The wood is kept chopped and arranged. After drying in the summer under the influence of sunlight and warm air, their humidity should fall below 20%. Dry firewood is cracked at its edges. The thickness of the wood should be according to the fireplace or stove for which they will be used (from 30 cm to 50 cm). The best way to store wood in which the wood is ventilated and dries quickly is to arrange it in layers.

Energy and fuel value of firewood


1. Caloric power of wood – there are two types of caloric power of wood:

High calorific power (VCM) – / energy power /When using the latent heat from the condensation of water vapor contained in the smoke produced during combustion. This heat is usable only in special cases / condensing units /, currently – only in installations using natural gas.Lower calorific power (LCM) – / fuel power /The latent heat is not taken into account and in practice it is expressed in a value that is reported / or calculated /.

The CCM is usually expressed in kWh / kg (or kWh / liter, or kWh / m3).

The relationship between the two types of energy is as follows:

NCM / VCM = 0.9

1.1. Fuel power

The CCM of wood is related to humidity, respectively. with drying conditions (according to indicative samples):

1 ton of wood = 2200 kWh fresh wood = 3650 kWh wood with 30% humidity
1 ton of dry wood = 5000 kWh

Biogas (domestic from animals) 1 liter = 10 kWh
Propane 1 ton = 12 800
Coal 1 ton = 8 500 kWh
Natural gas 1 cubic meter = 11,7 VKM (varies)
Wood (20% humidity) 1 ton = 3 900 kWh1 cubic meter (wide) = 1 500 kWh1 cubic meter (wide) = 150 литра биогаз = 1500 kWh
Electricity 1 kWh = 1 kWh
1 TEP (ton of oil equivalent)  =  11 600 kWh

Fuel value of different wood species

According to Kollmann, the energy output in kilocalories per kilogram of dry wood (15% humidity) for some tree species is as follows:


Beech :   4000 – 5000 Birch : 4200 – 4300
Oak :  4100 Alder : 4000
Clear : 4000 Maple : 3850

Average for deciduous trees: 4000 kcal / kg


Spruce :   4200 –  4500 Pines :         4500 – 4700
Fir :        4300 Larch : 4100

Average for conifers: 4400 kcal / kg

1 cal = 4,185 J ; 1 kWh = 860 kcal

1 000 Kcal = 1, 1626 kWh

As can be seen, coniferous tree species have a higher fuel power than deciduous, but due to their lower density, the yield is lower on the basis of one cubic meter. For deciduous trees, if we start from the density – hornbeam is the best wood for firewood, followed by beech, oak and others. hard deciduous, then from soft deciduous.

Conversely – the fuel power of wood decreases with increasing humidity level in it.


Wood                                                                   kWh/kg           kJ/kg______

fresh, about 50% water content                   2.09-2.32        7500-8400

air dry, 15-20% water content                       4.00-4.41        14400-15900

dry matter, 0% water content                       4.81-5.28        17400-19000

1 star fir wood, air dry ~ 350 kg                     ~1500 kWh     5.6 млн. kJ

1 sterile beech wood, air dry ~ 500 kg          ~1972 kWh     7.2 млн. kJ


Other combustible materials                          kWh/kg           kJ/kg______

coal                                                                        8.70                 31 400

anthracite                                                            9.05                 32 700

coke                                                                       8.12                 29 300

brown coal briquettes                                       5.57                 20 100

charcoal – lower value                                       7.19                 26 000

charcoal – upper value                                      8.93                 32 200

liquid fuel – super light                                     11.83               42 700

liquid fuel – light                                                 11.14               42 200

liquid fuel – heavy                                               10.21               36 800

gas in a bottle                                                      14.50               52 300

gas from the gasification system                    5.34                 19 300

natural gas – lower value                                   9.98                 36 000

natural gas upper value                                     11.37               41 000

Relative fuel value of different tree species

The values are given as a percentage of the beech wood, referred to the volume in the air dry state (Ster beech wood = 500 kg, ster spruce wood = 350 kg / Ster = Prostr.kub.m /)

The data are averaged from 10-12 values of different authors.

1.2. Energy power

The energy of the wood, related to the volume mass at the same humidity of the wood, changes in parallel with the specific weight.

Spruce and fir are high energy and beech is low energy. The differences reach up to 10%.

2. Weight of wood

The weight of the space cubic meter (stera) depends on the actual volume (of the order), on the humidity and the density of the wood in the dry state. When felling, the degree of moisture in the wood in% usually reaches 100%, especially in those at a young age and with porous wood.

After winter felling, the humidity level drops rapidly to 40-50% in early spring and to 20-25% in the summer of the following year, as the round wood dries in this natural way harder than split wood. It takes approximately 2 years for the natural drying of firewood to be completely natural (left in the air outside).

Split into small pieces, arranged under a roof, they will contain 17-18% moisture for 1 ½ year.

These figures may vary by region (as a function of temperature and hydrological status), tree species and tree integrity.

At 25% humidity, the weight of 1 ster (per cubic meter) by tree species is within:

hornbeam : 420 – 480 kg ;
oak : 380 – 450 kg;
beech and ash : 360 – 450 kg;
birch and maple : 300 – 380 kg;
pines : 300 – 380 kg;
spruce and fir : 260 – 320 kg.

Seven rules for drying firewood

1. As soon as the trees are felled, the firewood must be cut into one-meter sections, split and arranged;
2. Cut wood should not be kept for a long time in the forest for pre-drying without being well protected (exception – oak and chestnut);
3. Firewood to be removed from the forest as soon as possible after pre-drying;
4. The split sections should be cut, split and stored under a roof for at least 2 years.
5. Warehouses for firewood must be south-facing, ventilated, at a certain distance from the soil and between the individual layers of trees to have the necessary distance;
6. Additional drying of stored wood for burning in a dryer for several days to a week, helps to minimize humidity;
7. Unprotected (in the forest), not ventilated (in the basement, closet) and without additional distances between the individual rows, firewood loses its fuel value.

3. Residual ash from wood burning

Eco-friendly wood ash can be used as a fertilizer (even in nurseries). In ponies, the ash repels rodents. Ash from old wood, which is contaminated in some way, must be transported under control to the designated places (dumps).

Relative share of wood ash. For pure (permissible) wood – 0.5 to 1.5% by weight, and the higher the share of bark and leaves, the higher the percentage of ash. For old wood – 5% by weight.