Boilers for Firewood and Wood Briquettes

This paper is the English version of the booklet with the same title published by the Centre for Biomass Technology, as part of the information campaign "Small Woodstoves and Wood information Campaign FIRE AWAY" (1997).

The campaign was granted by the EU (contract no. 4.1030/A/94-121) and The Danish Energy Agency (contracts no. 51161/95-0018 and 51161/95-0054).

Text: Søren Houmøller, Centre for Biomass Technology
(dk-TEKNIK, Energy and Environment, Gladsaxe Møllevej 15, DK 2860 Søborg, Denmark, phone +45 39 55 59 99, fax +45 39 69 60 02)

This booklet tells you how you can use firewood and wood briquettes as fuel


The advantages of burning wood
If you get rid of an oil-fired central heating boiler and replace it with a woodburning one, you can save between 20 and 60% on your heating bills, because wood costs less than oil. And the Danish Energy Agency grants you up to 30% in subsidy.

At the same time woodburning units are eco-friendly. They only emit the same amount of the greenhouse gas CO2 as the tree absorbed when it was growing. So burning wood does not contribute to global warming. Less timber is felled every year in Denmark than is planted, so the area under forestry will not get less, even if we do burn wood.

Since wood contains less sulphur than oil does, less sulphate is discharged into the atmosphere. This means less acid rain and less acid in the environment.

Firing with wood benefits our forests
When you burn wood you benefit our forests. The smallest trees have to be thinned out to give more light and nutrients to bigger ones destined for use in building, the furniture trade etc. The thinned-out trees are sold for paper-making and for fuel. Thus the forests are nurtured and grow: nice to look at and a source of useful timber.

What is a boiler?
A woodburning boiler gives off its heat to your radiators in exactly the same way as an oil-fired one. In this it differs from a woodburning stove, which only gives off its heat to the room it is in. In other words a woodburning boiler can heat your whole house and provide hot water.

Never buy a boiler that is bigger than you need. For a single family home, a hand-fired woodburning boiler is usually the best and most economical investment.

In larger places such as farms the saving from burning wood is often so great that it pays to instal an automatic stoker unit burning wood pellets.

Automatic combustion units are described in the booklet entitled "Wood pellets and forestry chips in automatic combustion plant".

When burning wood it is important to know all about the process of combustion, so that you can burn it correctly.

Wood is called a "gassy fuel", because between 70 and 80% of its energy content comes off in the form of gas. Combustion can be divided into four phases:-

1. Water boils off. Even wood that has been dried for ages has as much as 15 to 20% of water in its cell structure.
2. Gas content freed from the wood. It is vital that these gases should burn and not just disappear up the chimney.
3. The gases emitted mix with atmospheric air and burn at a high temperature.
4. The rest of the wood (mostly carbon) burns. In perfect combustion the entire energy is utilised and all that is left is a little pile of ashes.

Regulating the right amount of air
Three things are needed for effective burning:

* high enough temperatures;
* enough air, and
* enough time for full combustion.

If not enough air gets in, combustion is incomplete and the smoke is black from the unburnt carbon. It smells terrible, and you get soot deposited in the chimney, with the risk of fire.

If too much air gets in the temperature drops and the gases escape unburnt, taking the heat with them.

The right amount of air gives the best utilisation of your fuel. No smell, no smoke, and very little risk of chimney fires. Regulation of the air supply depends largely on the chimney and the draught it can put up.

Basically, you should follow the manufacturer's instructions, and then find the best setting by trial and error.

Various types of boilers
Burn-through boilerNearly all old-fashioned cast iron stoves act on the burn-through principle: air comes in from below and passes upwards through the fuel. One example of this is the Salamander stove, of which we have thousands here in Denmark. But Salamander stoves were designed for burning coke, a low gas fuel. It is uneconomical to burn wood in stoves of this type, since most of the energy contained in wood is in the form of gas.

In burn-through boilers the wood burns very quickly. The gases do not burn very well, since the boiler temperature is low. Most of the gas goes up the chimney, and the energy with it. As can be seen from the diagram, the flue gases have a very short space in which to give off their heat to the boiler in the convection section. By and large, burn-through furnaces are unsuitable for wood. The useful effect of a burn-through boiler is typically under 50%.

Underburning boiler

An underburn boiler is very different from a burn-through one. The air is not drawn through all the fuel at once, but only through part of it. Only the bottom layer of wood burns; the rest dries out and gives off its gases very slowly.

Adding extra air (so-called "secondary air") direct to the flames burns the gases more effectively. In modern underburning boilers the combustion chamber is ceramic lined, which insulates well and keeps the heat in. This gives a high temperature of combustion, burning the gases most effectively. An underburning boiler typically has a useful effect of 65-75%.

Reverse combustion
Omvforb.jpg - 87,50 KIn reverse combustion too, air is only added to part of the fuel. As in underburning, the gases leave the fuel slowly and are burnt efficiently. Secondary air is also led into an earthenware-lined chamber, giving a high temperature of combustion. The flue gas has to pass through the entire boiler, giving it plenty of time to give up its heat. The useful effect is typically of the order of 75-85%.

Some reverse combustion boilers have a blower instead of natural draught. Such boilers often have slightly better combustion, with less soot and pollution than ones with natural draught, but their useful effect is not significantly better.

The efficiency of the boiler
How good a boiler is partially depends on the proportion of the energy in the fuel that it transfers to the central heating system. This proportion is called the "useful effect".

