All wood fuels need more storage space than fossil fuels (oil or LPG). The three types of fuel most commonly used are logs, wood chips and wood pellets. Of these, the latter two can be used in automated systems and stored in a hopper. Logs must be manually fed into the boiler and they are less automated, so their efficiency is more dependent on human input. If the wood isn’t adequately seasoned or contains too much moisture, or if the air supply is reduced too early, they will not burn as well and will create smoke and tar.
Wood pellets are made from compressed sawdust and wood shavings and other biomass products and are uniform in size and shape. They have a higher energy content and so take up less storage space than logs or wood chips. Stove and boiler manufacturers specify the size, shape and moisture content their products need to perform well. Wood pellet systems are the smallest, neatest and most like a mainstream boiler and require the least input from the user.
Wood chips are cheaper and abundant. They allow for more mechanisation than logs, but are not as efficient as wood pellets. It’s important that they are pretty uniform in size to work smoothly in a automated domestic system.
If you don’t have a hopper, you will have to load sacks manually into the boiler or stove’s feed system. How frequently you have to do this will depend on the system you choose. Not all suppliers are able to supply loose pellets.
It's important to think through carefully the supply, storage and handling of fuel before you invest in a biomass boiler. There is generally some trade offs between each element, and they will be specific to your site. You need to consider ease of access for the fuel delivery and how you are going to get fuel to the boiler.
To maximise the efficiency of your appliance it’s important to get the moisture content of the fuel right. Ideally logs and wood chips will have a moisture content of less than 25 per cent. Wood pellets need to be dryer – at around 8 per cent moisture. Your supplier should give you information about how to achieve this.