Dry or damp firewood: how to tell, and why it matters most for your chimney
Damp firewood is the main cause of creosote in the flue. Here is how to spot it, how long it takes to dry and which types work best in Catalonia.
Damp firewood is the main cause of creosote building up in the flue. And most people do not realise the wood they have is not dry. Here is how to spot it, how long it takes to dry and which types work best in Catalonia.
Dry firewood sounds hollow when struck, weighs less and has radial cracks at the ends; wet wood is heavier, has no cracks and hisses as it burns. It should be seasoned outdoors for at least 12 months before burning.
Why damp firewood harms your chimney
When you burn damp firewood, part of the energy from combustion goes into evaporating the water in the wood. That makes the fire harder to keep going, produces more smoke and generates far more soot and creosote in the flue. With properly seasoned wood, combustion is more complete, the fire is steadier and the flue dirties far more slowly.
The difference is not small: damp wood can produce up to twice as much creosote as dry wood. If you burn damp wood regularly, you may need the flue swept twice a year instead of once.
How to identify dry firewood: 5 methods
1. A wood moisture meter. The most reliable method. These devices cost between 15 and 30 euros and give you the exact moisture level in seconds. Firewood fit for burning must be below 20% moisture. If it reads over 25%, it needs longer to dry.
2. The knock test. Knock two logs together. Dry wood gives a sharp, ringing sound, almost like striking hardwood. Damp wood gives a dull, muffled thud.
3. The weight. Dry wood weighs considerably less than damp wood of the same size. If you pick up two apparently similar logs and one is much heavier, the heavier one is damp.
4. Cracks at the ends. Well-seasoned wood usually shows radial cracks on the cut ends — the marks of the drying process. Fresh wood has smoother ends and a brighter colour.
5. The bark. On dry wood, the bark tends to come away more easily. On fresh wood, the bark is more firmly attached.
The 20% rule: firewood fit for burning in a chimney must be below 20% moisture. Below 15% is ideal. Above 25% it should not be burnt — it needs longer to dry.
How long does firewood take to dry?
Freshly cut wood is between 40% and 60% moisture. Getting down to 20% takes time, depending on the species and how it is stored:
- Softwoods (pine, poplar): between 6 and 12 months.
- Hardwoods (oak, holm oak, beech): between 1 and 3 years.
- With good storage (covered, ventilated, raised off the ground): this can be cut by 20 to 30%.
If you buy firewood without knowing how long it has been drying, the safest thing is to test it with a moisture meter before using it.
The best woods for a chimney in Catalonia
In Catalonia the most common woods are holm oak, oak and pine. Holm oak and oak give plenty of heat and last well on the fire, but need longer to season. Pine dries faster but produces more resin and more creosote — with pine the flue needs sweeping more often.
Frequently asked questions
Can I burn bought firewood if I don't know how long it has been drying?
You can, but test it first with a moisture meter or the knock test. If it is above 20% moisture, let it dry in a covered, ventilated place before you use it.
Is pallet wood or building timber suitable for a chimney?
No. Building timber and pallets are usually treated with chemicals that release toxic gases when burnt. Never burn them in the chimney.
How should I store firewood so it dries properly?
Raise the wood off the ground so it does not draw up damp. Cover the top but leave the sides open for ventilation. Face the stack towards the sun if you can. Wood kept in a damp, closed place does not dry — it rots.
Does the flue dirty faster with certain types of wood?
Yes. Pine and other conifers produce more resin and more creosote. Hardwoods such as holm oak and oak burn more cleanly, though they need longer to season.
Looking for a chimney sweep near you?
We work throughout Catalonia and Andorra. Select your province: Barcelona · Girona · Tarragona · Lleida · Andorra
Want your flue checked?
If you have burnt damp wood for a season or more, the flue may hold more creosote than usual. We come out, check it and sweep it for you. Fixed price before we start.
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