Getting the best from your Wood Fire fuel

A Guide to Good Wood from the New Zealand Home Heating Association

Seasoned Fuel / Wood Is Important  >>

Choose A Suitable Fuel / Wood >>

Which Fuel Works Best? >>

Best Ash Disposal >>

Since the beginning of time, wood has been used to cook food, preserve food, fire pots and make weapons and implements for survival. But perhaps it's most common use, even today, is to provide warmth.  It would be easy to assume therefore, that burning wood has become second nature, but that is not the case as wood burning is a learned and practiced art. 

How often have you seen a stick of Manuka in a fireplace bubbling at the ends? Or wondered why there's not as much heat in the wood you are using now as there was in the last lot you used? People request answers to questions like these all the time. 

A number of things should be considered when using wood for heating. You should understand the various types of fuel wood that is available, it's advantages and limitations, and be aware of the various types of heating appliances available in order to select the best fuel for that application. It is also essential to know how to light a good fire. 

Seasoned Fuel / Wood Is Important 
Fuel wood ranges from soft woods like pine, to hardwoods like manuka. But whatever wood is chosen, the key to a successful fire is to ensure the fuel is as 'Dry', or as 'Seasoned', as possible. 
While surface water does not really matter because that will evaporate quickly, it is important to reduce the sap levels within the cell structure of the wood itself, and that takes some time. Softwoods will season quite quickly, in about 6 to 8 months, but it can take up to 12-18 months for manuka to dry to an acceptable level. 

Gathering and stacking wood in the open air over the summer period is advantageous because the warmth of the sun and good air circulation will automatically evaporate some of the sap. When the wood gets wet from seasonal rain, the rain water replaces sap and because water is more quickly evaporated, the fuel dries faster. Most reputable wood merchants will build their stocks of wood over the summer months in order to offer partially seasoned fuel, but it is generally not economic to hold stock for the full seasoning period. Users should purchase wood far enough in advance to allow for the extra seasoning it is likely to need. 
HINT – get a moisture meter & check firewood when delivered – if it shows over 20% moisture content send it back.

Choose A Suitable Fuel / Wood 
It doesn't really matter what fuel wood is used because all wood has about the same heat capacity per kilogram, whether it's a soft wood like pine, or a hard wood like manuka. The same volume of manuka will give more heat than an equivalent volume of soft wood, with less refueling.

Manuka is a good all round fuel, but it must be properly seasoned and burnt on a well established fire for peak efficiency. One option is to use a fuel made from a selection of mixed woods - a sort of fuel cocktail -generally gum, macrocarpa and pine. This excellent fuel is somewhat cheaper than manuka and it burns longer than straight softwood. 

Softwoods are perhaps the most common fuel woods available and while they burn well, the heating appliance or fire will need more regular refueling. One disadvantage of macrocarpa, which is classified as softwood, is that it tends to spark more than other fuels. 

Another fuel option is compressed sawdust 'logs' which burn well in all appliances. These are naturally dried during the production process and they also come in easy to handle boxes. These logs must be kept dry to prevent absorption of water and subsequent disintegration, but they burn well. 

Be strongly cautioned against using treated timber of any kind because the chemicals could pose a health hazard and be corrosive, even to brickwork. It is suggested that you make a careful examination of wood for a green or pink tinge, or markings to indicate chemical treatment. Old painted timber could also pose a threat if it has been painted with lead based paint. 

While dry, salt borne drift wood could be used in an open fire, it's use in any other type of heating appliances should be avoided because it will cause corrosion of the heater or flue system. 

Look carefully at bagged fuel offered by some operators, as the wood is often packed as soon as it is cut and will never season properly because the packing will prevent the air circulation required for drying. Good, reliable outlets ensure that the fuel is air dried for a considerable time before packing. 

Radial cracking at the end of logs is a good indication of dryness, as is a 'ring' when two pieces of wood are knocked together. A dull thud indicates excessive moisture is present. 

Good kindling is necessary to build a good fire. The best is dry, thin pieces of a soft wood like pine, or dry pine cones. Cones however are not recommended as a primary fuel, as they burn quickly and will need constant replenishing. 
 

Which Fuel Works Best? 
It's important to understand the capabilities and limitations of your heating appliance before selecting a fuel. 

Open fires generally have a maximum efficiency of only about 10-12%, which means most of the heat generated is carried up the chimney. Because a fireplace relies heavily on radiant heat for it's effectiveness, a mix of coal or carbonettes and hardwood is best. Annual fuel usage in an open fire will be considerably higher than other appliance options. 

A pot belly stove is a little more efficient, but the design of the combustion system and gaps around the joints can lead to high fuel consumption. These appliances provide radiant heat and it can be difficult to maintain an extended burn because of the amount of fuel they require. Because the fuel opening in a pot belly is small, many suppliers now produce specially cut fuel for these stoves. 

Best economy will be gained from a modern high efficiency double burning clean air approved appliance that offers both convected and radiant heat. These appliances have an efficiency approaching 70% and have the flexibility to safely heat a room or a whole house for weeks on end. Wherever practical, it makes sense to replace an open fireplace or pot belly with one of these. 

Best Ash Disposal 
So what about ash disposal? With the exception of a high efficiency heating appliance, which needs cleaning only every 3-4 weeks, ashes should be removed each day. Providing that only untreated wood has been burnt, ash can safely be dug into the garden as it makes an excellent addition to the soil. Ash which results from burning coal or coal based fuels, treated timber, or lead-based painted timber, should never be put into the garden, and needs careful disposal. Make sure the ash is cold before removing it from the heating source.