Test: Dry firewood makes for complete combustion

When you light a fire in your wood burning stove, it is important that the firewood has the correct moisture content and that it has been split and cut into an appropriate size. Scientific tests show that this is significant if you want to get the most heat out of your wood and if you want complete combustion in your wood burning stove. Read about the test from TFZ here.

Author: Line Nederby
Published: 17. October 2015
Fugtmåler i brændestykke


It’s good to know how important it is to you and the environment that you light a fire in a wood burning stove correctly. The German Technologie- und Förderzentrum (TFZ), which is an institution under the Ministry of Food, Agriculture and Forestry in Bavaria, has conducted a series of tests on:

  • Firewood efficiency at different moisture content levels
  • The level of particle emissions when lighting fires with firewood with varying moisture content.

The tests have been conducted in both stoves and wood burning stoves and the results are unambiguous:
To get the best result with your wood burning stove while being eco-friendly at the same time, you should always…

  • Light a fire using dry firewood with a moisture content below 18 %.
  • Use logs that have been cut and split into appropriate size and length.

TIP: Chop your firewood so each log has approximately the same size as a standard bottle of wine.

Read more about TFZ’s scientific tests below:

The moisture content in your firewood must not be above 18 % 

In this test, the lab has fired with wood with different moisture content percentages. They fired with wood which had a moisture content of 12 %, 18 %, 23 % and 30 %.

The test has been done in a stove.

Fuel: Beech. Length of logs: 33 centimetres.

Tabel der viser at partikeludledningen stiger, når du fyrer med vådt brænde

Table showing the amount of particle emissions when firing with wood with varying moisture content


You should always fire with wood which has a moisture content below 18% if you want to obtain the highest efficiency. As soon as the wood’s moisture content exceeds 18 %, the particle emissions increase drastically.

Further reading: Efficiency – make the most of your firewood

Moist firewood reduces the efficiency of firewood and the wood burning stove 

This test was made in a wood burning stove. Amount of firewood: 1.4 kilograms. Water content: 12%, 20% og 30%.

Fuel: Beech. Length of logs: 25 centimetres.

Tabel over virksningsgrad af brænde og brændeovn ved fyring med vådt brænde

The table shows how the efficiency of firewood and wood burning stoves is reduced when the firewood is moist


The higher the firewood’s mositure content is, the lower the efficiency will be. When firing with firewood with a moisture content of 30 %, the efficiency will be almost halved in comparison to the efficiency you can expect of a new wood burning stove. The optimal efficiency in a new wood burning stove is between 75-80 %. Furtheremore, the environmental impact increases considerably as soon as the moisture content in the firewood exceeds 18%.

Think about the environment – always light a fire using dry firewood 

The graph shows how pollution increases in line with the moisture content. An increase in moisture content from 18 % to 25% pollutes five times as much.

The test was done in a wood burning stove. Fuel: 7×7 centimetres beech wood.

Betydningen af fugtigt brænde ved fyring

The graph below clearly shows that the particle emissions increase if you fire with firewood which has a high moisture content.

Skærmbillede 2015-08-27 kl. 11.45.27

Use logs which have an appropriate size

– and make the most of your money 

This test shows how much the size of the logs matters when it comes to your wood burning stove’s efficiency which is it’s ability to exploit the firewood as much as possible to create heat.

The test was done in a stove. Fuel: Beech wood. The length of the logs: 33 centimetres. Fuel load: 4.5 kilogram.

Tabel over virkningsgrad ved forskellige brændestrørrelser

The size of the logs matters if you want the optimal efficiency for you wood burning stove and complete combustion


If the logs you use to fire with are too big, you’ll reduce the efficiency while logs too small can damage the environment.

Read more about the size of the logs and which impact the size has on particle and carbon dioxide emissions below.

In this test, they fired five times with 2.5 kilogram of firewood with logs of varying sizes.

The test was made in a wood burning stove. Fuel: Beech wood without bark. Fuel load: 2.5 kilograms. Moisture content: 15.5 %.

Billedet viser størrelserne på det brænde der er brugt i forsøget

The size of the logs matters in the combustion process

Grafen viser hvor stor betydning brændestykkernes størrelse har for den rene forbrænding

The size of the logs helps determine how complete combustion will be


The test shows that the most ideal log size is 7x7x25 centimetres if you want to minimise the particle and carbon dixode emissions during combustion.


If you use logs that are too small, you won’t get complete combustion. That’s because there won’t be enough oxygen to burn the gases which the firewood releases. The pollution happens when the gases disappear up the chimney as smoke instead of being transformed into heat in your house.

If you fire with logs that are too big, you’ll risk that the temperature in the stove will be too low.

Use neither too little nor too much firewood  

This test was done to establish how big the amount of firewood should be when firing in order to minimise particle and carbon dioxide emissions.

The test was done in a wood burning stove. Fuel: Beech wood without bark. The size of the logs: 5x5x25 (the amount of logs is increased for each test). The moisture content: about 14 %.

Mængden af brænde brugt ved hver indfyring.

Use neither too little nor too much firewood when firing.


The graph shows that the lowest particle and carbon dioxide emissions happen when the amount of firewood is 0.92 til 1.75 kilogram per firing.

The graph also shows that you should be careful not to put neither too much nor too little firewood in your wood burning stove when you fire.

Both the smallest amount of 0.45 kilogram per firing and the biggest amount of 2.30 kilogram per firing pollute more than the recommended amount of 0.92 to 1.75 kilogram per firing.

You can find more information on the TFZ tests here

*Source: The graphs in this article come from TFZ-Bericht no. 36. og 22:

Ellner-Schuberth, F.; Hartmann, H.; Turowski, P.; Roßmann, P. (2010): Partikelemissionen aus Kleinfeuerungen für Holz und Ansätze für Minderungsmaßnahmen. Berichte aus dem TFZ, Nr. 22. Straubing: Technologie- und Förderzentrum im Kompetenzzentrum für Nachwachsende Rohstoffe (TFZ), 135 Seiten, ISBN 1614-1008

Schön, C.; Hartmann, H. (2014): Nutzer- und Brennstoffeinflüsse auf Feinstaubemissionen aus Kleinfeuerungsanlagen. Berichte aus dem TFZ, Nr. 36. Straubing: Technologie- und Förderzentrum im Kompetenzzentrum für Nachwachsende Rohstoffe (TFZ), 173 Seiten, ISSN 1614-1008

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