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

Newburner 23 October, 2013

Quick question:
Do logs really need to be split?
I have quite a large multifuel stove that can take big logs on a bed of coal.
What are the advantages/disadvantages of burning logs that have not been split?

Karen Bateman 25 October, 2013

Very helpful thank you, also good point Simon I was thinking the same, centre of the log not the end. Just installed a multi fuel burner, it's only had a few logs and bags of coal used on it, best burn you know its there the heat is wonderful.

Neil 19 November, 2013

Scots pine is fine in a stove, if it really dry

Boris 8 December, 2013

I must say that a moisture meter is OK but you can check moisture content by weight - trim a piece to weigh 1Kg and place in the middle of the stack. Weigh it monthly and log (pun!)and plot the weight. The closer it gets to 500gm the better, when the curve flattens out, the drying process is pretty well complete and the wood is ready for burning. Stay warm!

Andrew Birnie 15 December, 2013

Thanks for this interesting article,I am a professional hedge layer and coppice worker and I am sure that hornbeam (carpinus betula) burns the hottest. I imagine that moisture content is important but different species dry at different rates.Firewood is sold by volume not weight so this should be taken into account.
I stack the cordwood in piles by species and keep a note of when the wood was felled as this also effects the rate of drying.
I would like to see a study comparing species in a scientific manner.
Diesel fuel in the UK became more expensive as the sales of diesel cars increased. To believe that its price relates to energy content is a quaint way of viewing multinational corporations and the governments that serve them.

Brendan 15 December, 2013

Must say I've had great success with Leylandii..... Was split and stacked outdoors for 14 months and dried to 20% or less... Burns really well with excellent heat output in my Boru boiler stove

Nigel 26 February, 2014

Diesel fuel is taxed more highly in part because of the high levels of pollution created by diesel engines, even recent ones with particulate filters. Diesels produce Oxides of Nitrogen that were outlawed in petrol engines a long time ago and Particulate pollution against which the human has no defence. Both these pollutants are very bad for human health, in fact the WHO has recently classified diesel fumes as a Class 1 Carcinogen. Not that wood smoke is particularly nice stuff, even if it can smell nice, but diesel is horrible - not at all eco-friendly.

Dianaashworth.com 2 April, 2014

We burn Leylandii but we clean our flue at least once every year as it tends to block with brandy-snaps -- tarry webs. We do it ourselves as soon as I notice that the fire isn't drawing so well. We are trying to eradicate the leylandii but it is difficult to keep up with!

Stuart 27 April, 2014

Why do you say in the 'types of wood' section that Douglas Fir is poor wood and little flame and heat but then in 'burning softwood ' it is described as a 'great firewood '???

slightly confusing?

Glynn 20 May, 2014

I have had a wood burner for years and burn anything from woodland and garden, such as Birch,Oak,Scotts Pine, Leylandii, Sycamore, laurel and Cob or Hazel. I do not have a problem burning pine its just means you have to clean your flue more often. But I always find it handy to mix it with a slow burner such as oak and laurel which take at least two years to really dry out. I would have to say my favourite out of these woods is Hazel and Sycamore. Its always also helpful to add a just a little amount of coal especially the heat beads which are probably the best!!

Add your comment

This helps to discourage spam