Heating qualities

There is very little calorific difference of wood types based on their dry weight. Woods with a greater density burn for a longer time, as it has a higher calorific density. The most important (part/ aspect) of burning wood is to make sure it is dry.

Moisture content in wood

When a tree is felled in winter, the moisture content of the wood will be over 50%, and even higher if it is felled in summer. By splitting the logs and/or cutting them to short lengths, you can accelerate the rate at which the wood will dry out. You can check how dry it is using a moisture meter. The picture below shows the spikes on the end of my moisture meter - you just jab these into the end of a log and it gives you a reading.

The reason the wood needs to have dried out is that when you put a log into a fire or stove, before it can burn, all of the moisture must be driven off, and this uses some of the heat from the fire - so, the wetter the log, the less heat you get out. But there's another problem too - wet wood doesn't burn as cleanly, which is bad from the point of view of pollution, and also can deposit tar in the chimney, potentially creating a fire risk.

So how dry should your wood be before you burn it? This varies from one stove to another, but the moisture content should be 30% at most, and ideally should be 20%. How long it takes to get this dry depends on how the wood is stored. Ideally it should be split as soon as possible, then stored off the ground in a place where air can circulate through it and the rain is kept off it. Drying to 30% should usually happen within a year, and 20% in two years, although in ideal conditions these times can be reduced.

Here's an example of how important it is to split logs. These two logs were both felled about two years ago, and have been freshly sawn from the centre of 2m lengths. The first one was split straight after felling, and the moisture content is just over 20%:

The second one has only just been split, and the moisture content is still over 40% (the meter only reads between 3 and 40%):

The good news is that a short split log like this dries out very quickly in the summer - a week of being in the sun and the wood had dropped to 30%.

Many thanks to Mike Pepler for this information.

Types of firewood.

Alder:  Opinion varies, works best well seasoned.
Apple:  Splendid/ It bums slowly and steadily when dry, with little flame, but good heat. Good scent. Must season well
Ash:  Best burning wood; has both flame and heat, and will burn when green, as it has a low moisture content. Will burn even better dry.
Beech:  Best when well seasoned
Birch:  The heat is good but it burns quickly with a bright flame.  Nice smell, works well when mixed with other woods that burn more slowly.
Cedar:  Good when dry. It gives little flame but much heat, and the scent is beautiful.
Cherry:  Burns slowly, with good heat. Another wood with the advantage of scent and does not spit.
Chestnut:  Mediocre. Apt to shoot embers. Small flame and heating power.??
Cypress: Burns well but fast when seasoned, and may spit
Douglas Fir:  Poor. Little flame or heat.
Elder:   Mediocre. Very smoky. Quick burner, with not much heat.
Elm:  To bum well it needs to be kept for two years. Even then it will smoke.Very high water content – more water than wood.
Hawthorne: burns well
Hazel:  Good, burns fast without spitting. but has other uses, so you might not want to burn it
Holly:  Good, will burn when green, but best when kept a season.
Hornbeam:  Good, burns well
Horse Chestnut:  Good flame and heating power but spits a lot.
Laburnum:  Totally poisonous tree, acrid smoke, taints food and best never used.
Larch:  Crackles and spits, scented, and fairly good for heat. Oily soot in chimneys
Laurel:  Has brilliant flame.
Lime:  Poor. Burns with dull flame.
Maple:  Good.
Oak:  Dry oak is excellent for heat, burning slowly and steadily with a good heat. Seasoned for 2 - 3 years is best.
Pear:  Slow and steady, good heat and a good scent.
Pine:  Bums with a splendid flame, but apt to spit.  Needs to be well seasoned. Gives off a large number of resins.
Plane:  Burns pleasantly, but is apt to throw sparks if very dry.
Plum:  Good heat and scent.
Poplar:  Burns slowly with little heat – better for making matchsticks
Rhododendron:  The thick old stems, being very tough, burn well.
Robinia (Acacia):  Burns slowly, with good heat, but with acrid smoke.
Rowan: Burns well
Spruce:  Burns too quickly and with too many sparks.
Sweet chestnut: burns well when seasoned but sends out sparks. Only for use in a stove with door closed!
Sycamore:  Burns with a good flame, with moderate heat. Useless green.
Walnut:  Good, and so is the scent. Aromatic wood.
Willow:  Poor. It must be dry to use, and then it burns slowly, with little flame. Apt to spark.
Yew:  Last but among the best. Burns slowly, with fierce heat, and the scent is pleasant.

Burning soft wood

by David Blake, Cranborne Chase and Westwiltshire AONB

For folk with wood burning stoves, soft woods are in some ways better than the more expensive hard wood logs. For small scale producers, being able to market soft wood logs to particular customers can avoid the necessity of the awful "mixed load"; it says to the customer that what you are offering is second quality, which is not a good marketing ploy. The important thing to realise is that soft woods need just as much attention to their seasoning as hard woods. Douglas is a great firewood, but needs two years outdoors under cover if it is to perform well. Norway and Sitka spruce need seasoning for at least a year and cedars, firs and cypresses are the same. All soft woods will spit as they have within their tissues the sticky substance called resin. This is a highly volatile hydro-carbon composed (mainly) of terpenes. It differs from species to species: some are essential oils like balsam while some are positively explosive, but they will all cause logs to spit. While proper seasoning reduces spitting enormously, this is why they are more suitable for wood burning stoves than open fires. What makes them great for wood burners is that folk with wood burning stoves tend to use them more than open fire users, especially if the stove is connected to the central heating or hot water systems. Soft woods will generate less heat per kilo purchased, but should cost less; do the numbers and see if it will drive down your costs.

