Are you trying to find the best traditional logs for your wood-burning stove? Buying logs can be a minefield, so here’s our guide to knowing what to look for from your log supplier.
Guess the weight?
This ‘tonne bag’ of mixed logs (20% moisture content) only weighs about 200kg.
Logs are sold by volume, so it’s virtually impossible for you to compare logs with any other form of energy…AND you pay for water!
A ‘builder’s bag’ of logs looks like a lot of energy but it’s all smoke and mirrors. The logs only weigh about 200kg.
In terms of calorific value it takes 5.6 ‘builder’s tote bags’ (measuring 82cm x 82cm x 82cm) or 4.2 full cubic metre bags, of mixed hard/softwood (dried down to 20% moisture content) to equal one pallet (960kg) of our wood briquettes.
Not only that, but briquettes are clean, consistent, dry, hot, easier to handle, stack and store, are cheaper than logs and have no spiders.
There is absolutely no question that in terms of heat output, briquettes are better value than logs.
If you do decide to burn logs instead of briquettes, be an informed buyer. Here’s how!
The firewood log supply market is largely unregulated and serviced by many small, often part-time suppliers.
Most sellers have absolutely no idea of the moisture content or weight of their product, and care even less. So long as there is a market out there, they will supply.
There is considerable inconsistency in product description: load size, weight, calorific value and cost per kilowatt hour are all important issues that the supplier and the customer need to know.
When comparing prices between suppliers it is essential to compare like with like. However, due to many suppliers’ lack of clarity this is often impossible.
WOODSURE and HETAS are the official bodies recognised by the Government to approve biomass solid fuel and bring clarity and consistency to the market place.
“In 2011 HETAS launched an industry leading biomass fuel quality scheme covering logs, wood chip, pellets and briquettes. The scheme will enable the public to easily identify quality wood fuel suppliers, increasing customer confidence that there is a reliable supply chain for a safe fuel that will burn efficiently within their appliance.”
So, what should you look for?
Wood contains moisture (water). Moisture content (mc) is a means of expressing the measured amount of water contained within the pores of any product, wood, coal, grass, etc. It is expressed as a % figure.
Moisture content is the key to quality firewood; the lower the better.
The more moisture in your wood the slower it burns, the more smoke and pollutants it gives off and the quicker it soots up your stove’s glass and chimney. Burnt together with fossil fuels like coal, damp wood creates acids that quickly corrode your stove and any metal within your chimney.
All stove manufacturers prefer you to burn wood with a moisture content under 20%.
Dry wood substantially increases the life of a stove, it runs cleaner and hotter and greatly reduces the cost to heat a given room area or volume of water.
New felled timber ranges from 40% mc through to about 60% mc. An average of 50% mc means that half of the wood weight is water.
Seasoning wood (for around one year) should lower the moisture content to between 20% and 25% depending on ambient conditions.
Kiln drying should lower the moisture content to under 18%.
Wood left to season outside can only become as dry as the ambient weather conditions allow. Therefore, wood air dried in the dry south east of England will be drier than wood air dried in the wet north west of Scotland. Wood can only become as dry as the surrounding atmosphere.
Make quite sure you are comparing like-for-like between suppliers, especially regarding quantity and seasoning.
Sub 20% moisture content is good; 20% to 30% – store for weeks; 30% to 50% – store for months.
Always try to bring logs indoors for a few days to ‘house dry’ before burning.
Firewood logs are sold by volume rather than by weight. This is because the wetter the wood, the heavier it is, but the lower the calorific value. You need to have a high calorific value consistent with reasonable cost, so you need dry wood, which is therefore light in weight.
However, the conundrum for log retailers is that they have to buy their raw material (unprocessed logs) from the forest by weight, not volume. So, they might buy one tonne of wet, newly felled logs with a moisture content of 50% for, say £50.00 delivered into their yard, then they have to process it into cut and chopped logs. They should really allow it to dry down to 25% before they sell it to you, but that means they have to sell you two tonnes of what they bought originally to be equal to one tonne dried down to 25% moisture content. This is getting complicated!
So, the log man has to buy two tonnes of wet wood in order to sell you one tonne of dry wood. He has to process, bag and store the wood until it is dry enough to burn. The one tonne you are buying has cost the log man £100 delivered into his yard, plus processing costs, plus drying time of six months when his money is sitting idle (or he is paying his overdraft). So, for anyone out there thinking they are buying a tonne of dry logs for £60.00. Think again! You are either buying wet logs for about £120.00 tonne or dry logs for about £300.00 tonne.
The volume measurement used in the industry is cubic metres (m3) for large loads or bags and litres (ltr) for small bags.
Not all suppliers appear to know what constitutes a cubic metre. A cubic metre is 100cm x 100cm x 100cm.
