This section covers the issues involved in running your stove safely and efficiently.
The techniques used to burn wood are quite different to those used for burning solid fuel. To get the best results, you must have the right stove for the job, set it up correctly – and run it in accordance with certain basic principles. To do otherwise invites disappointment and may even create a fire hazard.
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The Right Way To Burn Wood
To get the best out of your stove you must set it up correctly, burn only dry, fully seasoned logs – and be prepared to experiment with your technique.
The diagram below shows a modern ‘clean-burn’ stove running as it should.
The first thing to notice is that a ‘sandwich’ has formed in the fire-box. At the top is freshly added fuel – as much as the fire-box will accommodate. The centre consists of a layer of charcoal, produced by exposing dry fuel to a high temperature while at the same time limiting airflow. The fire is at the bottom and consists of a deep mass of glowing charcoal resting on a soft bed of ash or sand.
The soft bed traps all embers allowing them to burn out to a fine ‘talcum powder’. At the same time some of the glowing embers get submerged and starved of oxygen so that combustion is slowed right down. This reservoir of heat helps to smooth out the burning cycles and speeds up recovery of a fire that has been allowed to burn too low.
It is normally only necessary to remove ash every 2 – 3 weeks and you should always leave a soft bed at least 1″ deep.
The pre-heated air-supply at top has been adjusted so that it admits just sufficient air to create a slight disturbance in the fire-bed. By pre-heating the air-supply, using a double-glazed door and lining the fire-box with insulating firebricks, combustion temperatures have been boosted to a level that produce completely clean combustion. Yes – this stove is burning smokelessly! And it will continue to do so except for brief periods when fresh fuel is added.
One last point. Note that although the air-supply has been turned right down, a steady stream of heat is still being ‘bled’ to the chimney. This is important. Your chimney should be thought of as an integral part of your stove installation and its demands must always be taken into account. If you fail to bleed sufficient heat to it to keep it running sweetly you will run into a downward spiral in which draught suffers, combustion efficiencies fall, smoke is produced and the chimney starts to foul up. All things to be avoided!
• Long, slow, over-night burns are both wasteful of fuel and foul up the chimney since temperatures in the fire drop below the levels needed for clean combustion. It is much better to let your stove burn right down towards the end of the evening, then load it with dry timber and close the air-supply off completely. If you get your settings right you will produce a wonderful batch of charcoal and tinder-dry fuel to light the stove with that will eliminate the need for kindling and give you a light-up cycle almost as fast as an oil or gas appliance.
• If you must turn the stove down to a very low setting (i.e., as when you are out shopping), run it fast for several minutes upon your return to send a wave of heat up the chimney. This will help boost temperatures to a level that will (hopefully) dry up damp patches in the chimney and vaporise any creosote that has formed.
• Be aware that a chimney fire is a bald statement that something is wrong. If you do not fit a masonry chimney with a low-mass flexible liner before commissioning your stove you are asking for trouble. But there may also be something wrong with your technique – either you are burning damp/unseasoned wood, or you are burning the stove too low, or you are failing to ventilate the chimney by burning the stove brightly for a few minutes each time you light it up from cold. A badly designed chimney may be a contributory factor – particularly if large sections of it are exposed to the elements. (Expect the worst if you have a poorly insulated chimney on a north-facing wall).
The Wrong way to burn wood
Bad technique can boost your fuel consumption and burn out grate bars and deflector plates. It can even burn out the stove itself and create a major fire hazard. Keep an eye open for the warning signs, all listed here.
The air-supply to a stove should be controlled to produce a mild brightening of the embers and a slight disturbance of any flames produced. If the stove is roaring away and is filled with leaping flames you are flooding it with cold air which is carrying most of the heat straight up the chimney. This is a mistake nearly everyone makes when they first buy a stove – particularly if they have bought one too small for the job in hand. There is a strong instinct to produce a mass of crackling flames much as you would in an open hearth – and an even stronger instinct to believe that the more flames you produce the more heat the stove will produce.
To an extent this is true, but look at the diagram. The deflector plate is glowing bright cherry red, as is the flue connector. And the chances are that the bottom section of the chimney will fail soon under the assault of constant ‘combustion-chamber’ temperatures. Meanwhile there is no charcoal in the fire-box, and no prospect of any being created. The logs are burning straight onto open grate bars so that as soon as glowing embers are produced they fall through into the ashpan which is emptied at regular intervals in the mistaken belief that this is ‘correct procedure.’ It is for solid fuel, but not for wood.
If you suffer any of the problems highlighted in this section, click here to remind yourself how things should be!
WOOD - Slow-burning – the dangers
Wood is an excellent clean-burning, smokeless fuel when burned at high temperatures. It is a diabolical polluting fuel when burned at low temperatures. Worst of all it becomes a fire hazard when green or unseasoned logs are ‘cooked’ in a hot stove for hours at a time. In such conditions highly flammable tars and creosotes are produced that stick to the flues like glue and are almost impossible to remove.
There is a special temptation to under-run a stove when you want to wake up to a warm house – or if you are away all day and want to return to a home at a comfortable temperature. In theory a woodburning stove may be able to deliver what you want. In practice it is all to easy to set up the conditions shown here. As the hours pass, the temperature in the stove drop steadily and the production of smoke and tars increases. As when wet grass is dropped onto a low bonfire there just isn’t enough heat to burn up the fuel and a pall of dense smoke is produced. By the time you return to the appliance it is barely warm, the glass is as black as pitch (it is pitch – literally!) and there is often an unpleasant, sour smell hanging around that catches your throat.
