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Wood Burning Solid & Multi-fuel Stoves

WOOD v MULTIFUEL.

In short, wood burns best from the top downwards on a thick bed of ash. When burnt in this manner nearly all the fuel is used and all that is left is a very fine white ash, and there is very little of that.

‘Coal’, (usually referred to as solid fuel because of the wide variations) on the other hand, burns more fiercely and produces more ash so it is best burnt on a grate so that the bars can be kept cool by the rising air. This also allows the ash to fall through, not clogging the fire and restricting the supply of oxygen. As wood can burn on a grate, what most manufacturers call multi-fuel, is in fact just a solid fuel grate with or without riddling facility. One of the few exceptions to this is probably the ‘Charnwood’ & Aarrow Ecoburn ranges of multi-fuel stoves. With these, a clever grate system can be turned from either a flat bed, or in the second position, an open bar grate. Moving between the two acts as a riddling system, so in effect they can be either wood or solid fuel burners. Nobody yet makes a stove that can burn solid fuel and/or gas or oil, these have dedicated gas or oil burners in them so they have neither a bed nor a grate.

AIR WASH.

In recent years, efforts have been made to reduce the emissions of all boilers and stoves. By allowing the combustion air to be preheated and to enter from the top it was found to give a more complete combustion. As a spin-off it was also found, that, if arranged to be forced down in front of the door, an air curtain kept the glass remarkably clean. This is now known as the 'air wash system'. With 'cleaner staying' glass it has allowed the designers the opportunity to increase the glass area in the door design thus allowing the view of the fire and the infra red radiation to be uninterrupted. Stove designs that incorporate two small glass doors, to be opened to view the fire, ( which loses control of the burning rate and lets all the heat up the chimney) are now few. Nothing in life is 100%, but air wash certainly does work well on most stoves to which it is fitted, and even if smoking up does occur because damp green wood has been left smoldering near the glass, a good blast with fresh wood burning usually clears it off.

CHIMNEY LINING/TAR FORMATION.

There are three stages in the combustion of wood. The first stage is when the water boils off. Yes, there is a lot of water even in dry, seasoned wood. A small log will contain over 1/2 pint even when it registers only 20% moisture content with a meter. During the second stage, the resins in the wood are boiled to form hydrocarbon gases, these form the yellow licking flames when burnt (just like a gas effect fire!). It requires a hot secondary air supply preferably 300-500 deg.C. (as in air wash or properly designed wood burning stoves) to burn these gases, but remember, if you do not burn them you waste approximately 1/3 of the energy in the log and you will send the boiling resins up your chimney to condense on the chimney walls. Finally you will be left with the charcoal, clean heat, almost pure carbon, that will combine with oxygen from 150 deg.C. plus to form carbon dioxide and heat. Of course all the three stages are going on simultaneously at any time during the normal running of the stove or fire, but the understanding of the process highlights some of the design and system running methods.

• Burn only dry wood. If you are serious about burning wood you must sort out supplies and the relevant storage problems. The wood must be bought, sawn and stored under cover the season before you require it.
• Make sure you have a stove with a hot secondary air supply.
• Open the stove draught up every time you reload to allow the gases to be burnt.

Chimney design: Insulated chimneys are essential for burning wood. Even the best stove, run properly with the best dry wood, will let boiling resins up the chimney. These condense at approx. 150 deg.C., if the chimney is well insulated they will not condense out, but a standard chimney will gradually get saturated in foul smelling tar, which is also a fire hazard. It will first be noticed in the loft area but will gradually get lower until tar stains appear in the upstairs chimney breasts. See notes under PRODUCTS- CHIMNEYS for lining materials and methods.

CLEAN BURN/CATALYTIC CLEANERS/AFTERBURNERS.

In this age of cost consciousness & environmental awareness some stoves came with various optional secondary combustion systems. These were delicate though and the idea has been dropped as better combustion with primary secondary and tertiary air had been developed.

Read about the environmental Impact >>