A cubic metre is not a builder’s one tonne dumpy bag! That is less than 0.75 m3, and what’s more, a one tonne dumpy bag does not hold one tonne of wood, more like 200kg (1/5th tonne) assuming the wood is well seasoned and ready to burn. If it is heavier than that, it is too wet.
Also, remember that we are talking about a loose cubic metre, not a tightly stacked cubic metre. There are lots of gaps between the logs, which clearly affects how many logs you’re actually buying.
One loose cubic metre will stack into approximately 0.60 cubic metres of space.
You need to know the weight simply to be able to work out the calorific value of the logs with which you have been supplied. The lighter and denser the logs, the higher the calorific value.
By working out the calorific value in kWh you are able to make a direct cost comparison against other fuels; wood or fossil. Unfortunately very few suppliers will give you the weight or cost per kWh, most likely because they don’t know how to work it out, or because it would make their product look poor value for money.
The Wood Fuel Co-operative will always give you the cost per kWh of the wood fuel, whether it be briquettes or pellets, collected from our yard. Delivery depends on distance and that cost is variable.
More Facts About the Calorific Value of Wood
The heat locked up in a fuel is expressed as its ‘calorific value.’
Knowing the calorific value is important because it enables you to work out the cost of the heat contained in the wood fuel you have bought. You are then able to make a direct cost comparison with other fuels.
The two most important factors which have an effect on the calorific value, or available heat from a given volume, of wood are:
- Moisture Content
- Wood Density
See ‘Moisture Content’ section above. The lower the moisture content of your wood the higher the calorific value. Aim for sub-20% moisture content.
The denser the wood the more calories it holds per cubic metre. So, dense hardwood has more calories per cubic metre than lighter softwood, given a similar moisture content. However, a cubic metre of softwood is cheaper to buy than the same volume of hardwood, so the actual cost per kilowatt hour should work out about the same. See ‘Hardwood or Softwoods’ below.
How to work out the cost in pence per kilowatt hour of your fuel
You need to know the calorific value of your fuel and the cost.
We know Blazers briquettes have a calorific value of 4,800 kWh per tonne. Suppose one tonne of Blazer briquettes costs £324. Simply divide 32400 by 4,800 and the result is 6.75. So the cost per kilowatt hour is 6.75 pence. Now you can make a direct comparison with logs, electricity, gas etc. Remember though, this calculation does not take into account the lack of efficiency of your stove. An open fire might run at 25% efficiency whilst a good stove might run at 80% efficiency.
To compare Blazer Briquettes with logs we need to know the moisture content of the logs, the density (roughly, are they hardwood or softwood?), the weight and the cost.
I can tell you (because I have weighed several bags on an approved platform scale), that one builder’s tote bag measuring 82cm x 82cm x 82cm (see photo above) and brim full of mixed hardwood and softwood weighs an average of 200kg (a softwood bag weighs around 185kg and a hardwood bag weighs around 215kg) provided the logs are dried down to an average of 20% moisture content (the minimum level at which it is wise to burn).
I guess you will have to take my word for it when I tell you that one tonne of logs dried down to 20% moisture content has a calorific value of approximately 4,100 kWh (regardless of whether it is softwood or hardwood). Ref: Rural Developments Initiative Ignite Woodfuel Course.
If you are paying £60.00 for a bag of mixed hardwood/softwood as in the photo above, then in calorific terms 5.6 bags have the same calorific value as one pallet (960kg) of Blazer briquettes.
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. Hardwood logs come from broadleaf woodland
These are broadleaved deciduous trees and they tend to be slower growing and the wood is more dense and heavy than softwoods.
Softwoods used for firewood in the UK are mostly spruce, pine, fir and larch (There are several sub-species of each).
These ‘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. Softwood logs come from coniferous woodland.
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 be between 30% and 50% so it’s then 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.
Note: 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 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.
In Summary: Wood Briquettes and/or Traditional Logs?
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 stoves and open fires work exceptionally well fueled by 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.
Traditional Logs: It’s a minefield out there if you want to buy firewood logs.
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.
To avoid paying over the odds for low quality firewood, it’s worth making sure you are comparing like with like. At the price we can supply briquettes, they are a better buy for economy, cleanliness, heat output, consistency and security of supply than logs. They’re also better for your stove. But if you want something attractive to look at, logs are great.
Wood Pellets: In order to function consistently to the manufacturers’ standards, domestic wood pellet boilers need best quality Grade 1 premium pellets which conform to the latest European Standard EN Plus A1. These are the only pellets that the Wood Fuel Co-operative supplies and are supplied at less than 8%mc. Doubtless you have spent a lot of money on a fine boiler: don’t endanger it with unbranded fuel of unknown quality.