Wood is an excellent fuel. It is plentiful, and renewable. It can often be sourced very close to the point of use and is nearly carbon neutral. At a basic level it requires little or no processing – cutting and splitting. It can be used to manufacture other products e.g. pellets, briquettes or chips.
The use of wood as a fuel however, is not well understood. Consumers are generally
unaware of how to get the best from their wood fires / stoves. Most people think they know
how to burn a few logs but there are several very important factors which are often
overlooked.
Burning wood in a stove / woodburner
Stoves have become very popular in recent years but there is no cultural history of burning
wood in closed appliances in the UK. Many people have installed stoves with little or no
knowledge of the appliances, the chimneys or fuels. A significant number have used an
installer who has not complied with all the regulations. Some installers do not have a good
grasp of the facts concerning efficient combustion and either fail to inform or give incorrect
advice to their customers.
The problem
The basic problem is that the wood is often not burned in an efficient way. Much of what is
burning when you use logs are volatile hydrocarbon gasses. These take the form of
vaporized tars, creosote and resins and are an excellent fuel. If these vapours are not
completely burned in the appliance, they escape to the chimney. If your chimney is cool
enough, some of these tars will condense on to the inside of the chimney in the same way
as steam (water vapour) condenses on to a cool bathroom mirror. They immediately
solidify to form the tarry deposits which your chimney sweep tries to remove. Of course, not
all the tars condense inside the chimney. A great deal is emitted from the top of the stack or
flue as un-burnt hydrocarbon or smoke. This is a serious air pollutant. You have also lost
much of the available fuel which could have been turned into heat. Incomplete burning of
wood is bad for your chimney, bad for the environment and bad for your wallet.
How do I know if I am burning wood efficiently?
There are four basic factors which govern efficient combustion of wood.
- The potential efficiency of the stove / appliance
- The efficiency of the chimney
- The moisture content of the wood
- User operation or abuse of the stove
1. Stove efficiency
If the stove is well designed and built, it should be efficient. Basically, if it is capable of preheating
the combustion air and introducing it to the fuel in a turbulent manner, it should be
quite efficient. Link to blog article on choosing a stove
2. Chimney efficiency
Most people find it difficult to think of a chimney as being efficient or otherwise but it is a
key part of the system. It must exert an appropriate and consistent draw / pull throughout
its length. To achieve this it must be high enough. It must be the right size (cross section)
and be well insulated. Stoves in the UK are often fitted to chimneys which are too big and
cold and even a very efficient and clean burning stove will not reach its potential without
the right chimney above it. Lastly, the pot / cowl / terminal should not impede the exit of
combustion gasses.
3. Moisture content of wood
The moisture content of wood is a key factor for efficient burning. Any water in the wood
must be vaporized before the volatile tars etc. are vaporized and burned. This requires an
input of heat energy which is then lost to the chimney as steam. An average sized log may
contain well over ½ a pint of water and still seem reasonably dry. Consider how much
energy is required to boil a pan with ½ a pint of water dry on your cooker. You will find a lot
of conflicting information about the moisture content of wood for burning but anything less
than 20% is good and 15% even better. Kiln dried wood has a very low moisture
content. It costs a lot more but has much more available energy. If your wood is too wet,
the temperature in your stove can’t get high enough to achieve good efficient combustion
and lots of un-burned tars enter the chimney. To make things worse there is much more
water vapour in the smoke. All this has the potential to cool down enough inside the
chimney to condense and stick. The emission of hydrocarbon air pollutants is greatly
increased when burning wood with high a moisture content, as is the chance of a chimney fire.
Some people use a bed of coal or smokeless fuel to get their wet logs to burn. This is
very polluting and a waste of money. If you do this in a stove, the increase in water vapour
helps form acids which will etch the stove glass and attack the metal parts of the stove and
chimney. Not good.
4. User operation or abuse of the stove
This factor along with moisture content of the wood is within your control. Following the
manufacturer’s instruction on burning and operation is good but a basic understanding of
what is going on helps get it right. A reasonable load of appropriate fuel should be placed in
the stove along with any firelighters you may wish to use. Light the fuel, close the door and
make sure all air vents are fully open. A vigorous 15 to 20 minute burn will bring the stove
properly up to temperature. If your stove is designed to pre-heat the combustion air, it is
even more important to get it up to operating temperature. If it does not reach this
temperature before you begin to close the vents, the combustion will not be so complete
and you will be losing volatile tars to the chimney, wasting fuel and causing pollution. Once
the optimum operating temperature is reached you will probably need to re-fuel. Let the
fuel start to burn before beginning to close any vents. If your stove is able to pre-heat the
combustion air, now is the time to completely shut the vent which allows the room
temperature air in. You can then begin to set the rate of burn with the pre-heated vent
control. Don’t shut it too much! You have a box full of hot burning fuel and if the air is
reduced too much, combustion is incomplete and you’ll be losing fuel to the chimney again.
