The Science behind the need for dry wood

By July 14, 2020Food For Thought

‘My fireplace isn’t working as well as it used to’. This is an oft heard comment when the winter cold starts to bite. Rest assured, the previously 5 star fireplace is now not suddenly ‘broken’. In 99.99% of these cases, the result of the sudden poor performance is the use of poor quality fuel (in short – wet wood). While it is almost intuitive that wet and fire are almost opposites and should not go together, an insight into the science of combustion explains how vitally important having the right quality fuel is to ensure your expensive fireplace works as intended.

When any fossil fuel burns, after the initial combustion (flames) you are left with a mixture of gases including carbon monoxide and hydrocarbons in varying volumes. Up to 60% of the heat energy contained in wood is contained in these organic gases which are initially released as smoke. In an open fireplace these gases (smoke) are merely vented out of the chimney to disappear into the atmosphere – carrying with them a massive potential of unburned product (and lost heat). Enter the slow combustion closed stove – Whose ‘magic’ is based on a carefully designed combustion chamber which is focused on getting the maximum amount of heat out of each log by:

  1. Keeping the gases inside the fire chamber for longer to give them more TIME to burn.
  2. Creating TURBULENCE inside the chamber to ensure that the gases mix thoroughly with the available oxygen to enable proper and extensive combustion.
  3. Generating a high enough TEMPERATURE to enable combustion to occur.

Time, Turbulence and Temperature are known in the combustion industry as the three T’s.

The appliance designers spend an inordinate amount of time and energy in designing the stove to maximise the effect of each of these three factors, but all of this effort can be crippled and rendered ineffective by the choice of poor quality (wet) wood. The biggest influence of wood quality is the moisture content. All wood contains free water, this being water which is not chemically combined as part of the woods structure, and up to 50% to 60% of the weight of the wood can be water. Taking a 2kg log from a freshly cut tree can see up to 1.2 litres of free water present. When burning this wood (or trying to), this water would need to be burnt off (boiled away) before the wood will burn effectively. Using such fuel would produce a very small, low heat flame, blackening of the window, and constant hissing sound as the water in the wood boils away.

Burning this wet wood is fruitless and lots of blitz and much cursing later you will likely only have high blood pressure to show for it, and at best a lukewarm stove. By (trying) to burn wet wood, all of the three T’s are affected resulting in a very low heat fire, with most of the hydrocarbon gases (and 60% of the heat potential) going unburnt up the chimney into the atmosphere.

In the cliché of the car metaphor, would you use diesel in your new Ferrari? If you did, would you be surprised if it didn’t work properly? Using wet wood has exactly the same effect on the fireplace. Using the correctly type of wood with the right moisture content is not only preferable, but absolutely essential in maximising the efficiency and heat output of your closed stove.

Wood types and seasoning

Generally any well-seasoned hardwood will be great as fuel in a closed stove. We highly recommend blue gum which is available from most dedicated wood suppliers and it is our fuel of choice, but black wattle is as good and being an alien you are also doing some good by burning this fuel. Pine must never be burned in a closed stove, the resin creates a massive amount of creosote which will block the chimney in no time at all.
‘Seasoning’ wood can sound intimidating, but is fortunately easily handled by time and mother-nature. So if you are one of those forward planners who buy wood in advance of winter – OR have wood on your property which you cut and use, some handy tips:

  1. Cut the wood at the 1st opportunity. Water in the wood migrates through the ends of the wood, so smaller, shorter logs enable the water to pass out of the log more quickly than leaving huge uncut pieces and then cutting just before use.
  2. Consider the factors that speed drying time, so temperature, wind, and ventilation – Each additional 0.6°C doubles the drying rate. So dry it in a sunny spot in the open.
  3. Preferably have the wood raised off the ground to enable ventilation around the wood which helps speed up the drying time.

The recommended moisture content for wood in a slow combustion stove is between 15% and 18%. Generally it will take 2 years of correct storing to see the moisture reduce to these levels.

A moisture meter can be a useful tool, but a dedicated and professional wood supplier will know all of the above, so make certain you ask the right questions and get the right quality wood to get the maximum out of your ‘magic’ stove and keep you warm this winter.