A stove’s tolerance to being slow-burned depends on several factors. If your wood is bone dry and the chimney has been fitted with a low-mass liner back-filled with insulating material you may be able to turn down the heat for quite lengthy periods without incurring any penalty. But this will be much harder to do if the stove is too big for the job in the first place – or if it is equipped with a high-output central-heating boiler. (All boilers reduce the fire-box temperatures to a degree, but a worst-case scenario is a high-output boiler served by an out-of-control pump that circulates cold water through the appliance. As a matter of course a low-level thermostat should be fitted that disconnects the pump when the water temperature falls below 50º C.)
To sum up, a “dry” stove (i.e., no boiler fitted) burning fully seasoned logs in a warm (well insulated) chimney may tolerate fairly long, slow burns. A stove equipped with a central-heating boiler, fuelled on wet wood and connected to a cold chimney can never safely be run slow at all. If you are ever caught in this situation the safest bet is to switch to smokeless fuel without delay – assuming the stove is of ‘multifuel’ design and equipped to burn solid fuel.
SOLID FUEL - do it right
Many stoves on the market are designed with a ‘multifuel facility’. This offers you the option of burning wood or solid fuel according to availability – an attractive proposition. However the two fuels are quite different from each other and to get the best results you need to set up the stove correctly and alter your technique. This section explains the principles involved.
Choosing the best solid fuel for your stove can be quite tricky and you may have to experiment to get the best results. Household (“bituminous”) coal is best avoided altogether. It produces great volumes of soot and a long hot flame that can burn out deflector plates and flues. Anthracite can congeal and clog the grate bars – and some grades produce such intense heat that they may burn out grate bars rapidly. In general the most satisfactory fuels are the man-made ‘smokeless’ grades. The lumps are often ovoid (“egg-shaped”) and brands like Phurnicite and Homefire are well worth experimenting with.
It is vital to burn solid fuel straight onto an open grate and the grate must be riddled and ash removed at regular intervals. The air supply is generally controlled by a valve serving the ashpit so that a cool stream of air is directed upwards into the heart of the fire in a way that helps prevent the grate bars from over-heating.
Solid fuel likes to burn in a firebox that is narrow and deep and larger multifuel stoves don’t always provide ideal conditions – you end up with shallow coals scattered across the full width of the firebox. Mixing in some wood may help. Wood adds useful bulk and can help you build a deeper bed with modest quantities of solid fuel. Be warned though that wet wood and solid fuel with a high sulphurous content can be a bad mix. Under some conditions they precipitate supherous acid which will of course shorten the life of metal components (particularly boilers).
Smokeless fuel is far superior to wood as a central-heating fuel in domestic appliances. Given its short flame and its intense radiant output it can ‘punch’ heat straight into your boiler at a rate that will keep your radiators spanking hot for hours at a stretch. Wood just isn’t up to this task except when fed to a much bulkier specialised appliance typically located in a utility room or outhouse.
For these reasons, you should budget to burn mainly smokeless fuel in the depths of winter if your stove is heavily boilered. You may be able to spin out your supplies to a useful degree by mixing in some thoroughly dry wood, but don’t count on using wood alone.
Solid fuel is extremely sluggish in its response compared to wood. It takes longer to light, longer to deliver its full output – and longer to cool down. It is also much less forgiving than wood when burned at low output – often, it simply goes out. This is where mixing in some wood can help. The wood turns into charcoal which glows for long periods of time and helps keep the solid fuel ‘ticking over’.
SOLID FUEL – the wrong way to burn it
You may not intend to misuse and abuse your stove, but you can do so unwittingly – and the cost can be high. Soaring fuel bills and a constant need to replace grate bars and deflector plate are just some of the ills that will plague you.
Three things have gone badly wrong in the above scenario;-
• Household (“bituminous”) coal is being burned. It produces a great deal of soot and in a closed stove there simply is not enough air passing through the fire-box to carry the soot up the chimney, so that the flueways get clogged and frequent chimney fires can be expected.
• So much air is being admitted that the effect is of an open fire being burned in a closed box. You can see the result. When you fuel a stove with household coal and admit large volumes of air you get exceptionally long flames. The deflector plate and flue collar are both red hot and will burn out rapidly. Meanwhile most of the heat is disappearing up the chimney and the in-rush of cold air is chilling the base of the stove and causing uneven expansion in the metalwork. Running your stove in this manner will cause doors and panels to warp and shorten the life of the appliance.
• Ash has built up in the ash-pan. As a result the grate bars are trapped in a bed of red hot ash close to the temperature at which they were cast in the iron foundry. Naturally they will distort and burn out rapidly under these conditions.
What your chimney looks like - open fire
What an unlined chimney may look like when serving a closed stove
Soot particles travel up a chimney from a closed stove. Because the smoke is not diluted much, the particles are packed together at high density. Also the low volume of slow-moving flue-gases get chilled by the heavy mass of masonry in the chimney, causing creosotes to precipitate on the way up and create a major fire hazard.
What a lined chimney looks like serving a stove
Here the soot particles remain tightly packed as before but now the flue-gases stay hot to the top of the chimney because they are contained within a low-mass flexible liner back-filled with insulating material. The higher temperatures eliminate or drastically reduce precipitation of flammable tars and creosotes.