Try to maintain a moderate flaming combustion, where the stove has plenty of flame but it
is not being sucked up the chimney. Smoke (un-burnt fuel) should only issue from the top
when you light or re-fuel the stove. If it is present at other times then one or more of the
four factors governing efficient burning are not right. It’s a good idea to go outside and have
a look at the top of your chimney when the stove is up to temperature and the wood is
burning nicely. You should see no, or very little smoke. Return to the stove and shut the
vents then back outside and have another look at the top. You should now see lots of
smoke. There will be a point beyond which you should not close the vents. A little
experimentation and you should be able to find this quite easily.
Overnight burning / slumbering
Don’t do this. Many people think that it is great to fill up their stove with fuel last thing at
night and shut the air right down. I am always hearing phrases like “it keeps in all night”.
This is very bad practice and I will try to explain why: When a stove full of burning wood is
shut down, the volatile tar vapours (fuel) are still being given off but there is now not
enough oxygen to burn them. They escape to the chimney, sooting it up and causing lots of
air pollution. Inside the stove you have begun to manufacture charcoal. Once all the volatile
compounds have been driven out of the wood you are left with quite a pure carbon –
charcoal. This will then burn at whatever rate the available oxygen allows and the process
can take all night. Burning the same load of wood a bit faster will reduce or eliminate the
loss of fuel to the chimney. If you are in the habit of burning at night, then set your air
controls so that little or no smoke is coming from the chimney top. You are now burning
much more fuel in the stove rather than letting it escape un-burnt. The stove will deliver
much more heat to the building but over a shorter period of time.
I have had many customers with good stoves and good chimneys, using dry wood but they
have been shutting their stove down overnight. The amount of tar and creosote swept out
of some of these chimneys has to be seen to be believed. They have been creating huge
amounts of pollution, wasting fuel and greatly increased the chance of a damaging chimney
fire.
Type of wood
There is a great deal of confusion in the UK regarding the best wood to burn as well as
woods to avoid. All wood is good to burn in a stove as long as it is dry. Even seemingly
reliable sources of information get this completely wrong. You may read or be told that only
hardwood logs are suitable and that you should not burn pine or conifer as these contain
sap? or resins which tar up the chimney. This is rubbish. You may be told that there is more
heat in hardwood than softwood. Again, not true. The calorific (heat) value of a kilo of
softwood is almost identical to that of a kilo of hardwood, the softwood is just less dense so
it takes up more space. You will have to put more pieces of a given softwood into your stove
than you will hardwood, but weight for weight, the heat output will be the same. With this
in mind you should pay at least a third less for a given volume of softwood over hardwood.
Storing / drying your wood
You can easily dry your logs through correct storage. Stack your logs so the air can get at
them. If you cut and split them yourself try to do this when the wood is fresh cut as it is
much easier on you and your tools. Once split, you have greatly increased the surface area
of each piece and it will dry much faster. If you plan to fell trees it’s good to remember that
they contain much less water in the winter. Keeping the rain off your stacked logs is of
secondary importance – Ventilation is the most important consideration. That said, a well
ventilated log store with open sides and a roof on it is the best situation. Some people have
a pile of logs on the lawn with a tarpaulin over them. Unless they were very dry to start with
they will quickly become mouldy and will not dry out. You should easily be able to achieve
moisture content of 15% in a few months if your logs are the right size and properly stored.
Beware of the word seasoned. The only important consideration is the moisture content,
not how long it has been cut or stored. If you are a novice, get yourself a good moisture
meter. See links for a suggestion. To test the moisture content of any log, split it first and
then test the split surface. If you are buying wood think ahead and buy early.
Open fires
Open fires are nice to look at but they are very inefficient. Even a poor stove will be three or
four times as efficient as an open fire and this does not include the huge heat loss from a
building when open fires are not in use. The amount of wood burned on an open fire in an
evening to heat a room can practically heat an average house if burned in a good stove. If
you burn wood on an open fire avoid conifers, willow, sweet chestnut as these tend to spark
a lot. Kiln dried wood may burn too fast and hot on an open fire with a strong draw. Try to
burn your wood on a bed of ash rather than in a grate or basket. Grates are for burning coal
which must have a supply of air from below. Wood burns hotter and more slowly on a bed of ash and the glowing ember stage of burning lasts much longer. Remember that an open
chimney for an open fire acts like a giant vacuum, constantly sucking large volumes of air
from the building. If you are running your central heating when the fire is not lit, an awful
lot of your nice warm air is being sucked up the chimney. To put it into perspective the heat
loss is similar to having a large window open 24 hours a day over the winter. You are
however not normally aware of the losses because the chimney sucks warm air out rather
than letting cold air in.