Firewood moisture content: the enemy of efficiency

Understanding wood moisture content and its direct impact on efficiency

What wood moisture content actually measures (logs, pellets, wood chips)

Moisture content indicates the share of water contained in the wood at the moment you burn it. For so-called "dry" logs, in practice you aim for low moisture, often around 15 to 20%. Pellets are more consistent, generally under 10%. Wood chips vary more depending on storage and season, frequently between 20 and 35%.

Why overly damp wood burns more fuel and produces less heat

When the fuel is damp, part of the energy first goes into evaporating the water. As a result, flame temperature drops, combustion is less complete and the useful heating power decreases. You load more for the same comfort, and the appliance's actual efficiency falls.

Field signs of damp wood: smoke, fouling, uneven burning

On site or at a customer's, it's quickly spotted: hard starting, crackling, persistent white steam, darker smoke. The firebox fouls faster (glass, heat exchanger, flue), with tarry deposits. Burning is uneven and heat arrives late.

Measuring wood moisture on site: reliable methods and mistakes to avoid

Moisture meter: setting it correctly, where to probe, and how to read the result

A pin-type moisture meter gives a quick reading if you set it to the species (and, if possible, the temperature). Insert the pins along the grain, on a clean face, a few centimetres from the ends. Look for depth rather than a surface skin. Take 5 to 10 readings and keep an average value. The result is a percentage of moisture by mass. The higher the value, the greater the risk of shrinkage, warping and mould.

Weighing and drying: the simple method for checking a batch of wood

To settle any doubt, the oven-dry method remains the reference. Take a small sample from each batch, weigh it, then dry it in an oven at around 103°C until the mass stabilises. Weigh it again and calculate the moisture content: (wet mass − dry mass) / dry mass × 100. This gives you a clear calibration point for your moisture meter.

Common pitfalls: frozen wood, dry surface, mixed species and sections

Frozen or very cold wood gives a distorted reading. A surface heated by the sun gives a value that's too low. Mixed species, sections or treatments call for recalibration. When in doubt, measure at the core on several pieces and note the storage conditions. That's often where the real moisture content is hiding.

Moisture thresholds to aim for in 2026 to preserve appliance efficiency

Firewood logs: recommended rate by use (stove, insert, boiler)

For a stove or an insert, aim for wood around 15-20% moisture. Beyond that, part of the energy goes into drying the fuel, draught deteriorates and efficiency drops. For log boilers, follow the same logic, with split logs and ventilated storage sheltered from rain.

Pellets and storage: keeping a consistent, high-performing fuel

Pellets are designed to be very dry, in practice 8-10% moisture. Keep them on a pallet, off the ground, away from a cold wall or a damp slab. If the fuel swells, breaks apart or produces a lot of dust, the auger and ignition can quickly run into trouble.

Wood chips: managing moisture to limit breakdowns and losses

With wood chips, aim instead for 20-30% for stable combustion. Too damp, and you're transporting water while gaining fouling, with more shutdowns and ash. Too dry, and dust increases while settings become more finicky. A check at delivery avoids unpleasant surprises.

Storing and drying wood to hit the right moisture levels without extra cost

Basic rules: ventilation, elevation, rain protection

To avoid overly damp wood, rely on three habits. Raise logs on pallets or battens, leave gaps so air can circulate, and protect only the top from rain. A tarp that wraps everything blocks ventilation and traps moisture. Keep storage off the ground, away from walls, with an air intake on the windward side.

Drying times by species and log size: practical benchmarks

Target for domestic heating: under 20% moisture. As a benchmark, hard hardwoods (oak, beech) often need 18 to 24 months after splitting. Lighter species or softwoods can get there in 6 to 12 months. The longer and less split the log, the longer drying stretches out. A moisture meter costs little and avoids guesswork.

Yard organisation and stock rotation: protecting efficiency all season

Organise two zones. A "green" zone for freshly cut wood. A "dry" zone for ready wood, under ventilated shelter. Work on simple rotation, first in, first out. Prepare a buffer of a few weeks to smooth out rain and demand peaks.

Advice to pass on to customers to avoid damp wood and callbacks

Delivery checklist: checking a stère or a pallet before payment

Before paying, suggest a simple check on 3 to 5 logs. Dry wood can be spotted, but the measurement settles it.

• Split a log and measure moisture on the fresh face with a tester. Aim for 20% or less.
• Look at the appearance. Cracks, bark peeling off, a "clear" sound when knocking two logs together.
• Reject a batch stored on the ground, under a closed tarp or with a glossy look. It's often a sign of trapped water.

Good lighting and fire-tending practices with dry wood

For a clean burn, top-down lighting works well. Air is opened at the start, then reduced once the firebox is properly hot.

• Very dry kindling and firelighters. Avoid a smouldering fire.
• Reload in small amounts and keep clean flames, not lukewarm embers.

Maintenance and prevention: limiting creosote, corrosion and efficiency loss

Damp wood cools the flue and encourages creosote and corrosion. Advise a lively fire, an ash pan managed without smothering the air, and chimney sweeping suited to use, following local rules, often 1 to 2 times a year.