The useful effect of a boiler is defined as the relationship between the energy in the hot water and that in the wood: the higher the useful effect, the more of the energy in the fuel is transferred to the water in the boiler. A high useful effect gives the best operating economy.

So it does matter which type of boiler you choose: some have a higher useful effect than others. For instance, if a boiler has a useful effect of 50%, only half the energy in the wood is transferred to the water.

Good boilers have a useful effect of the order of 70-80%.

Fire away - and store the heat
It almost always pays to buy a storage tank when installing a woodburning boiler. A storage tank holds water that has been heated up by the boiler. The extra cost repays itself very quickly, and it is easier to fire properly. Wood chip boiler connected to heat storage tank

Shortly after lighting up, combustion is clean and the boiler starts producing masses of heat. Without a storage tank to take up the heat, the water will rapidly get too hot and the damper will have to be shut to stop it boiling. The reduced amount of air leads to smoky, incomplete combustion. But with a hot water tank you can fire away and store the heat. The water in the boiler cannot overheat because it goes into the tank. The damper remains open and combustion continues at high efficiency. When you need heat in the radiators, it comes from the storage tank.

The size of the storage tank will depend on the amount of heat the house needs and the useful effect of the boiler. A small, well-insulated home with a small boiler will obviously need a smaller storage tank than a house on a big farm with a large boiler. Your local heating engineer, plumber or manufacturer of boilers and hot water tanks will be able to give you details of sizes and prices.

Burning wood combined with solar heating
If you do decide to instal a woodburning unit, you should also consider putting in solar heating. The woodburning boiler and the solar panels can frequently use the same storage tank, reducing the cost of the system as a whole. Make sure first that the storage tank is suitable for the purpose.

Solar heating and wood boiler At the same time it makes it unnecessary to have a fire going in summer just to get hot water. And it is cheaper to "burn" solar energy than wood!

What fuel is best?
Whatever fuel you decide to use, it must be dry. Newly felled timber has a water content of about 50%, which makes it uneconomical to burn. This is because a proportion of the energy in the wood goes to evaporating the water off, giving less energy for heat. So wood has to be dried before it can be burnt.

The best thing to do is to leave the wood to dry for at least a year, and preferably two. It is easiest to stack it in an outdoor woodshed so that the rain cannot get at it.

Never burn wood that has been pressure-treated, painted or glued, since toxic gases are formed on combustion. Nor should one burn refuse such as waxed paper milk cartons and that sort of thing.

You can also burn wood briquettes. They are made of compressed sawdust and wood shavings, about 10 or 20 cms (4-8") long and 5 cms (2") in diameter. Because they are compressed and have a low water content they have a higher energy density than ordinary wood, so they need less storage space.

Your chimney - the engine that drives your boiler
Your chimney is responsible for the draught going through your boiler. The difference in the density of the air between the top of the chimney and the outlet on the boiler is what creates the draught. So the height of the chimney, the insulation, and thus the temperature of the smoke all contribute to the draught. Bends and horizontal bits of piping reduce the draught. They create resistance, which the hot air has to overcome. So the idea is to have as few horizontal flues and bends as possible. Some boilers have a built-in blower, ensuring a proper draught at all times.

Not all chimneys are suitable for woodburning. Get expert advice on this.

Boiler maintenance
A boiler must be installed and maintained properly. This increases its life and your safety.

Most countries have regulations about siting: in some places boilers have to be put in a separate room.

The chimney will need sweeping at least once a year. This reduces the risk of fire. Ask your local firm about maintenance of your boiler and chimney. Your chimneysweep can tell you if you are firing properly. Too much soot up the lum may mean you are not letting enough air through.

Up to 30% in State grants for woodburning stoves
It pays to burn wood in a boiler built for it. If you burn nothing but wood, the annual cost is less than half that of oil. And a woodburning boiler costs about the same as an oil-fired unit to buy. On the other hand a woodburner can cost more to maintain.

* A woodburning boiler with a storage tank costs from DKK 20,000 up, including Danish VAT but not counting the State subsidy. You have to reckon with the cost of installation on top of this, typically of the order of DKK 15,000 including VAT.

* The Danish Energy Agency grants a subsidy of up to 30% of the cost of boilers approved for burning wood or straw, provided they are installed in areas with no access to district heating or natural gas. The amount of the subsidy depends on the useful effect of the boiler and on how much dust and carbon dioxide it emits.

* An updated list of approved boilers, including prices and percentage subsidy, can be obtained from the Information Secretariat for Sustainable Energy (see below for address).

* Subsidies are also available for existing boilers for fitting them to burn fuel such as wood-chips or pellets.

* If your house has not got central heating, you will need radiators and a boiler room put in. Ask around for prices.

* If your home is heated by electricity, new regulations for subsidies for installing central heating came into force on Jan. 1, 1996. Ask at your local Energy and Environmental Office.

More information

For more about the use of bio-fuels for heating, read:

* Firewood and heating: the forests have it

* Correct firing: how to utilise your fuel better

* Open fires and woodburning stoves - Building regulations for single family homes

* Boilers burning firewood and wood briquettes

* Danish Standards for woodburning stoves

* Standard grants for sustainable energy

* Subsidies for small-scale bio-fuel units

* Bio-fuel boilers: type-approval licensed units entitled to subsidy

* Manufacturers of open fires and woodburning stoves



FORCE Technology. Last update 09. marts 2005