For small woodland owners and anyone managing small woods, proactively targeting customers that own wood burning stoves and offering them a high quality soft wood product could have real advantages.

  • it shows your customer that you are offering them a bespoke product suited to their needs, encouraging customer loyalty.
  • it gives unmarketable small round soft wood a outlet
  • it provides a more profitable market for soft wood than fuel wood chip / amenity chip.it could speed up your restoration of a broad-leaved woodland by making soft woods more saleable.
  • it could give you opportunities for market expansion and a competitive edge.

For owners of wood burning stoves, soft wood logs can save money. And they really do work: the millions of North Americans and Scandinavians who burn little else can't be wrong!

Heating qualities,  a comparison of the energy in various fuels.

(The units are MegaJoules MJ)

By Gervais Sawyer, wood scientist

3.6 MJ is the same as  one kiloWatt hour = one unit of electricity.

Dry Wood @ 15% M/C       about 16 MJ per kg
Green wood                      about 10 MJ per kg
Coal                                  about  35 MJ per kg
Peat dry                            about 16 MJ per kg
Diesel fuel                         about 46 MJ per kg
Petrol                               about 43 MJ per kg

Natural gas is rather difficult at about 39 MJ per cubic metre which equates to about 54 MJ per kg
LPG (mainly propane) is even better at about 85 MJ per kg

Note that the calorific value of wood is pretty constant no matter what the species. Variations in extractives can make it more ignitable.

This answers the question 'Why does diesel cost more than petrol?  Answer - it contains more energy.

Further reading

Trees and their uses.

Ashden Awards guide to woodstoves

Comments on this article

Susan Davis 15 November, 2012

What are the advantages and drawbacks of using Scots Pine. I have some woodland with a very great deal of it which I am slowly thinning. I wondered if it is possible to sell it for wood burners as I thought it produced too much creosote etc in the chimney. I would be very grateful for any advice you have. The woodland is in West Sussex.

Simon 22 November, 2012

Your demo on this page of moisture meters is poor practice, you do not test at end of log as this is always going to be drier than the rest of the log.You should be testing by splitting the log then test the centre of the log. Also a number of logs should tested to get an average.

Also industry standard for seasoned firewood is 25% or below, not 30%.

Hugh 4 January, 2013

I have lodgepole pine and balsam and chinese elm
I had spruce Its all good wood.
Dry is good. Too dry is bad

Kevin Twelvetrees 8 January, 2013

Nice to see a good artical on wood, you seem to have quoted some of my best sales ploys(Most original wood stoves come from Scandinavia or North America thus Pines and Birch areas).I have been selling fire wood for over 40 years (Started seling a Bushel of logs delivered in my go-cart in Southwater and Shipley West Sussex age9).How do you rate Leylandii when dry ?.I find it very hot. Rgds K12T

Tommy 17 January, 2013

In my stove I have been burning wood for many years,I burn any wood that comes my way,but I have a woodshed in which I season all my timber for at least one year before use,the only wood I do not bother with is Leylandii,that stuff is useless and its a waste of time to try and burn,I dont bother with it,however the key to getting wood to burn well,is seasoning for at least one year......Thats it.

Adrian Hepworth 29 January, 2013

Good artical but I note the comment from Gervais Sawyer that "the calorific value of wood is pretty constant no matter what the species. Variations in extractives can make it more ignitable." Does this mean that the calorific value of all wood is simply by weight. A heavier wood simply means its more dense so two hardwoods like oak and balsa will have the same calorific value per kg. (Balsa would be rather expensive as a fire wood but I think its deciduous so is technically a hardwood.) I'll have to Google 'Extractives'

Kevin - We've been burning Leylandii for many years and its excelent if well seasoned. Although it doesn't spit much if dry, I wouldn't leave the stove doors open if not in the room. Recently cut stuff does ooze very sticky sap that hard to remove from hands/clothing etc. This probably depends on the time of year when its cut.

John 27 March, 2013

I quote from this article "Soft woods will generate less heat per kilo purchased, but should cost less; do the numbers and see if it will drive down your costs." I assume this is a typo and you meant to say less heat per volume purchased. As you said in the opening paragraph, wood is wood and cv does not vary much between species

Sentin White 10 May, 2013

I'm just wondering what Yew wood costs to purchase. Say, by the kilo? I know that logging is quite good business, and I just wondered if Yew holds any special value, as I've heard it does.

Jenny 20 August, 2013

Great and informative article about wood species. But I don't agree with your view on why diesel fuel is more expensive than petrol. The real answer (in UK) is that it is taxed more. In many other EU countries, diesel fuel is cheaper than petrol.

Rob & Sarah 27 September, 2013

Just installed a log burner and found this all very helpfull .

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