e.g. in a trailer). Conversely, when you get a trailer load of loose logs, 1m3 "shrinks" to 0.6m3 when you stack it in your shed or shelter and is only 0.4m3 (1/2.5) worth of solid wood.
LFS uses a whole range of conversion factors depending on the variables mentioned above.
Example: The LFS tipping trailer properly filled (5.6m3) with 500mm logs will be 2.7 stacked pallets (3.5m3) and because there is still around 28% of air gaps between the logs, this load is worth 2.2m3 of solid wood.
Apart from these facts wood also shrinks during the drying process (see next paragraph).
2. Moisture Content (MC) - measure it yourself without instruments!
Water does not burn! Therfore the MC of split logs should be below 20% (air dry). LFS tries to season the products to 15 - 17% (wet basis). Damp wood:
can cause the fuel chamber to rust through
can damage the flue
results in lower heating energy output, because after lighting a fire the first heat produced is used to evaporate the remaining water (see below).
The water in the wood is either so called "bound water" in the fibres (the cells that run the length of the tree like straws) or "free water" filling up the cavities between the fibres. During the drying process first the free water evaporates. Only when the cells start to lose bound water, wood starts to shrink. This is the case when the MC gets below 25%. Air-dried SW shrinks about 5 - 6% while HW shrinks 6 - 9%.
Wood placed in an environment with stable temperature and relative humidity will eventually reach a defined MC. This point is called the equilibrium moisture content (EMC). Wood dried indoors will eventually stabilise at 8 - 14% moisture content; outdoors at
12 - 18%. There are graphs on the Internet to read the exact EMC for a given temperature and relative humidity.
For SW a MC < 20% can be reached within one year when it is in a sheltered and well-ventilated store. For HW it takes at least 2 years.
The MC is measured in two ways:
The method "MC wet basis" (Mar) expresses the percentage of water over the weight of the log. It is calculated as follows:
Ww - Wo Ww - Wd x (100 - EMC) / 100
Mar = ------- x 100 = --------------------------------- x 100
Ww = Weight wet = mass of wood to be measured for MC
Wd = Weight dried = mass of wood dried in a warm room until it does not lose any further weight
Wo = Weight absolutely-dry = mass of wood dried at 103° Celcius until it does not lose any further weight.
For a short time a piece of wood treated like this does not contain any water
Ww - Wo = mass of water
Mar is the MC that needs to be declared for RHI / BSL. LFS measures the probes for all the customers. The EMC for the conditions in our boiler room (temperature, rel. humidity) where the samples are dried is 8% (Equilibrium MC).
The method "MC dry basis" (Ud) expresses the percentage of water over the dry weight of the mass of fuel. It is calculated as follows:
Ww - Wo Mar
Ud = --------- x 100 = ----------- x 100
Wo 100 - Mar
On the Internet you will find tables showing the relation between Mar and Ud. Make sure you are always given the Mar-value.
3. Density - a variable of the type of wood and the MC
The density is the mass per volume [kg / m3]. The density of wood is a variable of
the type of wood
the Moisture Content (MC).
That is the reason why whenever a density is mentioned, the MC must be shown jointly
(e.g. densities based on absolutely dry (p0 = 0%) wood).
The most popular MC is 12% because that is the EMC for normal heated inside rooms. The density of fresh-felled oak is approx. 1000kg/m3. The standard density p12 for oak = 670kg/m3.
The most popular hardwood is beech, followed by ash, birch, oak. The most popular softwood is spruce followed by pine and larch. The following table shows the values p12:
4. Caloric Value and Energy
This is the final variable. The key question is:
"How much energy will my log supplier sell me for the price he is asking?"
So at the end of the day the important thing is always to determine the absolute dry weight of the delivery and then to multiply it with the caloric value for Mar = 0%. But there is a more practical method to calculate the energy as absolute dry only exists in laboratories. With every % of Mar the energy is reduced by 0.68 kWh, as this energy is used to evaporate the water.
In the table shown you can see that 1kg of softwood has more energy than 1kg of hardwood!
(100 – Mar) * max. Caloric value – Mar * 0.68
Caloric value [kWh / kg] = ------------------------------------------------------------------
Example: softwood, moisture content wet basis - Mar = 20 %
(100 – 20) * 5.2 – 20 * 0.68
Caloric value [kWh] = 4.02 = --------------------------------------- -> a reduction of 22.5 %
Just to remember: when you buy a tonne of roundwood it will only be approximately 650kg of firewood without the mud or soil from the transport, the produced sawdust, fallen of bark and the evaporated water.