Essentially wood fuelled boilers operate in exactly the same way as a conventional boiler. The fuel, pellet or chips, is fed to the main boiler unit where combustion is controlled and monitored to ensure efficient combustion by adjusting the in-flow of air. A heat exchanger, placed in the flue gas, then exchanges heat with the water which is circulating through the tubes.

Automatic wood-fired boilers consist of four key components:

1. Boiler: A wood fuelled system uses a conventional boiler, similar to those used in gas and oil fired systems. They comprise either a simple water jacket or a number of fire tubes positioned horizontally or vertically. This helps exchange the heat in the flue gas with the water in tubes. The boilers are highly efficient with typical efficiencies of 85% to 92%.

2. Burner: A short auger meters the fuel into the combustion zone either below or above the grate. Moving or rotating grates are used in very large systems. Wood typically produces 0.5 % to 1% ash that is collected at the bottom of the grate in ash pans, larger systems have automatic de-ashing devices. Combustion quality is usually monitored to maintain correct fuel/air ratio.

3. Fuel feed systems:

• Wood chips - This is not a free flowing material and storage bunkers are fitted with agitators. Small systems use two rotating spring loaded arms that drag the material towards an auger. Larger wood-chip storage systems require moving floors that push the material towards an auger at the end of store.
• Wood pellets - These flow very easily and do not need special agitator systems. The fuel is removed from the store by an auger which lifts the fuel and deposits it in a drop cell or zone before it is metered to the burner by another auger. The drop cell is a safety feature that prevents unwanted air being drawn into the boiler, and prevents burn back.

4. Fuel delivery and storage: Fuel is supplied by an external contractor or as a self supply operation, for example a farm or estate with its own wood harvesting operation. The chips or pellets can be tipped directly from a tipper truck or trailer into an bunker. Other options include using dump bag, front end loaders, purpose built containers, or feed blowers (pellets only). The storage system can account for a large proportion of the cost of installing a biomass heating system.