You can expect a calorific value of approximately 4.1 kWh per kilogram from logs. This will depend on how dry the wood is and the type of wood you’re using. As a guideline, for a batch-burning boiler such as the Guntamatic BMK or the Effecta Lambda, we recommend logs that are 500mm long and 12-15cm thick.


Make sure that you only use sustainable wood. When you’re doing such a lot to reduce emissions, it makes sense to boost your eco input! Check that the wood doesn’t have any coatings or preservatives and that it’s as dry as possible: wet wood will reward you with excess tar and soot which can wreak havoc on your flue. 

See if you can get your hands on a moisture meter as this will help you test how dry wood is. If you’re dealing with harder woods, you’ll need around two years to season it properly, whereas soft wood needs closer to one year. 

Sourcing wood

We recommend the Biomass Suppliers List (BSL) for quickly and efficiently sourcing local wood suppliers. The directory lists Wye Valley suppliers as well as others across the UK. 

This table gives you an idea of how wood compares to other types of fuel:

Fuel kg/m3 g CO2/kWh kWh/ Kg
Logs 350 7 4.1
Wood Chips 250 7 3.5
Wood Pellets 650 15 4.8
Coal n/a 484 8.6
LPG n/a 323 6.6
Heating Oil n/a 350 10
Electricity n/a 530 n/a
Source, Biomass Energy Centre