The Wood Considered Unfit as Fuel Is Green, Unseasoned Wood

#Why the water matters so much

Water in wood does not merely dilute the fuel. It actively consumes heat.

To turn one kilogram of water at 20 °C into steam requires:

• Heating to 100 °C: about 336 kJ
• Latent heat of vaporisation: about 2,260 kJ

Around 2,600 kJ per kilogram of water, all of it taken from the fire and carried up the flue as invisible vapour.

Burn one kilogram of green wood at 50% moisture:

Dry wood content: 0.5 kg x ~19 MJ/kg  = 9.5 MJ released
Water present:    0.5 kg x ~2.6 MJ/kg = 1.3 MJ consumed
Net available:                          ~8.2 MJ/kg

Against seasoned wood at 15% moisture delivering roughly 16 MJ/kg of usable heat, green wood gives about half. You buy twice the wood, carry twice the weight and stack twice the volume for the same warmth.

#The knock-on effects

The heat loss is only the beginning, because the cooler fire causes everything else.

Lower flame temperature. Wood burns in two stages: water and volatile gases are driven off, then those volatiles burn above the fuel bed. That second stage needs high temperature. A fire cooled by evaporating water never gets hot enough to burn its own volatiles properly.

Heavy smoke. Unburnt volatiles leave as visible smoke — which is fuel you paid for, going up the chimney unburned. Wood smoke is also a significant source of fine particulate pollution, and in many areas the reason for burning restrictions.

Carbon monoxide. The same incomplete combustion that produces smoke produces carbon monoxide. In an appliance with a poor flue, that becomes a household hazard rather than an efficiency problem.

Creosote in the flue. Cool, tar-laden flue gases condense on chimney walls as creosote — a flammable deposit that builds into a layer capable of igniting. Chimney fires burn at extreme temperatures and can crack liners and spread into the building structure. Burning wet wood is the single largest contributor.

Difficult lighting and poor control. Green wood is hard to light, hisses, spits and produces an unresponsive fire.

#How to season wood properly

1. Split it first. Bark resists moisture loss; split faces let water escape. Splitting is the single biggest accelerator of drying.
2. Stack off the ground on pallets or rails so moisture is not drawn up from soil.
3. Cover the top only. Rain must be kept off, but sides must stay open for airflow. Wrapping a stack in a tarpaulin traps moisture and prevents seasoning entirely.
4. Choose a sunny, breezy spot.
5. Allow enough time. Six to twelve months for softwoods, eighteen to twenty-four for dense hardwoods like oak.
6. Verify with a moisture meter. They cost very little. Split a log and test the freshly exposed inner face — testing the outside of a log gives a falsely dry reading.

#Signs of well-seasoned wood

• Cracks radiating from the end grain
• Bark loose or falling off
• Grey, weathered colour rather than fresh
• Noticeably lighter than a green log of the same size
• A sharp knock when two pieces are struck together, rather than a dull thud

#Other wood that should not be burned

Green wood is the standard answer, but several other categories are genuinely unfit and worth knowing:

• Treated or painted timber — releases arsenic, chromium, lead and dioxins
• Plywood, MDF, chipboard — adhesives release formaldehyde and other toxics
• Driftwood — salt content produces corrosive and toxic compounds on burning
• Rotten or mouldy wood — low calorific value and airborne spores
• Large quantities of unseasoned resinous softwood — accelerates creosote build-up

The underlying principle is calorific value and how moisture destroys it, and the smoke it produces is incomplete combustion in action.

#The compact answer

The wood considered unfit as fuel is green or unseasoned wood — freshly cut wood with high moisture content, often 45–60% water. Much of the heat produced is used up in evaporating this water, so its usable calorific value is roughly half that of seasoned wood. It burns at a lower temperature, produces heavy smoke and carbon monoxide, and deposits creosote in chimneys.