Hardwood Logs or Softwood Logs
For the purposes of firewood, hardwoods are most commonly oak, ash, elm, beech, birch, sycamore, aspen, cherry and alder. Of course there are many more hardwood species.
These are broadleaved deciduous trees and they tend to be slower growing and the wood is more dense and heavier than softwoods.
Softwoods used for firewood in the UK are mostly spruce, pine, fir and larch (There are several sub-species of each).
Softwood trees tend to be faster growing and the wood is less dense and lighter than hardwoods. The wood has more resin which may spit when burnt, so it is not really suitable for an open fire, but when dry, is perfect for stoves. The resin adds to the calorific value.
Hardwoods and softwoods have almost identical calorific value for a given moisture content and weight.
The difference to note is weight: the average density of UK hardwoods is about 700kg per solid cubic metre whilst the average density of softwoods is about 500kg per solid cubic metre. This means that you need about 1.35 cubic metres of softwood to be the equivalent of 1.0 cubic metre of hardwood.
So, there is no difference in calorific value (weight for weight, mc for mc) between hardwood and softwood, it’s just that you need about 30% more volume of softwood than hardwood for the same heat output.
Kiln Dried Logs
There are a few businesses kiln drying logs in the UK, but the majority of kiln dried logs sold in the UK are imported from Europe. Some of this is very good and of excellent quality and consistency. Clearly logs of this quality are more expensive than you would be paying your local log supplier.
Trees should be felled over winter whilst the moisture content is lowest. They are then processed into logs and stored in open mesh log bags or wooden crates inside open sided barns where the natural movement of air dries them during summer months ready for use the next winter.
Beware of logs that are being processed on a just-in-time basis for sale immediately. Their moisture content is sure to be between 28% and 50%.
Fresh Felled Timber
Trees are felled over winter when the sap (water) within the tree is at its lowest. The moisture content over winter is usually between 30% and 50% so it is necessary to reduce the moisture content to around 20%.
Recently felled trees are hopeless for burning as the fire has to use lots of energy just to dry the wood before it will burn properly.
Wet firewood can be extremely damaging when mixed with coal. Most coal and smokeless fuel has a high sulphur content. When water from the wood combines with sulphur, then sulphurous acid forms on the cooler surfaces within your system leading to rapid damage of metals.
Damp wood burnt alone leads to soot deposits in the chimney, blackening of the stove glass and much more wood is required for the same level of heat.
After 200 years of burning fossil fuels in the UK, wood is at last moving back into mainstream use for space and water heating.
This is greatly encouraged by the Government’s Renewable Heat Incentive (RHI) that pays householders to produce renewable heat.
In order to maximise the calorific value of wood fuel, and therefore its effectiveness, it is essential that the supply is consistent in moisture content and size.
HETAS and Woodsure have just launched a Quality Assurance Scheme for biomass based on the Draft European Standard. Firewood log production can be assessed under this scheme and that will give customers confidence that their HETAS assured supplier consistently produces wood fuel to exacting standards required by modern biomass stoves and their ever more informed and discerning owners.
Burning fuel is all about calorific value, and in particular the cost in terms of pence per kilowatt hour (p/kWh). Given this figure you can compare value between fuel products and suppliers.
Wood Briquettes: Modern wood burning stoves and open fires work exceptionally well with wood briquettes. If you can buy good quality briquettes for a similar cost to firewood logs (p/kWh) then briquettes will give a hotter, longer, more controllable burn, be easier and cleaner to handle and will take up less space to store than logs. What’s more, most briquettes are made from softwoods and it is far better for the environment if we burn a sitka spruce tree than an oak tree.
Here are some of the more common sources of confusion:
One tonne builder’s dumpy bag: It’s not a tonne of wood you get, as some suppliers claim, more like 220kg if it’s hardwood dried to 20% moisture content, and it’s not “nearly one cubic metre”, more like 0.56/0.75 cubic metre.
Crate measuring 1m x 1m x 1.9m: That’s the outside measurement but you don’t get 1.9 cubic metres of firewood, more like 1.6 when you measure the internal stack.
Truck, pickup, trailer load: It’s open to interpretation as to how much you’re getting. It may be good value but it’s taking a lot on trust, so assess it carefully. At least there is no packaging, but now you have to carry and stack.
Seasoned for one/two years: Perhaps, but what is the moisture content (mc)? Under 20%mc and it is ready to burn now. Over 25% and the calorific value starts to plummet, and the steam to rise.
Burning damp or green wood is very bad for your stove and can cause the flue to tar up quickly.